Clean production and properties of geopolymer concrete

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Journal of Cleaner Production 251 (2020) 119679

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Journal of Cleaner Production

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Review

Clean production and properties of geopolymer concrete; A review

Y.H. Mugahed Amran a, b, *, Rayed Alyousef a, Hisham Alabduljabbar a,Mohamed El-Zeadani c

a Department of Civil Engineering, College of Engineering, Prince Sattam Bin Abdulaziz University, 11942, Alkharj, Saudi Arabiab Department of Civil Engineering, Faculty of Engineering, Amran University, 9677, Quhal, Amran, Yemenc Department of Civil Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400, Serdang, Selangor, Malaysia

a r t i c l e i n f o

Article history:Received 22 September 2019Received in revised form8 December 2019Accepted 11 December 2019Available online 16 December 2019

Handling editor: Prof. Jiri Jaromir Kleme�s

Keywords:Alkali activation solutionCarbon dioxide emissionGeopolymer concreteGeopolymerPropertiesSupplementary cementitious materials

* Corresponding author. Department of Civil EnginePrince Sattam Bin Abdulaziz University, 11942, Alkhar

E-mail addresses: [email protected], m(Y.H.M. Amran).

https://doi.org/10.1016/j.jclepro.2019.1196790959-6526/© 2019 Elsevier Ltd. All rights reserved.

a b s t r a c t

The incessant production of cement has increased the amount of CO2 being released into the atmo-sphere; thus, aggravating the issue of global warming which has an adverse effect on the environment.Therefore, a more sustainable approach and a careful review of the existing admixtures used to replaceconventional concrete have become highly imperative. To this end, many investigations on geopolymerconcrete (GeoPC), which exhibit similar or better durability and high strength when compared to con-ventional concrete, have been carried out by various researchers. GeoPC concrete has the advantage ofcement replacement with supplementary cementitious materials that are combined with alkali activatedsolutions. GeoPC is a relatively new, innovative and sustainable engineering material with many ad-vantages over ordinary concrete. For example, it exhibits higher early strength, lower natural resourceconsumption, low cost and ability to form various structural shapes. GeoPC is an essential material thatcan be used for concrete building repairs, maintenance of road transport infrastructure and reducing thenegative environmental effects. Therefore, this paper presents a comprehensive review of GeoPC ma-terial, its constituents, production techniques, curing regimes, properties and its potential applications inthe construction industry.

© 2019 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22. Constituent materials of GeoPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

2.1. Source of by-product materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.1.1. Fly ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.2. Silica fume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.1.3. Rice husk ash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.1.4. Red mud . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.1.5. Ground granulated blast slag . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.1.6. Fine aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92.1.7. Coarse aggregates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2.2. Alkaline liquids system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103. Clean production of GeoPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

3.1. Composition of GeoPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2. Curing regimes of GeoPC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2.1. Ambient curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.2.2. Steam curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

ering, College of Engineering,j, Saudi [email protected]

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3.2.3. Oven curing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124. Fresh properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

4.1. Stability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.2. Compatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.3. Setting time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.4. Workability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.5. Deformability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

5. Mechanical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155.1. Compressive strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165.2. Splitting tensile strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175.3. Flexural strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185.4. Modulus of elasticity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 185.5. Stressestrain behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195.6. Rate of strength development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

6. Physical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206.1. Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206.2. Dry shrinkage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 206.3. Porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216.4. Sorptivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

7. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

List of acronyms

GeoPC Geopolymer concretePOFA Palm oil fuel ashRHA Rice husk ashRM Red mudFA Fly ashSFA Silica fume ashCDD Concrete digital dilatometerRCA Coarse recycled aggregateSTS Splitting tensile strengthMoE Modulus of elasticityGGBS Ground-granulated-blast-furnace-slagsC&D Construction and demolitionNCA Natural coarse aggregatesALS Alkaline liquids systemAAS Alkaline-activator solutionSS/SH Sodium silicate to sodium hydroxideSPs Super-plasticizersCDD Concrete digital dilatometerDS Dry shrinkageAAC Alkali-activated cementLOI Loss in ignitionPFA Pulverized fuel ashITZ Interfacial transition zoneOPC Ordinary Portland cement

List of nomenclaturesAl AluminumK PotassiumNa SodiumSi SiliconPo(Ss) Poly(sialates)CO2 Carbon dioxideH2O WaterNa2SiO3 Sodium silicateNaOH Sodium hydroxideK2SiO3 Potassium silicateKOH Potassium hydroxideSiO2 Silicon oxideAl2O3 Aluminum oxideCaO Calcium oxideMgO MgO oxideFe2O3 Iron oxideSO3 Sulfur trioxideS Sulfide SulfurIR Insoluble ResidueNa2O Sodium oxideCa(OH)2 Calcium hydroxidAl(OH)3 Aluminium hydroxideNa2SiO3 Sodium silicate

1. Introduction

Concrete is an important material used in construction in-dustries all over the world (Shaikh, 2016). It is considered to be themost commonly utilized building material due to its considerablylow price, durability, availability of constituentmaterials and abilityto be formed into any shape or size (Basha S et al., 2016; Shaikh,2016). The binding techniques and materials used for producingconcrete are also considered essential in construction technology

(Lakshmi and Nagan, 2011). For instance, cement is the mostextensively used binding material in plain concrete and reinforcedconcrete applications (Basha S et al., 2016). The production ofPortland cement (PC) keeps increasing by 9% annually, worldwide.This rate of increase poses a great danger to the environment due tothe large volume of CO2 being released into the atmosphere duringcement production (Madheswaran et al., 2013). Specifically, theannual greenhouse gas emissions from PC production are about 1.5billion tons, or an average of 6% of the total emissions, from

Table 1Concrete volumes and target embodied-energy CO2 emissions (A. Castel, 2016).

Strength(MPa)

Structural element Quantity(m3)

Emissionfactor(t CO2

�e/m3)

Emissions(t CO2

�e)

15 Blinding 589 0.20 11932 Footings 489 0.24 11932 Slabs 1948 0.27 53340 In situ columns and

walls235 0.27 63

40 Precast walls 1067 0.33 351e1185

Fig. 1. Production of GeoPC in the Ancient Roman era (Davidovits, 2015).

Y.H.M. Amran et al. / Journal of Cleaner Production 251 (2020) 119679 3

multiple sectors around the world (Fig. 1) (Castel, 2016; Dhakal,2009; Madheswaran et al., 2013). The greenhouse effect preventsthe reflection of solar radiation back into space; thereby, keepingregular temperature on the earth surface limited between 15 �C and18 �C (Shalini et al., 2016). The CO2 concentration in the atmospherehas recently increased by about 30%, or 467 Mt, of which 8% camefrom the UK in 2012 (Basha S et al., 2016; Davidovits, 2002; Josephand Mathew, 2012; Li and Xu, 2009; Madheswaran et al., 2013;Shaikh, 2016; Shalini et al., 2016; Soltaninaveh, 2008). Theseemissions produce a greenhouse effect, a natural phenomenon thataccounts for around 65% of global warming (Bhikshma, 2012). Thevolume of CO2 emitted during the production of various concretecomponents is summarized in Table 1 (A. Castel, 2016) and can becomputed using equation (1). Economic development requires ahighly effective application of renewable and non-renewable re-sources (Shalini et al., 2016). Sustainable development of a newadmixture to replace ordinary concrete has become increasinglyimportant as the world continues to face serious environmentaldegradation (Han et al., 2014; Shalini et al., 2016).

A suitable alternative to ordinary concrete is geopolymer. Thismaterial has been used in the past during the ancient Roman Em-pire (Fig. 2). The Romans were known for their historic monu-mental structures, especially their pioneering use of limestonebefore the advent of cement (Davidovits, 2015). Geopolymers aremostly produced by burning watershed materials, a techniquedeveloped by David Easton in 2011, to produce sustainablemasonrywith less cement using recycled materials (Palm oil fuel ash (POFA),red mud (RM), silica fume (SF), rice husk ash (RHA), and fly ash(FA)). These materials are known to enhance sustainability of

ðSi2O2Al2O2Þn þ H2O þ OH�/SiðOHÞ4 þ Al�OH

�4�/

0BBB@e

:jSijO

eOe

:jAljO

eOe

1CCCA

n

þ aH2O

structures that rely on concrete masonry units. Furthermore, Geo-polymerization includes a chemical reaction of aluminosilicateoxides Al3þ in IV-V fold synchronization with silicates, vassalagepolymeric-Si-O-Al-O-sialate connections similar to the following:

2(Si2O5,Al2O2) þ K2(H3SiO4)2 þ Ca(H3SiO4)2 / (K2O,CaO) (8SiO2,2Al2O3, nH2O).

Poly(sialates) (Po(Ss)) are labeled by the subsequentexperimentally-based formulation Mn[e(SiO2)ze AlO2]n. wH2O(Davidovits, 2015, 1999).

Where;

M is a monovalent cation such as potassium (Kþ) or sodium(Naþ),z is any 1, 2, 3 or � 3.n is the amount of multiple condensation.

The PoS are known as ring polymers and chain with Al3þandSi4þ in IV-crinkle harmonization with O2 and sequences in from airto semi-crystalline. In 1978, Davidovits proposed that the by-product of the reaction of alkaline liquid with aluminum (Al) andsilicon (Si) can be used for manufacturing geopolymers (Table 2)(Davidovits, 2015, 2002). The patchy in semi-oystalline 3D-Network silico-aluminate systems were known as “geopolymer” byDavidovits because they resembled plastics which are polymers inthe basis of carbon rather than silicon. Davidovits further catego-rized geopolymers depending on the Si:AI ratio as: Crystalline PoSwith SI:Al ¼ 1:1 ratio, Po(S-disiloxo) Mn-(SieO-A1-O-Si-O-Si-O)nwith SI:Al ¼ 2:1 ratio and Po(S-silcao) Mn-(SieO-A1-O-Si-O-)nwith SI:Al ¼ 3:1 ratio (Davidovits, 1999).

In addition, Davidovits stated that the atomistic proportion Si:Alin the PoS system controls its characteristics and utilization. Forexample, a small ratio of Si:Al (1.23) initiates a 3D-system that isextremely firm. A great ratio of Si:Al, higher than 1.5, causes greaterpolymerization. The Si:Al � 3:1 with Po(S-multisiloxo), the poly-meric building obtains from the irritated connecting of PoS sheets,networks or chains with a PoS link (3D or 2D-Networks)l(Davidovits, 2015, 1999). The subsequent reactions arise duringgeopolymerisation (Davidovits, 1999).

Geopolymer precursors refer to a family of inorganic pastes witha typical magnitude of not more than 20 mm. They have a smallloose density between 0.54 and 0.86 g/cm3, a high surface areafrom 300 m2/kg to 500 m2/kg, a bright texture and a round shape.

Table 2Typical properties of byproduct materials (ACI 233R-95 Committee Report, 1997; B. N. Sangeetha, 2015; Calder�on-Moreno et al., 2002; Kabir et al., 2015; Suresh and Nagaraju,2015).

Property/Element FA RHA GGBS SFA POFA

Fineness (m/kg) 450 ~450 m2/kg 350 to 550 15,000 to 35,000 4900e5200 cm2/gBulk density (kg/m) 1300 96e160 kg/m3 1200 1350e1510 2.40e2.50 g/cm3

Specific gravity 2.2 2.11 2.9 2.2 2.14Silicon (SiO2) 38 to 55 >90 30 to 40 >85 >80Aluminum (Al2O3) 20 to 40 >9 5 to 20 <2 16e18Iron (Fe2O3) 6 to 16 >2.8 <2 <1 8e10Calcium (CaO) 1.8 to 10 1e2.2 35 to 40 5e18Magnesium (MgO) 1 to 5 >1 5 to 18 >1.2

6%

16%42%

20%6%

10%

Residential, 6% Road transport, 16%Energy, 41% Industries, 20%Other transport, 6% Other sectors, 10%

A

30%16%

6%

7%

19% 11%

7%

4%

China, 30% EU, 16%Other HICs, 8% India, 7%USA, 195 Other MICs, 11Russia, 7% Japan, 4%

B

Fig. 2. Percentage of CO2 emitted by different sectors (A) and by countries (B) worldwide (European Commision, 2014).


These materials also contain dense spheres, cenospheres, porousunburnt carbon, irregular-shaped debris, and blast furnace slag(created by reducing iron ore with coke in a blast furnace at tem-peratures of 1350 �Ce1550 �C). They also have the same chemicalcompositions as zeolites with amorphous microstructures(Davidovits, 1999). Geopolymer precursors usually have a glasscontent in excess of 95% and are crushed into fine binder (ground-granulated-blast-furnace-slags (GGBS)) manufactured by calciningkaolin at a temperature range between 650 �C and 800 �C) (Yanget al., 2013). These materials mainly consist of amorphous SiO2

and Al2O3 with great pozzolanic activity (Khale and Chaudhary,2007). Apart from its filling effect, calcining kaolin at a high tem-perature can also react with calcium hydroxide, the NASH gel, andthe coexistance (NASH and CHS) in the final matrix, a product of PChydration, to produce calcium silicate hydrate gels. A combinationof these materials, plus other alternative raw materials with silicaand alumina content, can minimize CO2 emissions and reduce theharmful environmental effect of cement manufacturing (Alamet al., 2014). Thus, the percentage of CO2 emissions can be deter-mined using Eqn. (1) (Kavitha et al., 2016).

CO2 emission; ð100%Þ¼Ci � C0C0

� 100 (1)

where.

e C0 ¼ CO2 emission of the control mix; ande Ci ¼ CO2 emission of the blended GeoPC mixes.

In the design of GeoPC, another gel nuclei particle is used whichshould be stable enough to resist depolymerization and to start anew gel phase that will be responsible for enhancing the strengthand durability of GeoPCs. Moreover, geopolymeric materials haverecently attracted considerable attention from researchers due totheir environmental benefits, such as reduced CO2 emissions andreduced depletion of natural resources (Sashidhar et al., 2015). Also,geopolymeric materials can be used as an alternative cementitiuosmaterial due to its greater durability and advanced mechanicalproperties. Fig. 3 summaries the elementary difference betweenGeoPC and ordinary Portland concrete (OPC), and illustrates whyGeoPC is more preferable than OPC, particularly in terms of sus-tainability and durability. GeoPC is a self-compacting material; thatis, it is capable of consolidating under its own weight and it isconsidered as the most revolutionary progress in concrete tech-nology. This material, which is often used in complex structuralformworks, can reduce industrial wastes by at least 12.2 Mt everyyear, and can emit 5 to 6 times less CO2 when compared to PC(Anuradha et al., 2014; Reddy, 2015; Shaikh, 2016). Producing oneton of PC releases approximately one ton of CO2 to the air as aneffect of the decarbonation of limestone in a kiln at the time of theproduction of this cement (Chindaprasirt et al., 2014; Dimas et al.,2009; Talakokula et al., 2016). Several investigators have tried toreplace PC with a more environment-friendly concrete that usescertain byproduct materials which will be comprehensivelyreviewed in the following subsections. A paradigm shift to GeoPChas been witnessed in construction industries all over the worlddue to its indispensable role in reducing the amount of pollutantsand CO2 generated during PC production.

Fig. 3. Performance and properties comparison between GeoPC and OPC.

Fig. 4. Specific surface area and particle size of different binder materials (Sobolev andGuti�errez, 2005).


Therefore, the objective of this review paper is to provide acomprehensive review on the constituents, clean productiontechniques, heat curing methods and properties of GeoPC as well asto comprehensively review the literature to provide insights intothe potential application of GeoPC material in the constructionindustry today.

2. Constituent materials of GeoPC

Generally, PC is not required in the production process of GeoPC.The two key constituents of GeoPC are: (1) alkaline solutions suchas sodium silicate (Na2SiO3), sodium hydroxide (NaOH), potassiumsilicate (K2SiO3), and potassium hydroxide (KOH), and (2) Alumino-silicate sources of byproduct materials such as RM, SFA, RHA, FA,GGBS and fine and coarse aggregates.

2.1. Source of by-product materials

Geopolymer is a patchy silicate-alumino cementing materialthat is produced by poly condensation reaction of alkali and poly-silicates geopolymeric precursor known as the geopolymerizationprocess (Davidovits, 2015; Dimas et al., 2009). Geopolymerizationis an advanced process capable of converting several silicate-alumino materials into valuable products named inorganic poly-mers or geopolymers (Davidovits, 1999). Geopolymerization in-cludes an inhomogeneous chemical reaction between alkalisolutions and silicate-alumino oxides at mild temperatures. It is ahighly alkaline conditions squashy patchy to semi-crystallinepolymeric systems that contain SieOeSi and SieOeAl bonds(Davidovits, 2015, 1999; Dimas et al., 2009). In the geo-polymerization process, a Si- and Al-rich source material reactswith a highly alkaline solution to create a binding material like a 3Dnetwork of poly-(sialates), amorphous to semi-crystallinity ofgeopolymers. The development history of concrete technology re-veals that using various supplementary materials to reduce up to40% of waterecement ratio can result in subtle technological

changes (Table 2). Fine particles can be introduced into concrete byusing sustainable materials. However, nano-powders have aremarkably large surface area that can greatly change both surfacemorphology and surface energy (Fig. 4). By modifying their basicproperties and chemical reactivity (Klabunde and Richards, 2009),these factors can enhance the catalytic ability of nano-materials(Zhang et al., 1998). The new nano-materials that are based onmetals (lithium-ion, sodium-ion, lithium-ion and sulfur-ion), ox-ides (SiO2, Al2O3, CaO, MgO, Fe2O3, SO3, and S) and germaniumdemonstrate a superplastic behavior and require 100%e10000%elongation before it encounters failure (Boyd et al., 2002). A GeoPCwith ultrahigh strength and increased durability is expected to bedeveloped in the near future.

Table 3Frost resistance of FA (ASTMC666, 1997).

Fly ash mixtures Results at 300 cycles

Frost resistance in water, ASTM C 666 Method A[AASHTO T 161]

Expansion, % Mass loss, % Durability factor

Average of: Class C, 0.006 1.6 101Class F 0.004 1.8 102

Control mixture 0.002 2.5 101


2.1.1. Fly ashGeoPC may be formed by using low-calcium FA obtained from

coal-combustion binder stations as a by-product of bituminous oranthracite coal combustion (Adam, 2009; American ConcreteInstitute. and Malhotra, 2000; Talakokula et al., 2016). FA is iden-tified as “pulverized fuel ash,” and it is a byproduct of coal com-bustionwhich comprises fine particles that have been blown out ofthe boiler along with flue gases (flue gas refers to the burningexhaust gas formed at power plants) (Davidovits, 1999; Liu et al.,2016). FA is often used as a substitute to ordinary PC (OPC) inconcrete production (Anuradha et al., 2014; Castel and Foster, 2015;He et al., 2012; Kovacik et al., 2011; Liu et al., 2016; Talakokula et al.,2016; Ukwattage et al., 2013; Yildiz, 2004; Zhuang et al., 2016a).Compared with traditional concrete, FA concrete has a higherstrength and durability (Anuradha et al., 2014), can be pouredreadily, has lesser permeability, and resists the alkaliesilica reac-tion more efficiently; thereby, extending its service life andlowering its cost (Zhuang et al., 2016b). For example, the low-calcium FA has been positively used to produce GeoPC when thealuminum and silicon oxides constitute about 80% by weight, withthe Si:Al ratio of 2 (Adam, 2009; American Concrete Institute. andMalhotra, 2000). The iron oxide content is typically varied be-tween 10 and 20% by weight, while the calcium oxide content islower than 5% by weight (Adam, 2009). The FA carbon content, asdesignated by the loss on ignition by weight, is below 2%. Report-edly, the distribution of FA particle size is about 80% of the FAparticles which are less than 50 mm (Adam, 2009; Castel and Foster,2015; Law et al., 2015; Talakokula et al., 2016). FA can be 20%e60%cheaper than OPC in some countries, but in some cases, OPC can bemore than twice as expensive as FA (Chindaprasirt et al., 2014;Kovacik et al., 2011; Kumar and Kumar, 2013; Liu et al., 2016;Shaikh, 2016; Zhuang et al., 2016a). However, FA is rarely shippedat long distances and is more expensive than local OPC becausesome concrete durability requirements can only be fulfilled byusing FA. This material can also positively affect the environmentdue to the conservation of the landfill spaces, reduction of waterand energy consumption, and minimization of greenhouse gasemissions (Anuradha et al., 2014; Chang and Shih, 2000;Chindaprasirt et al., 2014; Talakokula et al., 2016; Ukwattageet al., 2013; Yildiz, 2004; Zhuang et al., 2016b). In this case, FAcan reduce the production of OPC that emits about 1 ton of carbondioxide for every ton of cement manufactured; in other words, foreach ton of FA used, the CO2 emissions are cut by one ton (Casteland Foster, 2015; Talakokula et al., 2016; Zhuang et al., 2016b). Infact, using an entire year’s supply of FA for concrete production isequivalent to eliminating 25% of the CO2 emitted by vehiclesworldwide (Poudenx, 2008). The fineness of FA is calculated based

Fig. 5. Approximate particle size distribution of FA (Li et al., 2014).

on ASTM C115 standard (ASTMC115-96, 1996) as illustrated in Fig. 5(Sanjayan et al., 2015). FA particles’ average size and blain surfacearea are usually 9 mm and 0.37 m2/g, respectively. Given the verysmall size of its particles, FA can enhance the density and frostresistance of GeoPC and reduce its permeability (Table 3)(ASTMC666, 1997).

FA production has become significant because of its key role inthe economic and green utilization of technologies (Fig. 6). Thismaterial can also be used in soil amendment (Ukwattage et al.,2013), nutrient retrieval (Kovacik et al., 2011), waste removal (asa low-cost absorbent) (RUBEL et al., 2005; Yildiz, 2004), and zeoliteproduction (as a source of Si and Al) (Chang and Shih, 2000; Lloydet al., 2012). FA has been adopted recently as an alternate materialfor producing geopolymers, a new cement or binder that is similarto hydrated cement in terms of reactivity, appearance, and othercharacteristics (Zhuang et al., 2016b). The chemical compositions ofOPC and FA are given in Table 4.

2.1.2. Silica fumeThe Silica Fume Association was established in 1998 to help

silica fume manufacturers to promote the application of silica fumein concrete (Anuradha et al., 2014; Saraya, 2014). As a byproduct ofSi metal or ferrosilicon alloy production (highly-reactive pozzolanand a fundamental ingredient in high-performance concrete), silicafume ash (SFA) was used to replace 10e40% of OPC content inconcrete (Toledo Filho et al., 2003). Based on its physical, chemicaland mineralogical properties, SFA is a highly reactive pozzolan thatmay come from either natural or artificial sources (Fig. 7)(Sreenivasulu et al., 2015; Triantafillou, 2016). The silica in pozzo-lana reacts with the portlandite formed during the hydration ofOPC and assists in its strength development (Singh et al., 2015;Sreenivasulu et al., 2015). Furthermore, this material progres-sively creates calcium silicate hydrate, a binder that takes up thespace in concrete materials and enhances their impermeability,durability, and strength (Sakulich, 2011; Sashidhar et al., 2016;

Fig. 6. Application fields of FA in the U.S. (Adams, 2017).

Table 4Chemical compositions of OPC and FA (Malathy, 2009).

Comp. LOI IR SiO2 Al2O3 Fe2O3 CaO Mgo So3 Na2O Specific surface

Cement 1.65 0.76 19.33 5.66 2.66 63.07 0.36 3.38 0.14 4.51 m2/kgFly Ash 2.88 0.30 54.92 23.04 6.62 3.84 2.82 0.76 0.73 3.60 m2/kg

Fig. 7. Artificial production of SFA (Naik, 2008).

Table 5Technical specifications of RHA (Junaid et al., 2014; Shalini et al., 2016; Zain et al.,2011).

Technical specifications Percentage/Size/Value

P 75% minimumHumidity 2% maximumParticle size 25 mm (d50)Colour GreyLoss on ignition at 800 �C 4% maximumpH value 8


Singh et al., 2015). The hydration of OPC can be written as

C3S þ H2O ➟ CeSeH þ Ca(OH)2

where.

‒ CeSeH ¼ calciumesilicateehydrate; and‒ Ca(OH)2 ¼ Portlandite.

Meanwhile, the combination of limestone with the silica ofpozzolana can be expressed as:

Ca(OH)2þSiO2 ➟ CeSeH

where.

‒ SiO2 ¼ Silica

Amorphous silica has a faster reaction to silica compared withcrystalline silica. Such difference accounts for the variation amongactive pozzolanas and materials of comparable chemical composi-tion that display minimal pozzolanic activity (Kovler and Roussel,2011; Liu et al., 2016; Nematollahi et al., 2017). However,concrete-containing SFA can demonstrate very high strength anddurability (Geopolymer, 1988, n.d.; Kovler and Roussel, 2011). SFAcan be obtained from concrete admixture suppliers and, whenstated, is easily added during the production of concrete (with alimit between 10 and 20%) (American Concrete Institute. andMalhotra, 2000; Karbhari, 2013; Subang Jaya et al., 2013). Theplacement, finishing and curing of SFA-based GeoPC require specialattention from concrete contractors (Geopolymer, 1988, n.d.; Poonet al., 2006). The smoke that is emitted during furnace operations iscollected and sold as SFA instead of being landfilled (Al-Qadri et al.,2009). SFA can also be included in concrete as a mineral admixture(Al-Qadri et al., 2009; Bhavsar et al., 2014; MATSAGAR, 2015; Moet al., 2016). Silica fume principally consists of amorphous (non-crystalline) silicon dioxide (SiO2) (Siddique and Iqbal Khan, 2011)and extremely small particles that are approximately 1/100 the sizeof an average cement particle (Greim and Kusterle, 2004). Given itshigh SiO2 content, large surface area and fine particles, SFA is ahighly reactive pozzolan applied in concrete (Al-Qadri et al., 2009;

Detwiler et al., 1996; Siddique and Iqbal Khan, 2011; Triantafillou,2016). The quality of SFA is specified by ASTM C 1240 (ASTM,2012) and AASHTO M 307. Concrete structures in the US oftendeteriorate by the corrosion induced by deicing or marine salts(Wolsiefer, 1991). SFA concrete with low water content is greatlydefiant to infiltration by chloride ions (Neville, 1987). Given itsdesirable properties, transportation agencies have begun to use SFAinstead of concrete for constructing new bridges or rehabilitatingexisting structures (Sakulich, 2011). The specifications of silica fumeconcrete with high durability or strength can be acquired from thesuppliers of SFA or other key admixtures (Anuradha et al., 2014;ASTM, 2012; Bhavsar et al., 2014; Poon et al., 2006; Saraya, 2014).Silica fume can be added in its wet or dry form during the pro-duction of concrete at a manufacturing plant (Atis et al., 2005). SFhas been effectively manufactured in both dry batch plants andcentral mix as an alternative to concrete (Kamath and Khan, 2016).Several guidelines have also been issued concerning the handlingand use of silica fume for producing high-quality concrete(Mazloom et al., 2004).

2.1.3. Rice husk ashRice husk ash (RHA) is a carbon-neutral green product (Table 5)

(Junaid et al., 2014; Mehta, 1977) that is mostly used as ash forgenerating power (Ajay et al., 2012), or as boiler fuel for processingpaddy with volume between 20% and 25% of the rice paddy is anindigestible outer husk that is removed and burnt either inhousehold stoves or in local power plants to produce steam forboiling rice (Sanjayan et al., 2015). In contrast, in the RHA-basedGeoPC structures, the presence of sodium silicates with alkali-activated at elevating temperature can lead to producing newcrystalline phases, for example Na-feldspars, albite (NaAlSi3O8),and nepheline (NaAlSiO4), contributing to a higher thermal stabilityat elevated temperatures (Saravanan and Sivaraja, 2016). Thecrystalline silica content of RHA has received wide concern becauseof the potential hazards of inhaling this mineral (Sanjayan et al.,2015). RHA is approximately 25% by weight of rice husk whenburnt in boilers (Fig. 8) (Raheem et al., 2013). RHA is an excellentsuper pozzolan that can be utilized to produce mixes of specialconcrete (Saravanan and Sivaraja, 2016). This material may beapplied as an alternative for cement in concrete production(Habeeb and Mahmud, 2010; He et al., 2013; Junaid et al., 2014;Mehta, 1977; Raheem et al., 2013; Saravanan and Sivaraja, 2016;Shalini et al., 2016). Fine amorphous silica is increasingly beingutilized in the manufacturing of special cement and low-permeability, high-strength and high-performance concretemixes as well as in the construction of nuclear power plants,

Table 6Chemical composition of RM (Dharmendra et al., 2017; He et al., 2013, 2012;Kumar and Kumar, 2013; Ye et al., 2014).

Oxides Approximated value, %

Fig. 8. Synthesis of RHA from rice husks (Gursel et al., 2016).


marine environments and bridges (Board, 2012; Sanjayan et al.,2015). Most of the SFA or micro silica being sold in the markethas been imported from Burma, China and Norway (Naji et al.,2010). In addition, due to supply shortage, the price of silicafumes in India increased to almost US $500 per ton, which is fargreater than the selling prices in China, Canada, and the US (Habeeband Mahmud, 2010; Shalini et al., 2016). The cement demand inIndia is also expected to reach approximately 550 Mt by 2020 witha shortfall of approximately 230 Mt (�58%), and such increasingdemand can be ascribed to the increasing number of infrastructuralactivities being conducted within the country (Singh et al., 2015).Given that RHA comprises about 18% rice husks, producing one tonof rice will generate approximately 45 kg of RHA, which has sub-stantial pozzolanic properties, rich silica content (~95%), and highsurface area (Gonçalves and Bergmann, 2007). The amount, crys-talline content and chemical composition of the produced silicastrongly depend on furnace design and the burning temperatures(Zain et al., 2011). In GeoPC, it is reported that RHA blended con-crete could lower the temperature influence that arises during thehydration of cement (B. N. Sangeetha, 2015; Shalini et al., 2016).RHA blended concrete can improve the workability of concrete incomparison with OPC and could also raise the setting time ofcement pastes (Detphan and Chindaprasirt, 2009; Mehta, 1977;Nazari et al., 2011). Moreover, RHA-based GeoPC can reduce con-crete’s total porosity, adjust its pore configuration, and consider-ably lower the permeability which permits the effect of dangerousions contributing to the weakening of the concrete matrix (Habeeband Mahmud, 2010; He et al., 2013; Naji et al., 2010). RHA cementincreases the compressive strength and assists in improving theinitial age mechanical and long-term strength characteristics ofGeoPC pastes. Specifically, partial substitution of cement with RHAdecreases the water diffusion into concrete by capillary action andeffectively enhances the resistance of GeoPC to sulfate attack (Ajayet al., 2012; Saravanan and Sivaraja, 2016).

Silicon Dioxide, SiO2 ~13.14Aluminium Oxide, AL2O3 ~20.26Iron Oxide, Fe2O3 ~42.25Calcium Oxide, Cao ~1.25Sodium Oxide, Na2O ~4.36Titanium Oxide, TiO2 ~1.9Finesse ~2200 cm2/gmSpecific Gravity ~2.09

2.1.4. Red mudRed mud (RM) is an offshoot of the Bayer process (Fig. 8) for

refining bauxite to aluminawith volume between 55 and 65% of theprocessed bauxite (He et al., 2013), and the resulting alumina servesas raw material for creating Al via the HalleH�eroult process (Ye

et al., 2016, 2014). An ordinary alumina plant generates one totwo times as much RM as alumina. RM consists of 30e60% ironoxides which account for its red color (He et al., 2013; Si et al.,2013). More specifically, Dharmendra et al. (Dharmendra et al.,2017) reported a value of 42.25% for iron oxide content in RM.Furthermore, this material is extremely basic and has a pH value of10e13 (Ye et al., 2016). About 2 to 3 Mt of RM are being usedannually in cement production [100], red sand used in the sub-grade and sub-base in road construction (Biswas and Cooling,2013), and iron production (Liu et al., 2009; Paramguru et al.,2004). This material can also be used in producing low-cost con-crete, studying sandy soils, improving phosphorus cycles, reducingsoil acidity, capping landfills, and sequestering carbon (Si et al.,2013). The solid constituents of RM mainly consist of alumina,iron oxides (mostly hematite), and some toxic heavy metals(Paramguru et al., 2004). This material also becomes somewhatradioactive if the original bauxite contains radioactive minerals(Kumar and Kumar, 2013; Paramguru et al., 2004; Si et al., 2013; Yeet al., 2016, 2014). Given the high alkalinity and water content ofthis material, the safe and economic disposal of RM poses a majorenvironmental concern for alumina refineries (Dharmendra et al.,2017; He et al., 2013, 2012; Kumar and Kumar, 2013; Paramguruet al., 2004). Table 6 displays the chemical composition of RM.Reportedly, the addition of RM improved the intensity of the re-action and structural restructuring; however, enhancement in bothcompressive strength and setting time are found when the speci-mens only possess 5e20% RM, and greater than that could causeadverse effects on the relative properties of GeoPC (Dharmendraet al., 2017; He et al., 2013, 2012; Kumar and Kumar, 2013).Henceforth, it may be deduced that the low addition of RM inGeoPC as a partial cement substitute can lead to higher solution andutilization of sodium trisilicate which can have a positive effect onthe Young’s modulus, failure strain and compressive strength ofRM-based geopolymers, leading to more reactive Si to the GeoPCsynthesis, resulting in an important contribution to facilitating thegeopolymerization reaction.

2.1.5. Ground granulated blast slagBeing a by-product of blast furnaces, Ground granulated blast

slag (GGBS) is often utilized in iron production (Li and Zhao, 2003;Suresh and Nagaraju, 2015). This material can be obtained at tem-peratures of almost 1500 �C and is fed with a cautiously controlledmixture of iron ore, coke, and limestone (Fig. 9) (Suresh andNagaraju, 2015). The melted slag has a content of about 40% cal-cium oxide (CaO) and 30e40% silicon dioxide (SiO2), which is nearto the chemical formation of OPC. When iron ore is reduced to iron,the residual materials create a slag that floats on top of the iron(Ananthayya andWP., 2014; Suresh and Nagaraju, 2015). This slag isregularly tapped off as a molten liquid and must be quenchedquickly in large volumes of water tomanufacture GGBS (Sahithi andPriyanka, 2015). This quenching process utilizes the cementitious

Fig. 9. Sources of GGBS (Spanlang et al., 2016).


properties of the slag and engenders granules comparable to coarsesand (Sumajouw et al., 2004). The granulated slag is subsequentlydesiccated and ground to fine powder. GGBS can replace the OPCcontent of concrete by 35%e70% (Kelham, 1996) and exhibitsexcellent properties when finely ground and combined with othermaterials to form GeoPC (ACI 233R-95 Committee Report, 1997;Jain and Pal, 1998; Shukla et al., 2009). The glass particles ofGGBS comprise Q0-type mono-silicates, which are comparable tothose being applied in ordinary Portland cement clinker anddissolve upon activation by any medium (Ganesh Babu and SreeRama Kumar, 2000; Prediction of Long-Term Corrosion Resistanceof Plain and Blended Cement concretes, 1993). GGBS’s glass contenttypically exceeds 85% of its total volume, while its specific gravityranges from 2.7 to 2.90 (lower than that of ordinary Portlandcement), and its bulk density varies between 1200 kg/m3 and1300 kg/m3 (Saraya, 2014). The standard chemical composition ofthis material is presented in Table 7. The chemical composition ofordinary cement shows more similarities to that of GGBS than ofother mineral admixtures, such as POFA concrete (Saraya, 2014).GGBS can be used for refining the pores and increasing the long-term strength, sulfate, and alkali silica reaction resistance of con-crete as well as for reducing the water demand, permeability, and

Table 7Chemical composition of GGBS.

Oxides Quantities specifiedPractically, % (Saraya, 2014;Shukla et al., 2009)

Quantities specified byASTM C989, % (Debet al., 2015)

Annotation

SiliconDioxide,SiO2

30e35 40.0 e

AluminiumOxide,Al2O3

8e22 13.5 HigherSlag

CalciumOxide,CaO

27e32 39.2 Lower Slag

ManganeseOxideMgO

7e9 3.6 e

Iron Oxide,Fe2O3

8e10 1.8 HigherSlag

Sulfur ion,SO3

3e7 4 e

Sulfidesulfur, S

e 1.0e1.9 e

LOI e 0.0 e

heat generation during the hydration process (Castel and Foster,2015; Dimas et al., 2009; Hadjsadok et al., 2012; Saraya, 2014;Suresh and Nagaraju, 2015). However, the addition of GGBS caninfluence the reaction, characteristics and GeoPC matrix. The in-fluence is varied in the basis of the volume of GGBS added (5e50%).It is found that the reaction at 27 �C is governed by the GGBSactivation. The reaction at 27 �C is contributed by precipitation anddissolution of CeSeH gel due to the alkali activation of GGBS(Ananthayya and WP., 2014; Li and Zhao, 2003; Sahithi andPriyanka, 2015). Moreover, in the production of GeoPC, thealuminum and silicon present in the GGBS are activated by amixture of sodium silicate and sodium hydroxide solutions toproduce the geopolymer paste that binds the aggregates (Islamet al., 2015, 2014). It may be deduced that the increased additionof GGBS can increase the ultrasonic pulse velocity, resistance to acidand compressive strength of GeoPC at all curing regimes(B. N.Sangeetha, 2015; Ganesh Babu and Sree Rama Kumar, 2000).

2.1.6. Fine aggregatesFine aggregates contribute to the production of GeoPC by 35%e

45% (Shaikh, 2016; Shalini et al., 2016). Foundries have successfullyrecycled and reused sand many times in the past. This study ex-amines the efficacy of foundry sand as a partial substitute (up to25%) for fine aggregates in GeoPC (Joseph and Mathew, 2012). Self-compacted GeoPC is a composite material made from a geo-polymeric paste, fine and coarse aggregates. Fine aggregates areusually mined from sand quarries, and red mud is a result of theBayer process for manufacturing alumina from bauxite (Fig. 10)(Soltaninaveh, 2008). Electronic waste sand can also be applied as apartial substitute for fine aggregates in concrete. A concrete mixthat contains sand showsmuch better properties than conventionalconcrete because of the even particle size, high reactive silicacontent, and high density of sand. Granite powder is a fine aggre-gate in GeoPC that is generated from the waste materials of thegranite industry (e.g., solid waste and stone slurry), which annualvolumes can reach as high as 12.2 million tons (Reddy, 2015).Table 8 gives the typical properties of fine aggregates. Meanwhile, itis reported that the appropriate choice for the ratio of fine aggre-gate to total aggregate content (60e75%) in GeoPC is 0.35, and thisleads to improvements in the Poisson’s ratio, modulus of elasticity,split and flexural tensile strength of GeoPC by 19.2%, 14.4%, 45.5%and 30.6%, respectively, compared to normal concrete (Joseph andMathew, 2012). Also, researchers reported that the pozzolanicmaterial for higher concrete strength indicated that optimumperformance is attained by replacing 7%e15% of Metakolin; how-ever, the increase in FA content led to decrease in strength (Basha Set al., 2016; Soltaninaveh, 2008). Specifically, it can be inferred thatthe mechanical properties of GeoPC can improve when the partialreplacement of cement does not exceed 20%, any more than that

Fig. 10. Bayer process (Dodoo-Arhin et al., 2017).

Fig. 11. Coarse aggregate particle distribution (Adewuyi et al., 2017).

Table 9Physical properties of coarse aggregates (A. Castel, 2016; Castel and Foster, 2015;Ganesh et al., 2016; Shrivastava and Shrivastava, 2015).

Property Approximated value

Specific Gravity 2.5e2.80Bulk density 1550e1570 kg/m3

Compaction Factor 0.85e0.95Flakiness Index 10.23e12.2%Elongation Index ~28.72%absorption of the aggregates ~1%

Table 8Properties of fine aggregates (different types of sands) (Soltaninaveh,2008).

Characteristic Value

Type Uncrushed (shape)Specific gravity 2.68Fineness modulus 2.50Water adsorption 1.02Grading zone II


will lead to a slight reduction in the concrete structure compared tonominal concrete design mix.

2.1.7. Coarse aggregatesAbout 75% of coarse recycled aggregate (RCA) is made from

concrete while the rest is made from masonry, asphalt, tile andothers (Shaikh, 2016). RCAs can partially replace natural coarseaggregates (NCA) in GeoPC by 15%, 30%, and 50% (KThu andMurthy,2015; Shaikh et al., 2015). Given the increasing use of concreteevery year, the extraction of NCA for producing concrete canadversely affect the natural ecosystem (KThu and Murthy, 2015;Shaikh, 2016). The dumping of construction and demolition(C&D) wastes poses another environmental concern that hasinspired several researchers to explore new methods of recyclingsuch wastes with an aim of increasing the available landfill spaceand reducing the present dependence on minerals and naturalaggregates (Kou et al., 2012; Zaharieva et al., 2003). Knowing thatfine and coarse aggregates comprise roughly 75%e80% concrete [9,121], C&D wastes may be used in the form of RCA to produceconcrete (Corinaldesi andMoriconi, 2009). Although this idea is notexactly new, many researchers have investigated the properties ofRCA-containing concrete and agreed that such properties areinferior to those of NCA-containing concrete (Corinaldesi andMoriconi, 2009; Etxeberria et al., 2007; Shaikh, 2014; Shaikhet al., 2015). However, only few researchers have tested the dura-bility and mechanical properties of RCA-containing GeoPC (Anuaret al., 2011; Nuaklong et al., 2016; Posi et al., 2013; Sata et al.,2013; Shi et al., 2012). These studies have also tested thecompressive strength of RCA-containing GeoPC that uses geo-polymer from wastepaper sludge ash instead of that from FA andslag. They found that the compressive strength of this concrete risesby almost 10% from 7 to 28 days and that the high molarity of so-dium hydroxide displays a higher compressive strength in GeoPCthan in conventional concrete. Shuang et al. (Shi et al., 2012)examined the mechanical properties of GeoPC that contains 50%and 100% RCA as a substitute for NCA, and compared such prop-erties with those of ordinary concrete. The coarse aggregate used inGeoPC comprises pea gravels with amaximum size of 10mm (dmax)(Fig. 11), a bulk dry specific gravity and absorption (ASTM C127)(ASTMC, 2004) (Table 9). The particle size distribution of thealumino-silicate source by-product can influence the compressivestrength of GeoPC as given in Table 10.

2.2. Alkaline liquids system

The alkaline liquids system (ALS) is an alkaline-activator solu-tion (AAS) for GeoPC such as Na2CO3, NaOH, Na2SO4 and Na2O$nSi2(Table 11). These alkaline activators can be applied in solid or liquidstate. Regularly, cements integrated with activator and precursorsare favored (in solid state) and water is used as a mixing liquid. Thegels matrix shaped by slag activation powerfully relies onnumerous chemical factors governing the reaction mechanism and,consequently, the improvement of durability properties and resis-tance to external attacks. Also, ALS is obtained by combining the

solutions of alkali silicates and hydroxides, except for distilledwater (Shalini et al., 2016). The alkaline liquids include concen-trated aqueous alkali hydroxide or silicate solution with solublealkali metals that are commonly potassium (K) or sodium (Na)based used in the preparation of alkaline activators for balancingthe negative charge of the alumina in four fold coordination withthe silica (Madheswaran et al., 2013). These liquids produce ageopolymeric binder by extracting and activating Si and Al atomsfrom Si- and Al-rich by-product materials (Madheswaran et al.,2013). In GeoPC, a large number of research focused on FA andGGBS as precursor of their blended cements, because they arereplete with Si and Al, and they are activated by alkaline liquids tomake the geopolymeric binder (Liang et al., 2019; Puertas et al.,2003; Ryu et al., 2013). Though, the nature of FA and GGBS (theirchemical andmineral compositions) disturbs the final performanceof the GeoPC gained. Also, the key to AAS is to melt the reactiveportion of source materials Si and Al found in FA and provide a highalkaline liquid medium for concentrated polymerization reaction(Temuujin et al., 2011; Yusuf et al., 2015). Furthermore, geo-polymers are amorphous to semi crystalline polymeric items madeby the alkali activation of alumino-silicate constituents with alka-line silicate solution at ambient or higher temperatures, leading topossible use as a substitute to ordinary cement (Usha et al., 2016).However, the effect of the chemical activation process on thecompressive strength of GeoPC is shown in Table 12.

Fig. 11 illustrates how the viscosities of alkali hydroxide solu-tions change along with their concentration at 25 �C. The datasetslack information for some cases because of the unavailability ofdata in the literature (Provis and Deventer, 2009). However, allsolutions show a consistent trend. Specifically, their viscositygradually increases up to 1.0 M, after which the viscosity no longershows any significant change from that of water, meaning that the

Table 10Influence of the particle size distribution of the binder phase on the compressive strength of GeoPC based on the type of poly(sialate) structure.

Types ofGeoPC

Compressive strength,MPa

Fineness Major findings Ref.

RHA-based 35e61.5 BA: 15.7, 24.5, and 32.2 mm Finer BA gives rise to higher strength Nuaklong et al. (2016)FA-based 39e75 FA with Blaine fineness of 2700, 3900, and

4500 cm2/gFA with highest Blaine fineness give rise to optimumstrength

Chindaprasirt et al.(2014)

15e45 FA: 75 and 3 mm RHA: 90 and 7 mm Combination of FFA and RHBA give rise to higheststrength

Ye et al. (2016)

RHA-based 34.5e43.0 RHA: 5%, 3%, and 1% retained on No. 325 sieve Finer RHA gives rise to highest compressive strength Dharmendra et al.(2017)

15e16 RHA: 100% passes 150 mm sieve 37.43% strength increment as compared to ungroundsamples

Board (2012)

Table 11Typical alkaline liquids, acids, bases, and their pH scale (Shalini et al., 2016).

pH value Hþ concentration relative to pure water Typical alkaline liquids

11 0.000 1 Ammonia solution12 0.000 01 Soapy water13 0.000 001 Bleach, oven cleaner14 0.000 000 1 Liquid drain cleaner


viscosity of a solution varies with a rise in the molarity of the so-lution (Usha et al., 2016). Further, the viscosity significantly in-creases after this point, and the steepness of such incrementdepends on the identity of the alkali cation. Fig. 12 is plotted onlogarithmic axes. When treated as a function of cation size, vis-cosity does not show any systematic trend but demonstrates asignificant degree of uncertainty given the lack of data for somesystems (Provis and Deventer, 2009).

Solute concentration refers to the amount of solute that is dis-solved in a specified quantity of solvent or solution (Hein andArena, 2010). The quantity can be stated in terms of volume ormass/molar amount (Pauling, 1988), while the concentration isusually stated in terms of molarity (m), mass percent, and molefraction (XA) as shown in equations (2)e(4) (Pauling, 1988). (See.Table 13)

Molarity ðMÞ¼ moles of soluteLiters of solution ðLÞ

¼ moles of soluteKilograms of solvent ðkgÞ (2)

Table 12Influence of the chemical activation process on the compressive strength of GeoPC.

Types ofGeoPC

Chemical activator Compressive Strength,MPa

Major finding

Ms NaOHmolarity

SS/SHratio

POFA-based 0.92e1.63

10 0.5e3.0 7e32 SS/SH ratio o65e69 Ms of 0.915 o

RHA-based e 14 and 18 1.9e5.5 15e40 SS/SH ratio o0.5e2.5 34e56 18 M NaOH o

2e6 2.5 8e15 2 M NaOH op4e12 20e30 12 M NaOH o

HMT-based 10e15 e 4e34 15 M NaOH oFA-based 0.75

e1.2510 39e57.3 Ms of 1.0 opt

1.0e2.0 e 5.0e63.4 Ms of 1.5 opt

e 7.5e12.5 25e45 SS/SH ratio o4.5e16.5 e 4e14 SS/SH ratio o10 0.33e3.0 7e25 Strength incr

16.5 M NaOH

e 3e9 0.4e2.3 12e23 6 M NaOH op

Mole fraction ðXAÞ ¼ moles of substance; ðAÞTotal moles of solution

(3)

Mass percentage of solute; ð100%Þ ¼ moles of soluteMass of solution

� 100

(4)

3. Clean production of GeoPC

GeoPC is usually produced by activating different alumino-silicate (AleSi)-based waste materials with a highly alkaline solu-tion, such as alkaline earth metal silicate components, alkali oralkaline earth metal hydroxide, fine and/or coarse aggregates, andwater (Calder�on-Moreno et al., 2002). For example: inorganicpolymeric ceramic formed from aluminum and silicon sources thatcontain AlO4- and SiO4 tetrahedral units, under highly alkalineconditions. The ratio of SiO2 to M2O and to sodium hydroxide solids(NaOH) must be at least 0.8% and 97%e98% of the total volume ofGeoPC materials, where the purity of the NaOH in pellet form was98%, respectively (Calder�on-Moreno et al., 2002; Hardjito et al.,2005a,b; Johnson, 2007). The concrete is initially cast into a mold,after which it consolidates (Fig. 13). The ratio of alkaline liquidmassto FA mass typically ranges between 0.30% and 0.45% (Hardjitoet al., 2005a,b).

3.1. Composition of GeoPC

Geopolymer cement is used as a substitute to typical OPC

s Ref.

f 2.5 optimal Yan et al. (2016)ptimal but insignificant Nazari et al. (2011)f 4.0 optimal Dharmendra et al. (2017)ptimal Yang et al. (2013)timal Board (2012)ptimal Ye et al. (2016)ptimal Sivaraja et al. (2010)imal Kelham (1996)

imal Sashidhar et al. (2016)f 1.5 and 7.5 M NaOH optimal Chindaprasirt et al. (2013)f 0.7 optimal Zhang et al. (2016)eased from 4.5 to 14 M NaOH, but decreased at San Nicolas et al. (2013)

timal Nematollahi and Sanjayan(2014)

Fig. 12. Viscosities of alkali hydroxide solutions as a function of molarity (Olsson et al.,1997; Provis and Deventer, 2009).

Table 13Chemical composition of the sodium silicate solution (Malathy, 2009).

Composition Na2O SiO3 Water Specific Gravity pH

By mass, % 7.5 ± 8.5 25 ± 28 63.5e67.5 1.53 g/cc neutral


(Komnitsas and Zaharaki, 2007). The manufacturing of geopolymercement needs an AleSi precursor material (e.g., FA), a user-friendlyalkaline reagent (Na or K soluble silicates with a molar ratio (MR) ofSiO2:M2O � 1.65, where M can either be Na or K (Fig. 12) (Adam,2009; Bakri et al., 2011b). Room temperature hardening can beeasily achieved by adding a source of calcium cations, such as blastfurnace slags. Commonly, the first alkali-activated FA-based GeoPCneeds heat hardening at 60e80 �C; furthermore, it cannot beformed separately but it can improve part of the FA-based concrete.NaOH (user-hostile) þ FA: FA particles entrenched in an alumino-silicate gel with Si:Al ¼ 1 to 2, zeolitic sort (chabazite-Na andsodalite).

The second slag/FA-based geopolymer cement requires headhardening at ambient room temperature. The produced silicatesolution þ blast furnace slag þ FA: FA particles are entrenched in ageopolymeric matrix with Si:Al ¼ 2, (Ca,K)-poly(sialate-siloxo) [37,152]. Geopolymer cements (FA, GGBS) can be cured faster than PC,and some mixes can even reach their maximum strength within24 h (Bakri et al., 2011b). The obtained compressive strength wasbetween 60 and 70 MPa at 28 days (for high early strength pro-duction, 20 MPa after 4 h and 25 MPa after 25 h). However, theseconstituents must be set slowly enough in order for them to bemixed with fine and/or coarse aggregates at a batch plant to createGeoPC concrete as shown in Fig. 12.

3.2. Curing regimes of GeoPC

The curing of freshly prepared GeoPC is the most important partof the entire geopolymerization process because of its key role inmaximizing the quality of concrete (Chithra and Dhinakaran, 2014).Proper curing can also positively influence the final properties ofGeoPC (Patil et al., 2014). GeoPC is often cured at elevated tem-peratures in three ways, namely, steam, ambient, and oven curingregimes. The result of each curing regime on the compressivestrength of the GeoPC is presented in Table 14.

3.2.1. Ambient curingAmbient curing is performed after casting the specimens and

letting them rest for a single day at room temperature of 20 ± 3 �C(Kumaravel, 2014; Nath and Sarker, 2012). Rest period refers to thetime from the completion of specimen casting to the initiation ofcuring at raised temperatures (Kumaravel, 2014). In ambientcuring, the compressive strength of GeoPC rises from age 7e28 days(Zhuang et al., 2016b). GeoPC can be cured without using elevatedheat and could be applied to other areas beyond precast members(Nath and Sarker, 2012). Furthermore, the addition of slag in the FA-based GeoPC mixture enhances the compressive strength and re-duces the setting time (Adam, 2009; Kumaravel, 2014; Phoo-ngernkham et al., 2014). Also, the addition of slag by around 30%of the overall binder results in compressive strength of approxi-mately 55 MPa at 28 days while the setting time condenses quicklywith higher amount of slag in the mixture and the slump of freshconcrete reduces a bit when the slag content increases (Deb et al.,2016; Nath and Sarker, 2012).

3.2.2. Steam curingThis form of curing is done in autoclaves at temperatures in the

160 �Ce190 �C range and pressures of 0.55e1.70 MPa. The condi-tion changes the interaction of the hydration making a concretethat has better sulfate resistance, no efflorescence, less shrinkageand creep, and lower moisture content after curing (ChennurJithendra, 2017; Komnitsas and Zaharaki, 2007). Steam curing isperformed after dismantling the specimens from their steel molds(after 24 h) and immediately storing them in a vacuum bagging filmto generate steam at temperatures of 40 �Ce100 �C (Kumaravel,2014; Soroka et al., 1978). By reducing drying shrinkage (DS) andcreep, this curing regime is particularly useful in cold weather orwhen trying to achieve early strength gain (Hardjito et al., 2005a,b;He et al., 2013). Steam curing can be performed under low atmo-spheric pressure and high pressure in autoclaves (Aldea et al.,2000). The specimens are placed inside the vacuum bagging filmfor 7e28 days (Adam, 2009; Shaikh, 2016), and the temperature isutilized as a compromise between the ultimate strength and therate of strength gain (Hemalatha and Ramaswamy, 2017). In GeoPC,creep and dry shrinkage could be reduced by adding the leastvolume of water possible in the concrete mix, increasing the rela-tive humidity of air, minimizing the cement paste volume, reducingthe water/binder ratio, and adding large coarse aggregates and alittle bit of steel fibers (Duan et al., 2016; Kirupa and Sakthieswaran,2015; Ridtirud et al., 2011; Yan et al., 2016).

3.2.3. Oven curingIn oven curing, the temperature in a hot air oven is maintained

between 40 �C and 120 �C for 24 h (Vijai et al., 2010). Afterwards,the oven is turned off to permit the cubes to cool down at roomtemperature; next, samples are removed from the oven and testedfor compressive strength. The oven cured samples can improve thecompressive strength of FA-based GeoPC under elevated temper-atures of up to 800 �C and oven curing regime at 80 �C by 2%e7%along with age (Kong and Sanjayan, 2010). Therefore, the strengthgains in ambient curing are considerably larger than those in hotcuring (Ganesh et al., 2016; Nath and Sarker, 2012; Vijai et al., 2010).Reportedly, the rate of strength gain was found lower at 60 �C incomparison to strength at 120 �C; thereby, a compressive strengthhigher than 60MPa can be attained by FA-based GeoPC in only 24 hof curing (Chithra and Dhinakaran, 2014; Kumaravel, 2014; Patilet al., 2014). In addition, the curing regimes perform a vigorousrole in the development of strength and the micro-structural sys-tem of GeoPCs, meaning that when samples are subjected toelevated temperature, they can attain higher strength.

Table 14Influence of each curing regime on the compressive strength of GeoPC.

Types ofGeoPC

Curing regime Compressivestrength, MPa

Major findings Ref.

RHAbased

Cured at room temperature after casting for 14, 28, 35, 42, and 49 days 2e12 Optimum strength achieved at 35 days of curing Ukwattageet al. (2013)

GGBS-based

- Steam curing (5 h humidity cabinet curing followed by 100 �C steam curingfor 8 h)- Autoclaved curing (24 h humidity cabinet curing followed by 210 �C, 2.0MPaautoclaved curing for 8 h), at 28days.

15e90 Steam-cured specimens exhibited higherstrength than autoclaved specimens

Zhang et al.(2016)

POFA-based

7 days 60e120 �C oven curing after casting 4e34 90 �C oven curing optimal Sivaraja et al.(2010)

FA based 24 h pre-curing period after casting followed by 36 h 50e90 �C oven curing, at28days.

49e60 80 �C oven curing optimal Ye et al.(2016)

24 h 65 �C curing; 5 min microwave curing þ 3/6/12 h 65 �C cures; ambienttemperature curing, at 14, 28 days.

20e42.5 5 min microwave curing þ6 h 65 �C curingoptimal

Dharmendraet al. (2017)

25 �C, 40 �C, and 60 �C curing for 24 h after 1 h of pre-curing period, 14, 28, 49days.

22e53 60 �C oven curing optimal (applicable for 7 and28 days strength development)

Nath andSarker (2012)

10e12 h room temperature curing upon casting, followed by saline-water,normal-water, and sealed-condition curing, at 28, 58 days.

49e91 Sealed-condition curing optimal, followed bysaline-water and normal-water curing

Soroka et al.(1978)

Fig. 13. Constituents used in the production of GeoPC (Hassan et al., 2019).


4. Fresh properties

Fresh GeoPC practical requirements differ from those of vibratedfresh concrete. A concretemixture is only categorized as GeoPC if itsthree key features, namely, resistance to segregation, passingability and filling ability are all satisfied according to European

guidelines [165]. The workability, stability, and flowability con-cerning the fresh properties of various GeoPC mixes are commonlyevaluated using slump flow, T50cm slump flow, V-funnel, L-box, andJ-Ring test procedures (Table 15) (Chennur Jithendra, 2017; Druta,2003; Subang Jaya et al., 2013; Ushaa et al., 2015). The freshproperties of GeoPC, including its stability, compatibility, setting


time, workability, and deformability, are described in the followingsubsections.

4.1. Stability

Stability is the ability of a material to keep its original formationand structure when facing various impacts, including environ-mental impacts (Gaochuang et al., 2016). A viscosity-modifyingadmixture (welan gum) ensures the sufficient stability of con-crete cast in deep structural members (Khayat et al., 1997). GeoPCmainly consists of covalent bonds and offers several advantages,such as low density, excellent volume stability, and ability to avoiddeformations (Su et al., 2016). Limestone powder is frequently usedas an inert mineral additive to reduce the content of energy-intensive reactive binders and to enhance the stability of freshconcrete mixtures (Triantafillou, 2016). Using a large quantity offine fillers and/or additives to increase viscosity can also maintainthe mixture design of GeoPC, reduce bleeding, and prevent theseparation of coarse aggregates (Ushaa et al., 2015). Hawes et al.(Hawes et al., 1992, 1990) found that adding pozzolans, such assilica fume and FA, can increase the stability of concrete that in-corporates phase change materials. Memon et al. (2013) reportedthat GeoPC becomes unstable and weak when the amount of silicafume exceeds that of FA by over 10%. Using super-plasticizers (SPs)can also influence FA-based geopolymer pastes at varying degreesbecause of the instability of these commercial materials in highlybasic media, such as NaOHþ Na2SiO3 (Memon et al., 2012; Palaciosand Puertas, 2004). In GeoPC, it is reported that when the use ofhybrid fibers, 80% GGBS and 20% RM, a 30% upsurge in flexuralstrength takes place as compared to normal concrete (Adam, 2009).Samples achieved their properties at ambient curing temperatureof 40e50 �C. Hence, GeoPC strength increased by about 23% whenused 17% GGBS parallel with 5% Na2SO4 and 5% H2SO4 whereasnormal concrete suffered losses. The presence of Al3þ in controlledSi/AleCa/Al or Si/AleNa/Al ratios can positively influence thechemical stability and durability of the GeoPC matrix (Kamseuet al., 2016), and reduce the amount of sulfur trioxide (SO3) byless than 1%; thereby, ensuring high volume stability (Gunasekaraet al., 2016). Commonly, the stability of GeoPC can be enriched bydecreasing its paste volume and cement content. Furthermore,aggregate typically suffers from shrinkage; thereby, the tendency ofstability can be upgraded by destroying the contraction ofaggregate.

4.2. Compatibility

GeoPC is well-known to be compatible and robust when itcomes to interaction between the various components in the mixdesign and other chemical admixtures (Anuradha et al., 2014;Kavitha et al., 2016; Sashidhar et al., 2016). Consequently, wherethere is no cooperation between various components in the matrix,

Table 15Limited values of fresh GeoPC tests according to the EFNARC guideline (ChennurJithendra, 2017; EFNARC, 2002).

Typical tests Units MinimumValue

MaximumValue

Refs.

Slump flow mm 650 800 (Aharon-Shalom and Heller,1982; Chennur Jithendra,2017; Subang Jaya et al.,2013)

T50cm slumpflow

Second 2 5

J-ring mm 0 10V-funnel Second 8 12L-box H2/H1 0.8 1.0U-box (H2eH1) mm 0 ‘30

the compatibility of GeoPC mortar would be condensed (ChennurJithendra, 2017; Druta, 2003; EFNARC, 2002; Subang Jaya et al.,2013). Hence, segregation may frequently occur when contact is-sues arise between the plasticizers and surfactant due to the in-compatibility of the admixtures in the mix design (Kamseu et al.,2016; Memon et al., 2013, 2012). GeoPC is synthesized from low-calcium FA and triggered by a combination of sodium silicate andsodium hydroxide solutions. Also, incorporating SPs improve theself-compatibility and influence the hydration of GeoPC (Ganeshet al., 2016; Han et al., 2014). In GeoPC, the self-compatibilityproperty is essentially obstructed by the features of materials (e.g. super plasticizer) and also the proportions of the mix; hence-forth, it becomes important to develop a procedure to improve mixdesign of self-compacting GeoPC (Huseien et al., 2015). A self-compacting concrete achieves consistency and self-compatibilityunder its own weight without the need for any external compac-tion and can be influenced by the characteristics of materials andthe mix proportions of various byproduct materials (e.g., RHA, FA,and SFA); therefore, a procedure for ensuring a suitable mix designfor GeoPC must be proposed (Moghadam and Khoshbin, 2012;Prabhu et al., 2016). In the limited oxygen index test, the alkaline-treated coconut tree leaf sheath fiber with phenol formaldehyderesin shows high compatibility in composite geopolymer (Bharathand Basavarajappa, 2014). Using FA as a substitute for 30% ofGGBS can also increase the compressive strength by 60% at 28 daysand compatibility of GeoPC in the fresh state by 12% (Ganeshan andVenkataraman, 2017). Lightweight aggregate concretes are usuallylinear to levels approaching 90% of the failure strength, therebyshowing the relative compatibility of the constituents and reducingthe formation of micro-cracking (Chandra and Berntsson., 2002).Al-Mulla (2010) reported that a 10% RHA absorbs a large amount ofwater from the mixture; thereby, reducing the concrete strengthdue to inadequate compatibility. Meanwhile, those binary cementsthat contain 40%e50% FA, nearly 10% silica fume, and 50%e70%GGBS show high compatibility with GeoPC in the fresh state,increased sulfate resistance in the hardened state, and relativelyslow strength development under normal or low temperatures(Gjorv and Sakai., 1999). Thus, it can be concluded that the per-formance of the self-compatibility of GeoPC relies on the ratio offine and coarse aggregates (50% by volume), super-plasticizerdosage (�2%) and the w/c ratio (0.9e1.0) (Huseien et al., 2015).

4.3. Setting time

Geopolymer mortar has an initial setting time of 35 min and afinal setting time of 600 min at curing temperatures of 20 �Ce80 �C(Vijai et al., 2010). Low-calcium FA has a much higher setting timethan high-calcium FA. Those paste samples with some formulationof alkali-activated FA have a setting time of less than 5 mine7 min,which can reach 20 mine40 min in the best case. Meanwhile, thoseslags and other materials activated with sodium hydroxide requireless than 3 mine4 min and less than 15 min to activate sodiumsilicate, respectively. Reportedly, the setting time of GeoPC mix-tures is having the same shares of alkaline activator and binder ofthe conforming mortar mixtures with the fine aggregate misplaced(Nath and Sarker, 2015). It is known that the sodium hydroxidereaction in solution is exothermic, and it is suggested that the mixdesign of NaeSi and NaOH should be completed one day beforemixing with the calcio-aluminosilicate constituents (Subang Jayaet al., 2013). This will guarantee equilibration and ensures that nounrestricted heat duringmixing that will affect setting of the GGBS-based GeoPC paste (Castel and Foster, 2015; Dimas et al., 2009; Jainand Pal, 1998; Suresh and Nagaraju, 2015). It is reported that whenNaOH is used as an activator, the concrete mechanical propertiesrise with the reduction in water/binder ratio. Another research on


FA-based GeoPC revealed that the increase in Na2SiO3/NaOH ratioto 2.5 contributed to a considerable decrease in mortar setting time(Ali and Zurisman, 2015). To examine the microstructure of OPCblended with FA-based geopolymers, two paste mixes that con-tained OPC as a substitute for 10% and 50% of the total binder wereprepared (Adam, 2009; Hardjito et al., 2005a,b; Hardjito et al.,2008). An alkaline solution with a 2.5 (Na2SiO3/NaOH) ratio and a2.0 solid/liquid ratio were adopted as a substitute for 40% of theoverall binder. It was observed that the initial setting time wasdoubled as a result of increase in alkaline liquid content from 35% to40%. The samples were cured at room temperature (20 �Ce23 �C) towhich a slow setting was observed at this temperature using a Vicatapparatus (Nath and Sarker, 2012). In addition, using GGBS andsilica fume improve geopolymerization by reducing the settingtime, while using alternate materials, such as RHA, class C FA,metakaolin, and RM, also leads to positive results (Ganeshan andVenkataraman, 2017). Generally, the setting time of GeoPC can beaffected by numerous factors such as chemical and physical prop-erties of the binder itself, mix design composition, mixing process,molarity of NaOH, Na2SiO3/NaOH ratio and ecological conditions.

4.4. Workability

The amount of extra water in GeoPC (a water content of 12% andSP dosage of 6% by mass) is an important gradient required forregulating strength and workability (Ganeshan and Venkataraman,2017; Ushaa et al., 2015). Nath and Sarker (2015) stated thatincreasing the alkaline liquid content can lower the strength andimprove concreteworkability because of the high liquidesolid ratioof the mixture with the highest liquid content. Therefore, using amixture with 5% OPC content can achieve a reasonably high earlyage strength, setting time, and workability for ambient-cured FA-based GeoPC (Nath and Sarker, 2012). Another research on FA-based GeoPC revealed that the increase on Na2SiO3/NaOH ratio to2.5 resulted in a substantial reduction in the workability of themortar (Ali and Zurisman, 2015). It is also reported that addingnaphthalene-sulfonate-based SP, the use of total 4% of SP per FA bymass, led to an enhancement of the workability of GeoPC in thefresh state; however, the compressive strength of GeoPC in thehardened state slightly declined when the SP dosage exceeded 2%(Hardjito et al., 2005a,b). Nevertheless, the workability of GeoPC inthe fresh state is remarkably enhanced by adding silica fume up to10% (Memon et al., 2013). Reducing the particle size can alsoimprove the workability of the mixture (Leong et al., 2016), whilethe addition of fibers can reduce the workability (Kovler andRoussel, 2011). The addition of naphthalene-based SP in GeoPCcan influence both theworkability and final setting time dependingon the percentage added between 1.5 and 3% of FA by mass(Memon et al., 2012; Nematollahi and Sanjayan, 2014). It is alsoobserved that the workability of FA-based GeoPC with SFAimproved when SP was limited to1.5%, which led to enrichment ofthe compressive strength and showed a slight degradation whenthe SP value was 2%. Sakulich (2011) found that adding slagenhanced the workability of the matrix because slag needs a lesseramount of liquid than metakaolin for particle wetting, which helpsin reducing the porosity and water permeability. In this case, thoseGeoPC mortars blended with flash metakaolin have better work-ability than those blended with rotary-kilnmetakaolin (San Nicolaset al., 2013). Among several mineral admixtures, the blast furnaceslag series have better workability than the silica fume series(Ushaa et al., 2015), while the fine aggregates fill the voids andincrease the workability of concrete (Sashidhar et al., 2016). Behzadand Sanjayan (Nematollahi and Sanjayan, 2014) found that usingdifferent commercial SPs (1.0%) does not enhance theworkability ofthe activated FA pastes in GeoPC. Furthermore, the proportion of

GGBS and silica fume in GeoPC rises to more than 30% and 15%,respectively, and does not meet the workability requirements ofthis concrete (Anuradha et al., 2014).

4.5. Deformability

Knowledge on the deformability of concrete is highly imperativein the calculation of structural deflections, computation of strainsand stresses, and in the development of constitutive models forsimulations. The concrete digital dilatometer (CDD) (Fig. 13) is oneof the tools used to measure the linear deformation of GeoPC inboth fresh and hardened states (Esping and L€ofgren, 2005).Meanwhile, slump flow-rate is a fast and a simple procedure that iscommonly used in laboratories and construction sites, and it ap-plies horizontal free flowmeasurements of the concrete in the freshstate (Memon et al., 2013). This method can favorably assess thedeformability or flowability of concrete in the fresh state and canbalance deformability with stability (Sashidhar et al., 2016). Inaddition, it is reported that the hardness of GeoPC is almost twotimes higher than that for normal concrete, and it could showhigher brittleness and less deformability (Kabir et al., 2015; Saraya,2014). Because the chemical admixtures of GeoPC can improve thedeformability and viscosity, this causes the mixtures to have highfilling capacities ranging from about 60% to 70%; thereby, indicatingexceptional deformability without blockage within obstacles thatare closely spaced (Aggarwal et al., 2008; Khayat and Guizani,1997). A low water content necessitates a relatively high dosageof high-range water reducers to gain the desired deformability(slump flow, 660 mme690 mm) especially when lower bindercontents are available because of high paste viscosity and highinter-particle friction (Sashidhar et al., 2016; Ushaa et al., 2015).Previous studies were performed to measure the behavior ofGeoPC, including the influences of chemical admixtures, CeSeHphase and curing conditions. It was found that the addition of FA/GGBS-based GeoPC pastes, alumino-silicate gel (N-A-S-H) andCeSeH; activated principally by NaOH at a low temperature of27 �C, thereby, the concrete paste is led by the N-A-S-H and CeSeH,depending on the alkalinity volume of activators used (Bakri et al.,2012; G€orhan and Kürklü, 2014; Kamseu et al., 2016; Sashidharet al., 2016; Singh et al., 2015). Rickard et al. (2016) found thatexpanded clay aggregates (quartz content <30%) have higherdeformability than quartz aggregates. The presence of poly-propylene fiber (0.1%) reduces the deformability of GeoPC byincreasing the surface area that must be lubricated by cement pasteand water (Muthupriya et al., 2014). Given that a GeoPC with a highw/binder-ratio (0.67) has a large crack area, the w/binder-ratio inthe region must be at least 0.55 (Esping and L€ofgren, 2005). How-ever, it can be concluded that decent understanding on thedeformability of GeoPC is an essential key towards the estimationof deflection curves, and the stress-strain relationship in order todevelop the solid rules for finite element modeling(See. Fig. 14).

5. Mechanical properties

After setting, it is necessary to ensure that the concrete is suf-ficiently hard to resist the applied service and structural loads.However, given that GeoPC is produced of high-quality materialsand is suitably proportioned, mixed, handled, placed, and finished,this concrete is among the most durable and strongest constructionmaterials available in the market. The mechanical properties ofGeoPC, including its compressive, splitting tensile and flexuralstrengths, modulus of elasticity (MoE), stressestrain behavior, andrate of strength development, are reviewed in the followingsubsections.

Fig. 14. Test arrangement of the CDD for linear deformation measurement (Esping and L€ofgren, 2005).


5.1. Compressive strength

GeoPC’s compressive strength (ASTM C39) is affected by wet-mixing time, curing time, curing temperature, particles size(Chang and Shih, 2000; Lakshmi and Nagan, 2011) and addition oftypical additives (Ca(OH)2, Al(OH)3, SF (Ye et al., 2016), nano-SiO2(Boonserm et al., 2012), nano-Al2O3 (Nath and Kumar, 2013), vinyl(Nematollahi and Sanjayan, 2014) and copolymer and polyacrylatecopolymer based SP (Puertas et al., 2003). The impact of theseadditives on the compressive strength of GeoPC is summarized inTable 16. FA-based GeoPCs exhibit a steady reduction in theiroriginal compressive strength at elevated temperatures of up to400 �C regardless of their molarities and coarse aggregate sizes(ASTME119, 2012; Chu et al., 2016; G€orhan and Kürklü, 2014; Kongand Sanjayan, 2010). The compressive strength of GeoPC also re-duces considerably when the amount of extra water exceeds 12% ofthe FA mass (Memon et al., 2012). Meanwhile, calcination slightlychanges the mineralogical composition of this concrete and re-duces its compressive strength by almost 30% (Temuujin et al.,2011). Replacing 40% of cement with GGBS can cause greater im-provements in compressive strength compared with a 20% or 60%replacement (Chithra and Dhinakaran, 2014; Nath and Sarker,2012). Numerous scholars have examined the mechanical proper-ties of GeoPC that contain 50% and 100% RCA and deduced that the

Table 16Influence of additives on the compressive strength of GeoPC.

Types ofGeoPC

Types of additives Compressivestrength, MPa

Major fin

POFA-based

15e25% Ca(OH)2, 5e10% Al(OH)3, 2.5e7.5% SF 15.67e44.74 20% Ca(O

RHA-based

ASTM Class C FA (0, 25, 50, 75, 100%), GGBS (0, 5,10, 15%)

5e55 50% FA a

FA-based 0e50% GGBS replacement by weight of binder 8.5e93.4 50% GGBthan FA-G

1e3% of nano-SiO2 and nano-Al2O3 by binderweight

20.2e56.4 2% nano-

1% addition by mass of binder of N, M, and PCbased SP

47e81.3 PC-basedreduction

0.5e1.5% addition of vinyl copolymer andpolyacrylate copolymer based SP

30e35 InsignificSPs

Addition of 2.5e5% of Al-rich waste calcined at400, 600, 800, and 1000 �C

27.4e34.2 Al-rich w

RM based 10e40% RM by wt of cement 50.91e27.74 25% wasGGBS

based,GGBS was replaced up to 40% Up to 30 The value

aggregateSF based SiO3/OH ¼ 0.5, AL/SF ¼ 0.25, up to 56e60% of SF

by wt of cement10.31e37.5 The stren

respectiv

compressive strength of this concrete rises by approximately 10%from 7 to 28 days (Brake et al., 2016; Nuaklong et al., 2016; Shi et al.,2012; Talakokula et al., 2016). Such compressive strength reachesits peak when a mixture of water-glass and NaOH is adopted as theactivator but decreases when the alkaline solution content in-creases from 35% to 45% of the total binder (Nath and Sarker, 2015).The sodium alumina silicate hydrate (NeAeSeH) gel for 100% FAcan also improve the compressive strength of GeoPC (Soutsos et al.,2016). To achieve a microstructural phase of NASH gel, or KASHwhen using potassium, it is stated that the strength development ofGeoPC mixtures based on sodium hydroxide and calcined kaolinhave shown lesser reactivity than the mixtures in which this astcomponent was substituted by potassium hydroxide (Nuaklonget al., 2016; Shi et al., 2012). Besides, the findings display thatthere is no direct linear bond between the strength and the calciumhydroxide content. Vaidya and Allouche (2011) found that theaddition of fiber can meaningfully enhance both the ultimateflexural capacity and ductility of FA-based geopolymers, particu-larly at the early ages, without adversely influencing their ultimatecompressive strength. Such influence can be maintained byreplacing FA with 30% palm oil FA (POFA), 30% GGBS, and 10% SFA(Anuradha et al., 2014; Ganeshan and Venkataraman, 2017; KThuand Murthy, 2015; Mo et al., 2015a; Prabhu et al., 2016). Bakrilso(Bakri et al., 2012) and Sanjayan (Nematollahi and Sanjayan, 2014)

dings Ref.

H)2, 5% SF and 10% Al(OH)3 optimal (Shi et al., 2015)

nd 5% GGBS replacement level optimal Ryu et al. (2013)

S replacement optimal and FA-GCS yielded higher strengthGBS

G€orhan and Kürklü(2014)

SiO2 and 1% nano-Al2O3 optimal Pauling (1988)

SP showed highest plasticizing effect and least strength Adam (2009)

ant changes in strength andworkability with addition of both Sata et al. (2012)

aste with 2.5% and 1000 �C calcined temperature optimal Chindaprasirt et al.(2014)

found the optimum percentage of replacement. Poudenx (2008)of MoE for 90% of normal concrete using the same type of.

Fernandez-Jimenezet al. (2006)

gth was increased by 84%, 38% and 15% at 7, 28 and 56 days,ely, for the whole AL/SF ratios.

Anuradha et al.(2014)


deduced that the compressive strength of concrete reduces by 29%when the optimum Na2SiO3/NaOH ratio is 2.5. The correlation be-tween the compressive strength and the SiO2/R2O ratio indicatesthat a rise in alkali content or a reduction in silicate content canincrease the compressive strength of geopolymers by formingAleSi network structures (Singh et al., 2015; van Jaarsveld andDeventer, 1999). Applying a silicate solution to GeoPC can in-crease its density from 79% to 93% and its compressive strength by35% at room temperature (He et al., 2010; Sakulich, 2011). However,the compressive strength of this GeoPC reduces with the additionof fiber (e.g., glass, carbon, polyvinyl chloride, and polyvinylalcohol) (Al-Majidi et al., 2017; Karbhari, 2013; Kovler and Roussel,2011; Li and Xu, 2009; Rickard et al., 2014). Furthermore, previousresearchers indicated that the compressive strength of RCA-basedGeoPC that uses geopolymer from wastepaper sludge ash insteadof that from FA and slag rose by roughly 10% from 7 to 28 days.Moreover, the high molarity of sodium hydroxide indicated ahigher compressive strength in GeoPC than that in conventionalconcrete (Anuar et al., 2011; Nuaklong et al., 2016; Posi et al., 2013;Sata et al., 2013; Shi et al., 2012). Also, the addition of slag by around30% of the total binder attained a compressive strength ofapproximately 55 MPa at 28 days. The setting time condensedrapidly with greater volume of slag in themixture, and the slump offresh concrete reduced a bit when the slag content increased (Debet al., 2016; Nath and Sarker, 2012). Using FA as a substitute for 30%of GGBS can also increase the compressive strength by up to 60% at28 days and increase the compatibility with GeoPC in the fresh stateby 12% (Ganeshan and Venkataraman, 2017). Moreover, it is re-ported that the compressive strength of FA-based GeoPC onlyshows a 12%e40% reduction when exposed to AE, while OPC con-crete shows about 40%e65% reduction in compressive strengthunder the same conditions and time (A. Castel, 2016; Azreen et al.,2018). GeoPC has potential to produce ultra-high compressivestrengths with better durability performance, leading it to be usedin the fabrication of concrete applications that suffer from aggres-sive environments. According to Neville (1995), the relationshipbetween the splitting tensile and compressive strengths of OPCconcrete can be given as

fct ¼ 0.3 (fcu)2/3 (5)

fct - principal tensile strength of concrete, N/mm2; and fcu -compressive strength of concrete, N/mm2.

S ¼ K

264 1�

1þ wc

�þ��

ac

�375n

(6)

K - Empirical constants, n ¼ Strength to gel-space ratio

S ¼ PO (1 - P)n (7)

So - The strength at zero porosity.

n - Balshin’s constant

S¼Ks ln�PcrP

�(8)

Pcr-Critical porosity at zero strength.

Ks - “Schiller’s constant

S ¼ Kgn (9)

K - The intrinsic gel strength.

g - The gel-space ratio (Power’s gel-space ratio)

5.2. Splitting tensile strength

The splitting tensile strength (STS, ASTM C 496) is a basic and animportant property of concrete which is weak in tension due to itsbrittle nature and is not typically designed to withstand directtension (Druta, 2003). The STS of self-compacting concrete isassumed to be almost 30% more than that of normal concrete(Anuradha et al., 2014; Chennur Jithendra, 2017; Druta, 2003;Kavitha et al., 2016; Neville, 1995; Subang Jaya et al., 2013; Ushaaet al., 2015). Using slag as a partial substitute for FA can improvethe STS of GeoPC, while incorporating 10% and 30% RCA (KThu andMurthy, 2015; Mo et al., 2015b; Nuaklong et al., 2016) or 10% palmoil shell aggregate can reduce the STS (Liu et al., 2016; Mo et al.,2015a). An SFA content of over 10% can also reduce the STS ofGeoPC (Memon et al., 2013). Islam et al. (Al-Majidi et al., 2017)found that POFA and GGBS can increase the STS of GeoPC byapproximately 6%e9% and 9%e11%, respectively. Siva Raja et al.(Sivaraja et al., 2010) examined the mechanical properties of sisal-fiber-reinforced GeoPC at a three-month interval and found thatsisal fiber only enhances the STS of this concrete by 8.4%. Using FAand granite slurry can improve the STS of concrete after 28 days by7.8% and 40%, respectively (Ryu et al., 2013; Sreenivasulu et al.,2015). Maochieh and Huang (Chi and Huang, 2014) found thatreplacing fine aggregates with circulating fluidized bed combustionash can increase the STS of GeoPC by 5%e10%. Moreover, the use of0.03 vol% steel, 10% synthetic, 2% sweet sorghum, 1%e5% carbon,0.5 vol% anon-metal, and 0.5 vol% polypropylene fibers can increasethe STS of GeoPC by 16%, 12.8%, 36%, 8.4%, 12%, and 14.4%, respec-tively (Bashar et al., 2016; Chu et al., 2016; Karbhari, 2013;Mazaheripour et al., 2011; Rickard et al., 2014). A research on 90%FA/and 10% GGBS-GeoPC with 0.25% steel fibers under variouscuring conditions found that the STS increases with an increase inthe volume level of steel fibers (Chithra and Dhinakaran, 2014; Nathand Sarker, 2012). Similar findings were reported when 1.5% steelfibers were added into the mix design of slag-based GeoPC. Theinfluence of these additives on the STS of GeoPC is summarized inTable 17. The Australian standards for concrete structures (AS3600)(AS, 2009) proposes the following equations for computing thecharacteristic principal STS (fct) of GeoPC:

ft ¼ 0.20 (fc)0.70 (10)

For density between 1400 and 1800 kg/m3

ft ¼ 0.23 (fc)0.67 (11)

fc ¼ 28 days compressive strength N/mm2

fct ¼ 0:4ffiffiffiffiffiffiffifcm

p(12)

where, P ¼ load at failure (N);

ft ¼ 2PpDL

(13)

D¼ diameter of specimen (mm); and L¼ length of specimen (mm).Further, the expression below is proposed for STS of fiber rein-

forced GeoPC with respect to the volume fraction level of steel fi-bers (Vf) for oven-curing;

STS ¼ fsc þ 0.743 Vf (14)

Table 17Influence of additives on the STS and flexural strength of GeoPC.

Types ofGeoPC

Types of additives Splitting tensilestrength, MPa

Flexuralstrength, MPa

Major findings Ref.

GGBS-based

1e15% addition of polymer resin 2.34e3.66 4.8e8.6 41% enhancement in flexural strength by 1%resin addition

Mo et al. (2015b)

FA-based 0e3% addition of nano- SiO2 and nano-Al2O3 4.14e4.67 3.66e5.12 1% SiO2 and 2% Al2O3 optimal Pauling (1988)0e8.3% addition of horizontally and verticallyoriented cotton fabric

8e32 8.3% and horizontally oriented cotton fabricoptimal

Mazaheripour et al.(2011)

1, 2, and 3% addition of sweet sorghum fibers 2.2e3.4 M 3.2e5.6 Addition of 2% of sweet sorghum fibersoptimal

Mo et al. (2015b)

0e15% addition of SFA 4.14e4.67 4.09e4.56 10% of SF optimal Khayat et al. (1997)


Where.

fsc ¼ the STS of GeoPC composites at 28 days,Vf ¼ the volume fraction level of steel fibers.

5.3. Flexural strength

According to ASTM C 78, the flexural strength of geopolymerscan be considerably improved through the incorporation of syn-thetic fibers like polypropylene and PVA. The bridging effect in themicro- andmacro-cracking of the geopolymer matrix under flexureimproves the interfacial strength. However, an excessive addition offibers can reduce the flexural strength (A. Castel, 2016; Brake et al.,2016; Bre~na et al., 2001; Sharafeddin et al., 2013). For instance, 2%addition of sweet sorghum fiber improves the flexural capacity ofASTM class F FA-based geopolymer specimens by about 40% (Chenet al., 2014). Also, 2% addition of nano-SiO2 and nano-Al2O3 byweight improves the flexural strength significantly in a high-calcium FA-based geopolymer specimens (Phoo-ngernkham et al.,2014). Meanwhile, adding 10% SF in an FA-based GeoPC improvesthe flexural strength by 11.09% (Memon et al., 2013). The flexuralstrength of concrete with an optimum addition of cotton fabric(8.3%) is nearly three times more than that of unreinforced GeoPCs(Alomayri et al., 2014a). The flexural strength of GGBS-basedGeoPCs also increases by incorporating 1%e15% polymer resin(Zhang et al., 2010). The addition of 1% resin greatly improves theflexural strength of GeoPCs by 41%. Prabhu et al. (2016) found thatconcrete’s flexural strength can increase by up to 25% whenreplacing cement with 10% FA, 10% GGBS, and 5% SFA. The hybridGeoPC with 40%e80% PFA content has a higher flexural strengththan a control mixturewith 0% PFA content (Dimas et al., 2009). Theimpact of these additives on the STS and flexural strength of GeoPCis summarized in Table 17. Also, research on FA-based GeoPC withaddition of 10 and 8 NaOH molarities found that the flexuralstrength increased by 3.5% as the concentration of NaOH molaritiesrose from 8 to 10 (Bakri et al., 2012; Somna et al., 2011). Anotherstudy found that the density of FA-based GeoPC increased with alonger curing period but the degradation of higher temperaturecuring condition caused a poly-reaction, leading to reduction in thedensity of samples at the same time (Aldea et al., 2000; Atis et al.,2005). But FA-based GeoPC with the addition of GGBC with Phos-phogypsum revealed an increase in flexural strength by 7.5%compared to normal concrete. However, it was observed that theflexural strength of GeoPC is smaller when compared with normalconcrete excluding when a lesser amount of alkali activator solu-tion as cement paste is replaced, which can lead to attainment ofhigher flexural strength.

5.4. Modulus of elasticity

Modulus of elasticity (MoE) (ASTM C 469) is highly correlatedwith the compressive strength of any concrete type (e.g. GeoPC),thereby a higher degree of geopolymerization can result in a densergeopolymer matrix, which in turn leads to a higher MoE (Topark-Ngarm et al., 2014). However, geopolymerization is known as theformation of polysialates that depend greatly on the mineralogical,chemical and physical properties of the raw materials, amount ofactivator and curing conditions (Ahmari and Zhang, 2012;Boonserm et al., 2012; Davidovits, 2015; Kumar and Kumar,2013). The MoE property does not rely totally on the dosage ofthe chemical activator but is also governed by the amount of ag-gregates in GeoPC mixtures (Khandelwal et al., 2013). LowMoE canreduce the degree of crack propagation initiated by the corrosion ofsteel bars. A high fine aggregateetotal aggregate ratio can producea GeoPC with an equal or higher MoE (A. Castel, 2016). Severalstudies have stated that high silicate content can raise the MoE ofGeoPC and reduce that of OPC concrete (Yusuf et al., 2015). Severalstudies of FA-based GeoPC obtained MoE values for specimensbetween 23.0 and 30.8 GPa (Hardjito et al., 2005a,b), 10.7e18.4(Sumajouw et al., 2004) and 30.3e34.5 GPa (Hardjito et al., 2008).Also, it was reported that the MoE of pulverized fuel ash (PFA)mortars was lower than OPC mortar due to the presence of alkaliactivated PFA mortar, but later at a long-term, the MoE of PFAmortars increased to about 5e20% higher than OPC mixes (Dimaset al., 2009; Palacios and Puertas, 2004; Puertas et al., 2003).Ngarm et al. (Topark-Ngarm et al., 2014) stated that high-calciumFA-based GeoPC displays equal or greater MoE with a low sodiumsilicateeNaOH ratio, which corresponds to a high amount of Na2O.The incorporation of various fibre types, polymer resin, SPs, andnano-materials can significantly improve geopolymers mechanicalproperties, including their flexural strength, STS, and MoE(Alomayri et al., 2014b; Chen et al., 2014; Nematollahi andSanjayan, 2014; Phoo-ngernkham et al., 2014). Furthermore, add-ing 2% nano-Al2O3 and nano-SiO2 by weight of binder can improvethe MoE by about 30% of high-calcium FA-based geopolymersamples cured at ambient temperature. After 90 days of curing, theE-value of these samples can reach to as high as 17.65 GPa, which iscomparable to that of OPC concrete (Phoo-ngernkham et al., 2014).Moreover, further research on FA-based GeoPC found that the MoEwas 15e28% lesser in comparison to normal concrete due to theaddition of a low volume of silicate content and sodium hydroxidesolution in GeoPC (Fernandez-Jimenez et al., 2006). Also, thecombination of FA-based- and GGBS based GeoPC at ambienttemperatures resulted in MoE values between 10 GPa and 21 GPa,which is parallel with the compressive strength of 30 MPa. How-ever, the value of MoE for GeoPC is almost 90% of normal concreteusing the same type of aggregate. MoE can be computed using


following equations;

E ¼ 33 W1.5 (f’c)0.5 (15)

It was used Pauw’s equation

ESCGC ¼ � 11400þ 4712ffiffiffiffiffiffifcu

p(16)

fcu ¼ compressive strength of FA-based GeoPC at 28 days.

E ¼ 5.31 � W - 853 (17)

Density is ranged between 200 and 800 kg/m3

Ec ¼ 9:10ðfcÞ0:33Ec ¼ 1:70� 10�6P2ðfcÞ0:33

(18)

fc ¼ compressive strength of concrete.

Р ¼ plastic density (kg/m3)

E ¼ 0.99 (f’c)0.67 (19)

It used when FA utilized as fine aggregate

E ¼ 0.42 (f’c)1.18 (20)

It used when Sand utilized as fine aggregate.

5.5. Stressestrain behavior

The stressestrain behavior of GeoPC materials (ASTM C 469)must be investigated to completely categorize their performanceunder field implementation and design (Noushini et al., 2016). In-vestigations have shown that the stressestrain behavior dependson the type of concrete and confinement (Ganesan et al., 2014;Haider et al., 2014). (Hardjito et al., 2005a,b) examined thestressestrain behavior of three sodium silicate FA-based geo-polymer matrixes containing 408 kg/m3 FA at 28-days compressivestrength of 41 MPae64 MPa. The stressestrain behavior of heat-cured specimen at 48 h and 50 �C were observed. The resultsshowed that FA and slag-based GeoPCs with high FA/GGBS contentbetween 570 and 620 kg/m3 and Naesilicate solution with MoE of0.75 < Ms < 1.5 indicated more brittle fracture compared with PCconcrete [236]. More investigations on the stressestrain behaviorof confined GeoPCs pastes have witnessed a tremendous increaseover the years. Haider et al. (2014) carried out empirical in-vestigations and observed the stressestrain behavior of sodiumsilicate FA-based geopolymer paste under constant levels ofconfinement. It was determined that geopolymer paste indicatedlower deformation in the axial direction compared with OPC con-crete under similar confinement (Bakri et al., 2011a; Malathy, 2009;Nath and Sarker, 2015; Noushini et al., 2016; Shi et al., 2011;Venkatanarayanan and Rangaraju, 2014). Ganesan et al. (2014)carried out investigative comparisons between OPC concrete andconfined sodium silicate FA-based GeoPC and observed theirstressestrain behavior. Initially, GeoPC samples were cured atambient temperature over 24 h before heating at 60 �C in an ovenfor another 24 h. It was observed that the stressestrain modelproposed in the literature for confined OPC can serve for GeoPCthrough curve fitting and adjustment of the curve factor (Manderet al., 1988). Furthermore, it has also been determined that thestress-strain behavior of GGBS/RHA-based GeoPC under

compression is similar to that of normal concrete (Mo et al., 2015a;Nematollahi et al., 2017; Noushini et al., 2016). Also, adding 0.05%Polypropylene fibres was determined to increase the stress-strainby 60% at peak stress of GeoPC (Chu et al., 2016; Mazaheripouret al., 2011; Rickard et al., 2014). Another study of RM-basedGeoPC with sodium silicate solution found that the stress-straincurve increased at the peak, indicating a strong and a moreductile behavior of the GeoPC matrix (Ganesan et al., 2014; Haideret al., 2014; He et al., 2013; Paramguru et al., 2004; Thomas andPeethamparan, 2015). Also, the increase in RCA content was re-ported to slightly improve the ultimate axial stress-strain behaviorof the unconfined GeoPC samples because of the increase in thestrain softening (Ganesan et al., 2014; Haider et al., 2014; KThu andMurthy, 2015; Shaikh, 2016; Shaikh et al., 2015; Thomas andPeethamparan, 2015). Collins et al. (1993) suggested that thestressestrain behavior of concrete in compression can be antici-pated as follows:

sc ¼ fcmεc � n

εcm n� 1þ

εcεcm

nk(21)

where.

‒ fcm ¼ peak stress;‒ εcm ¼ strain at peak stress;‒ n ¼ 0.8 þ (fcm/17); and‒ k ¼ 0.67 þ (fcm/62) when 3c/ 3cm>1 or 1.0 when 3c/ 3cm�1.

5.6. Rate of strength development

The strength development rate and the chemical reaction ofGeoPC are affected by a number of factors depending on thechemical composition of source materials, the alkaline activators(KOH, Na2SiO3 and NaOH), the curing conditions, and the miner-alogical phases (Diaz et al., 2010). A rise in temperature corre-sponds to a rise in the strength development rate, while the ratio ofalkaline liquid to binders does not influence the GeoPC (Chithra andDhinakaran, 2014; Gjorv and Sakai, 1999; Hardjito et al., 2005a,b;Hemalatha and Ramaswamy, 2017). In general, the strengthdevelopment of GeoPC becomes steady after 28 days (Kabir et al.,2015). In order to attain an appropriate chemical composition inthe growth of geopolymers, the favored procedure is to mix FAwitha great silica material. Reportedly, the mechanical properties ofGeoPC are intensely influenced by the connection between aggre-gate and cement paste at the interfacial transition zone (ITZ)(Brough and Atkinson, 2000). Thus, it is found that the strengthdevelopment and ITZ in GeoPC improved through the creation ofdense ITZ between the binder matrix and aggregate at a greater SPamount (Mazloom et al., 2004). Several researchers have observedthat an FA-based GeoPC specimen shows higher compressivestrength when cured at high temperatures instead of ambienttemperatures (Satpute Manesh et al., 2012; Thampi et al., 2014). Forinstance, a 53% rise in the strength of GeoPC was reported after thegeopolymer was smoked using heat. Similarly, another studyrevealed an increment of strength between 80% and 60% at 7-daysand 28-days for ambient and oven-cured samples, respectively.Also, Manesh et al. (Satpute Manesh et al., 2012) reported that thestrength development rate of concrete remains constant for up to16 h at 600 �C and 900 �C. Higher than this temperature, concretestrength continues to increase at a lower rate, while at 1200 �C, thestrength of concrete remains constant at all periods. GeoPC can


rapidly gain strength after 6 h of heating at 1200 �C and shows an80% strength gain after 24 h. Such strength development can bemainly attributed to the development of C-S-H cementitious gel onthe pore space and can improve the density of the resultant geo-polymer binder matrix (Nath and Kumar, 2013). It is also reportedthat the higher development of strength gained in external contactcuring was because of the increase in polarization of OH- to breakSi-O and Al-O bonds on the FA surface. Meanwhile, it is determinedthat the greater the Si content of the samples cured at 60 �C, thegreater the strength achieved. Likewise, the strength of GeoPC re-duces by about 15-23%with a rise in thewater to geopolymer solidsratio by mass (Calder�on-Moreno et al., 2002; Hardjito et al.,2005a,b; Zhuang et al., 2016b). Reportedly, the use of 100% GGBS-based GeoPC mixes were shown to increase the strength by 41%between the results recorded at 7 days and 112 days at ambientroom temperature curing and a 78% improvement in strength ofRHA-based GeoPC, depending on the RHA fineness, the ratio of FA/RHA, and sodium silicate to NaOH (Ajay et al., 2012; B. N. Sangeetha,2015; He et al., 2013; Mehta, 1977; Naji et al., 2010; Shalini et al.,2016). Moreover, the RM-based GeoPC led to increases instrength depending on the red mud and alkaline liquid contents;however, no substantial increase in strength was observed with theaddition of more than 30% RM (Dharmendra et al., 2017; Kumar andKumar, 2013; Paramguru et al., 2004; Ye et al., 2016). The propertiesof GeoPC-raw-materials (binders), additives, producing techniques,and chemical compositions were seen to have a positive influenceon the strength development of GeoPCs in both the short-term andlog-term.

6. Physical properties

GeoPC’s physical properties are influenced by many factors dueto its mixed proportion of binder, sand, aggregates, water, andalkaline liquids. As GeoPC matures, it continues to shrink depend-ing on the design density because of the ongoing reaction in thematerial. However, the rate of shrinkage decreases rapidly andcontinues to reduce with time. The physical properties of GeoPC,including its density shrinkage, porosity, and sorptivity, aredescribed in the following subsections.

6.1. Density

GeoPC has an average density (ASTM C 567) of 2020 kg/m3 to2700 kg/m3 (A. Castel, 2016) depending on the components in thestructure. The amount of aggregate in GeoPC has the main influ-ence on its density, particularly the content of fine aggregate whichimproved the density when greater volume was included in thematrix. The results of the slump test for GeoPC are within the80mm ± 20mm criterion, which is not only reliant on rheology butalso on the aggregate density (Nuaklong et al., 2016) and the den-sity of the fibers (Kamseu et al., 2016; Lee et al., 2016;Mazaheripouret al., 2011). The GGBS-based GeoPC with fine particles providesdensity improvements of around 7.5% in comparison with POFA-based GeoPC; whereas GGBS-based and FA-based GeoPCs den-sities are similar to that of normal concrete. It was observed that themortar density varies between 2014 kg/m3 and 2163 kg/m3. Thecombined aggregates mass is taken in the range between 75% and80% of the concrete mass (Castel and Foster, 2015; Hardjito et al.,2005a,b; KThu and Murthy, 2015; Shaikh, 2016; Shaikh et al.,2015) because the fine aggregates are limited to 30% of the totalaggregates (KThu and Murthy, 2015; Sreenivasulu et al., 2015) andthe addition of silicate solution increases the relative density from79% to 93% (He et al., 2010).When added to OPC concretemixes, theBayer-derived geopolymer mortar aggregate shows a 30% lowerdensity and a 50% higher compressive strength (Bakharev, 2005).

The reduction in the compressive strength of geopolymer mortarswith up to 10% of dry wastepaper sludge can be attributed to theexistence of surfactants (dissolved lignin residues) in the sludge;such decrease can cause the production of low-density geopolymermortar (A. Castel, 2016; Yan and Sagoe-Crentsil, 2012). The spher-ical particles (Eq. (11)) of all materials can lead to a higher packingdensity compared with the crushed particles in a wet state;therefore, the water retention and water demand in the sphericalcase are lesser than those in the crushed case (Sakai, 1997). To in-crease the actual packing density, the grading span must beincreased by using a certain amount of finer particles, utilizingcompact or rounded particles, and continuous grading withincreased ratio of fractions of larger particle size in GeoPC (G€orhanand Kürklü, 2014). Such effect is accredited to the fact that thedelayed pozzolanic reaction of FA increases the compressivestrength and density of concrete over time and ultimately leads to acarbonation resistance that is higher than that obtained in theaccelerated test (A. Castel, 2016). The density decreases as theproportion of POFA increases (Kabir et al., 2015). Furthermore, RHAcan reduce the bulk density (Ajay et al., 2012; Chennur Jithendra,2017; Provis and Deventer, 2009), and the small particle size ofFA, which has a D50 of 7.6 mm, can increase the density of GeoPCconcrete (Zhang et al., 2016). Some studies have applied the particletechnology in manufacturing OPC to enhance the granular distri-bution of geopolymer materials and to attain high packing density,which in turn can help reduce the required amount of activebinders and alkaline activators (Ismail et al., 2011). Thus, comparedto the mortar with POFA, GGBS with fine particles can increase thedensity of GeoPC by approximately 7.5% (Islam et al., 2014).Wardhono et al. (2017) found that increasing the packing density ofthe AleSi gel matrix can positively affect the elastic modulus andstrength development of FAGP concrete between 90 days and 540days. These results altogether imply that reducing the concretedensity can also reduce the transport cost, vehicle wear, and roaddeformation (A. Castel, 2016). Furthermore, noteworthy cost sav-ings are reported because of density decrease as POFA concrete hasalmost 17e25% lower density than ordinary concrete. It is alsofound that metakaolin-based GeoPC can achieve higher strengthand microstructure; therefore, it is applied for the formation of thegeopolymeric gel of 1.45 g/cm3 density (Kong et al., 2007; Sakulich,2011). Also, it was revealed that SFA-based GeoPC with the additionof aluminium particles have led to a reduction in the bulk densityby about 15.5%. In general, the average density of GeoPC is equiv-alent to that of normal concrete, depending on its composition andthe underestimation integral to laser particle analysis whereirregular particles are presumed to be impeccably spherical (Eq.(22)) (Shi et al., 2011).

Geometric specific surface area

0B@SSA

1CA ¼

0:6D

Particle density

(22)

where, D ¼ size of particle, mm.

6.2. Dry shrinkage

Dry shrinkage (DS) (ASTM C 596) refers to the reduction ofvolume during the drying and hardening processes (A. Castel,2016). The shrinkage of GeoPC up to the age of 6 months wasdetermined to be similar to that of normal concrete of comparablestrength (Duan et al., 2016). The strains of FA-based GeoPCs dependsignificantly on the period of exposure in sulfate solution of vari-able concentrations (Satpute Manesh et al., 2012). If the formation


of these strains is prevented, then the concrete faces tensile stressand develops cracks. Increasing the proportion of slags above 10%can result in cracking due to DS (Sakulich, 2011). This phenomenoncan be reduced and controlled by adding fibers to the mix design ofGeoPC (Rickard et al., 2014). A concrete made of lightweight ag-gregates (e.g., RCA (Kovler and Roussel, 2011)) is known to havehigher DS (up to 50%) compared with conventional concrete (Moet al., 2015b). Autoclave curing can effectively reduce the DS ofAAS (A. Castel, 2016). Yan and Crentsil monitored the DS behavior ofdry wastepaper sludge FA-added geopolymer mortars for up to 91days (Yan and Sagoe-Crentsil, 2012). Heat-cured FA-based GeoPCshows a very low DS of about 100 micro strains after a year(Hardjito et al., 2005a,b). The addition of up to 10% dry wastepapersludge can reduce the DS of the resultant geopolymer mortars(G€orhan and Kürklü, 2014; Hardjito et al., 2005a,b; D Hardjito et al.,2005a,b; Hardjito et al., 2008; Noushini et al., 2016; Ryu et al.,2013). Chindaprasirt et al. (2010) reported that using high-calcium FA as a geopolymer source material can improve the DSof geopolymer mortars. They also found that high-calcium geo-polymer mortars demonstrate ten times improvement in terms ofDS compared with OPC mortars, thereby demonstrating theextraordinary dimensional stability of high-calcium FA geo-polymers (A. Castel, 2016). From the results of a moisture lossanalysis, the growth of cellulose fibers in the presence of moisturein GeoPC can increase the degree of DS, while a continued curing at70 �C for 7e28 days can reduce the strength of mixes with morethan 20% GGBS (Soutsos et al., 2016). The DS (0.025%) of GeoPCconcrete becomes lower than that of OPC concrete (0.09%) after 12weeks (Singh et al., 2015) and the incorporation of nano-TiO2 par-ticles refines the microstructure and lowers the DS of GeoPC (Duanet al., 2016). Another study of 20% FA-basede and GGBS-basedGeoPC with sodium silicate to sodium hydroxide (SS/SH) ratiorevealed that shrinkage reduced with the rise of slag content andreduction in SS/SH ratio in GeoPC cured at room temperature,leading it to be comparable to that of normal concrete of similarstrength (Deb et al., 2015). Meanwhile, the use of 50% RM indesigning GeoPC is seen to greatly increase the strength; however,exceeding the 50% percentage can cause a lot of shrinkage cracks(Dharmendra et al., 2017; He et al., 2012; Kumar and Kumar, 2013;Paramguru et al., 2004). Furthermore, the low drying shrinkage ofGeoPC provides support to the long-term performance of GeoPCelements, leading to use in infrastructure applications.

6.3. Porosity

Porosity (ASTM C 830) which is usually caused by several relatedfactors, such as relative humidity, degree of reaction, and macro-scopic properties of GeoPC, is vital to the development of GeoPC(Noushini et al., 2016; Provis et al., 2005). Pore size distribution andporosity are the most important characteristics to be analysed inorder to study corrosion in GeoPC. Porosity of GeoPC is highlyinfluenced by the volume of binder added. For example, in thedesign of RHA-based GeoPC, an increase in the amount of RHAnecessitates an upsurge inwater/cement ratio. Due to that, RHA is agreatly porous material (B. N. Sangeetha, 2015; Habeeb andMahmud, 2010; Naji et al., 2010). Baltazar et al. (2014) found thatthe effectiveness of surface treatment is more significant in con-crete with higher porosity because treatment agents can penetratemore easily and deeply into this concrete. Those geopolymericmortars with sodium hydroxide molarities of 14 M and 18 M canabsorb around 20% of water because of their open porosity (A.Castel, 2016). In alkali-activated cement (AAC), the curve of 100% FAbinder has presented dominant pore diameters of 10e100 nm intotal porosity of 20% geopolymer binders (Diaz et al., 2010;Gunasekara et al., 2016; Hemalatha and Ramaswamy, 2017). In

this binder, the maximal diameters distribution was around 27 nmand the pores in the mesopores interval, 10e50 nm, accounted for80% frequency of total porosity. This curve differs from the bimodalprofile of the pore size distributions in alkali-activated FA that iscured for 28 days, where the pores are located at 100 nm and1000 nm (Khale and Chaudhary, 2007; Ma et al., 2013). Increasingthe number of cotton fabric layers also gradually increases theporosity of composites by 20%e30% (Yan et al., 2016). Provis et al.(2012) observed that adding GGBS not only reduces porosity butalso produces a pore refinement effect. This finding indicates thatthe excess GGBS can increase the porosity of concrete (KThu andMurthy, 2015; Mo et al., 2016; Sreenivasulu et al., 2015). Porosityand cracks also have harmful effects on the modulus of elasticity ofRCA (Zhang and Bentley, 2003). Moreover, the use of fine RCA iscategorized by a greater porosity than coarse ones because of thevolume of enclosed cement mortar. Reportedly, the use of raw RHAparticle size in GeoPC at 1000 �C heating displayed 87% porosityand 450 m2/g SSA but the larger particles improved strength (Ajayet al., 2012; Chu et al., 2016; Provis and Deventer, 2009). Asmentioned earlier, adding slag increases the workability of thematrix because this material needs less liquid compared withmetakaolin for particle wetting, which in turn can help reduceporosity (Sakulich, 2011). Reducing the alkalinity of the bindermatrix and adding aggregates and Si carbide sludge can also helpreduce porosity (Almeida et al., 2013; Druta, 2003; Nuaklong et al.,2016; Pouhet and Cyr, 2016; Prud_homme et al., 2015). Reportedly,the addition of SF by up to 5% improved the strength of GeoPC, butadditional increase in SF initiated a reduction in strength,contributed to better microstructure and revealed lesser porosity(Atis et al., 2005; Bhavsar et al., 2014; Mazloom et al., 2004; Memonet al., 2013). Meanwhile, the 40% POFA-based GeoPC with 2.5%NaOH showed an increase in strength by 95% and a reduction inporosity by 5.27% at 28 days (Kabir et al., 2015; Salih et al., 2014).Another research revealed that the addition of less than 30% RM inGeoPC caused higher porosity performance. In GeoPC, porosity isgenerally affected by the pores formed with the diameter sizedepending on the volume of binder included, in particular, whenthe binder added with the volume was more than half of OPC.

6.4. Sorptivity

Sorptivity (ASTM C1585) refers to the capability of concrete toabsorb and transmit water by means of capillary suctions(Davidovits, 1999; Mo et al., 2016; Rickard et al., 2014). The sorp-tivity of geopolymers (5e30 mm/s1/2) greatly depend on their watercontent and forming pressure (A. Castel, 2016; Nuaklong et al.,2016). The sorptivity of GeoPC can increase with a rise in grade;for instance, the increase in FA content can lead to improvements insorptivity (Thokchom et al., 2009). The defiance to the sorptivity ofGeoPCs is inversely balanced to the water/binder ratio, fineness ofparticles, and extra water (Deb et al., 2016; Thokchom et al., 2009).The initial sorptivity of GeoPC is measured during the first 6 h ofwater absorption (Shaikh, 2016). The water sorptivity of 100 mmGeoPC specimens is usually measured at 28 days (Soutsos et al.,2016). The measurement begins by placing the specimens insidean oven and drying them at 105 �C ± 5 �C for 48 h until a constantweight is recorded. These specimens are then greased at all sidesand covered with cling film in order for the water to be absorbedvertically when these specimens are placed 5 mm under the watersurface (Fig. 15) (Hadjsadok et al., 2012). Reportedly, the 25%replacement of natural aggregate by RCA in GeoPC presented bettersorptivity than normal RAC. Another research reported that thePOFA-based GeoPC revealed a lower sorptivity due to contributionto the rise of particles’ surface area (Detphan and Chindaprasirt,2009; Somna et al., 2011; Wongpa et al., 2010).

Fig. 15. Setup for the sorptivity test of GeoPC (Hadjsadok et al., 2012).


Meanwhile, the use of 50% GGBS and 50% FA in GeoPC canreduce the sorptivity in GeoPC with about 36% compared to normalconcrete (Thokchom et al., 2009) Afterwards, the specimens wereweighed at 1, 4, 9, 16, 25, 36, 49, and 64 min to determine theirweight gain. Those GeoPC specimens with lower alkali contenthave higher water sorptivity (Thokchom et al., 2009). Meanwhile,increasing the metakaolin content and activator concentration aswell as adding RCA, GGBS, 2% nano-silica dosage, metakaolin, FA,and 8% Na2O can reduce the water sorptivity of GeoPC by 20%(Bernal et al., 2012; Davidovits, 1999; Deb et al., 2016; Mo et al.,2016; Shaikh, 2016; Singh and Siddique, 2015; Thokchom et al.,2009; Wardhono et al., 2017). Increasing the flowability propertyof GeoPC can also lower the porosity of concrete and subsequentlylessen its absorption of water (Chi and Huang, 2014), However, thesorptivity of GeoPC exhibited lower rate when compared to normalconcrete of different grades. The sorptivity of the concrete speci-mens is calculated using Eqn. (23). The same experimental setuphas been adopted in other studies (Hadjsadok et al., 2012; Liu et al.,2016)

i ¼ A þ St1/2 (23)

where;

e S ¼ sorptivity coefficient, mm/min1/2;‒ i ¼ cumulative volume of water absorbed at time, mm/mm2;‒ A ¼ surface area of the test specimen, mm2; and‒ t ¼ recorded time, min.

7. Conclusion

GeoPC is an inventive construction material that provides analternative to OPC, and it is made by the chemical activity of inor-ganic particles of waste materials. GeoPC has become more com-mon in past decades due to being more environmentally friendly asopposed to conventional OPC. Further, economic and environ-mental reasons necessitate the amendment of current concreteproduction materials (OPC). This can be achieved through the cleanproduction and use of GeoPC. It is found that the production of OPCand GeoPC using secondary industrial raw waste materials, such asFA, SFA, GGBS, and RM, is a better substitute to traditional OPC, andbecause of that, GeoPC can provide ultra-high early strength,greater durability, improved economic benefits, less CO2 emissionsduring production, lower use of sodium silicate solution, andlengthier service life in a number of RC applications, in particular,

for use in transportation infrastructure construction. Thus, theproduction of industrial-by-products-based geopolymer greatlydepends on alkali activated geopolymerization occurring undermoderate conditions and is deemed as a cleaner procedure becauseof the higher reduction of CO2 emissions during manufacturingwhen compared to OPC. It is shown that the production of GeoPCneeds intensive care and exact material composition. During theactivation process while producing the geopolymer, great alkalinityalso obliges a safety danger and improved energy consumption andproduction of greenhouse gases. Moreover, GeoPC is highly influ-enced by the curing temperature and time as well as by the prop-erties and proportions of the constituent materials. In this study, itwas revealed that GeoPC has excellent compressive strength andthis led GeoPC to be identified as a high potential product used inthe fabrication of several structural concrete applications. It wasalso determined that GeoPC has a significant resistance to acid,excellent resistance to sulfate attack, experiences low creep, andslightly suffers from drying shrinkage. The outstanding factors thataffect the properties of the fresh and hardened GeoPC have beendiscussed. Based on this review study, it was observed that the vastmajority of previous research work focused on the specific prop-erties of geopolymer, such as compressive strength, rather thanfocusing on their characteristics such as alkaline activator solutions.Also, components such as natural and artificial fibers in GeoPC andtheir effect on the strength of the GeoPC have not received signif-icant attention over the years. Therefore, based on this review, thefollowing conclusions are drawn:

� Binder paste containing small diameter carbon nano-fiberdemonstrates high sensitivity and stable sensing properties inGeoPC.

� Gel nuclei particles must be sufficiently stable to resist depoly-merization and in order to begin a new gel phase that will beprimarily responsible for strength and durability enhancementof GeoPC.

� Silicate and Aluminate monomers that condense stabilizationmechanism in a GeoPC network are considered among the mostimportant components of concrete that guarantee fast chemicalreaction of SieAl minerals under alkaline conditions.

� The desired density and strength of concrete depends on themethod of proportioning materials, design codes and con-struction guidelines.

� The stability of GeoPC production depends on numerous factorssuch as curing temperature, setting and curing time, molarity ofalkaline activator and mix ratio.

� The properties of GeoPC in the fresh state determine variousaspects of workability that control segregation resistance,passing ability and filling ability of GeoPC which must becautiously controlled.

� GeoPC exhibits high early strength and has been effectivelyutilized in precast industries. This is demonstrated by its abilityto produce large-scale GeoPC structure within a short durationwith minimal tendency to breakage during transportation.

With this in mind, it can be said that GeoPC is a superior alter-native material to cement and can be effectively used to replaceOPC for practical use in construction industries worldwide. Futurestudies should focus on the improvement of strength and durabilityof GeoPC in hardened state through inclusion of fibers. Further-more, future studies should also consider investigating the influ-ence of aggregate content and additives on the engineeringproperties of GeoPC. Also, suitable guidelines for selection ofaggregate contents in GeoPC should be developed with clear mixdesign procedure. Investigations on the porosity property of geo-polymer foams prepared by using mixed-foaming method should


be carried out. The properties of these foams are no better thanthose of traditional porous materials, such as glass foam orautoclaved-aerated concrete. These issues limit the potential use ofgeopolymer foams as thermal insulation materials and preventthem from competing with traditional porous inorganic materials.Lastly, further studies are required to compare the cost of GeoPC tothat of conventional concrete.

Acknowledgment

The authors gratefully acknowledge the financial support by theDepartment of Civil Engineering, College of Engineering, PrinceSattam Bin Abdulaziz University, Saudi Arabia; and the Departmentof Civil Engineering, Faculty of Engineering and IT, Amran Univer-sity, Yemen, for this research.

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Clean production and properties of geopolymer concrete