1-s2.0-S0950061815002834-main

7/17/2019 1-s2.0-S0950061815002834-main

1/13

Review

Geopolymer concrete: A review of some recent developments

B. Singh , Ishwarya G., M. Gupta, S.K. Bhattacharyya

CSIR-Central Building Research Institute, Roorkee 247667, India

h i g h l i g h t s

An overview of geopolymer ispresented alongwith its processing

parameters.The hardened properties and

durability of geopolymer concrete are

discussed.

The design guidelines for OPCconcrete are applicable to

geopolymer concrete also.

Geopolymeric building productsdeveloped at CSIR-CBRI are

highlighted.

Ambient cured single componentgeopolymer may enhance its wider

use in the field.

g r a p h i c a l a b s t r a c t

Conversion of fly ash into geopolymers/concrete.

a r t i c l e i n f o

Article history:

Received 26 November 2013

Received in revised form 16 February 2015

Accepted 4 March 2015

Available online 31 March 2015

Keywords:

Geopolymer concrete

Activator

Bond strength

Compressive strength

Durability

a b s t r a c t

An overview of advances in geopolymers formed by the alkaline activation of aluminosilicates is pre-

sented alongwith opportunities for their use in building construction. The properties of mortars/concrete

made from geopolymeric binders are discussed with respect to fresh and hardened states, interfacial

transition zone between aggregate and geopolymer, bond with steel reinforcing bars and resistance to

elevated temperature. The durability of geopolymer pastes and concrete is highlighted in terms of their

deterioration in various aggressive environments. R&D works carried out on heat and ambient cured

geopolymers at CSIR-CBRI are briefly outlined alongwith the product developments. Research findings

revealed that geopolymer concrete exhibited comparative properties to that of OPC concrete which

has potential to be used in civil engineering applications.

2015 Elsevier Ltd. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

2. An overview of geopolymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

2.1. Constituents effect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

2.2. C-S-H phase effect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

2.3. Effect of admixtures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

2.4. Curing conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

3. Geopolymer mortars . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

4. Geopolymer concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

http://dx.doi.org/10.1016/j.conbuildmat.2015.03.036

0950-0618/ 2015 Elsevier Ltd. All rights reserved.

Corresponding author.

E-mail address:[email protected](B. Singh).

Construction and Building Materials 85 (2015) 7890

Contents lists available at ScienceDirect

Construction and Building Materials

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c o n b u i l d m a t
http://dx.doi.org/10.1016/j.conbuildmat.2015.03.036mailto:[email protected]://dx.doi.org/10.1016/j.conbuildmat.2015.03.036http://www.sciencedirect.com/science/journal/09500618http://www.elsevier.com/locate/conbuildmathttp://www.elsevier.com/locate/conbuildmathttp://www.sciencedirect.com/science/journal/09500618http://dx.doi.org/10.1016/j.conbuildmat.2015.03.036mailto:[email protected]://dx.doi.org/10.1016/j.conbuildmat.2015.03.036http://crossmark.crossref.org/dialog/?doi=10.1016/j.conbuildmat.2015.03.036&domain=pdf

7/17/2019 1-s2.0-S0950061815002834-main

2/13

4.1. Fresh and hardened properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

4.2. Interfacial transition zone. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

4.3. Bond between reinforcing bars and geopolymer concrete . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83

4.4. Fire behaviour . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

5. Durability studies. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

5.1. Alkali-silica reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

5.2. Effect of acid attack. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

5.3. Effect of sulphate attack . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

5.4. Carbonation and permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 865.5. Corrosion of steel reinforcement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

6. Research and development at CSIR-CBRI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

7. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

1. Introduction

The concrete industry faces challenges to meet the growing

demand of Portland cement due to limited reserves of limestone,

slow manufacturing growth and increasing carbon taxes. It is

reported that the requirement of cement in India is likely to touch550 million tonnes by 2020 with a shortfall of230 million ton-nes (58%) and the demand for cement has been constantlyincreasing due to increased infra-structural activities of the coun-

try[1]. One effort to combat shortfall is the development of alter-

nate binders to Portland cement aiming at to reduce the

environmental impact of construction, use of greater proportion

of waste pozzolan, and also to improve concrete performance.

Search for several alternatives such as alkali-activated cement, cal-

cium sulphoaluminate cement, magnesium oxy carbonate cement

(carbon negative cement), supersulphated cement etc. are being

made with the advantages of Portland cement [2]. As the family

of the alkali-activated cement is growing, the alkaline cement is

classified based on a phase composition of the hydration products:

R-A-S-H (R = Na+ or K+) in the aluminosilicate based systems and R-

C-A-S-H in the alkali-activated slag or alkaline Portland cements

[3]. In recent years, geopolymer has attracted considerable atten-

tion among these binders because of its early compressive

strength, low permeability, good chemical resistance and excellent

fire resistance behaviour [49]. Because of these advantageous

properties, the geopolymer is a promising candidate as an alterna-

tive to ordinary Portland cement for developing various sustain-

able products in making building materials, concrete, fire

resistant coatings, fibre reinforced composites and waste

immobilization solutions for the chemical and nuclear industries.

2. An overview of geopolymers

Geopolymer is considered as the third generation cement afterlime and ordinary Portland cement. The term geopolymer is

generically used to describe a amorphous alkali aluminosilicate

which is also commonly used for to as inorganic polymers,

alkali-activated cements, geocements, alkali-bonded cera-

mics, hydroceramics etc. Despite this variety of nomenclature,

these terms all describe materials synthesized utilising the same

chemistry[4]. It essentially consists of a repeating unit of sialate

monomer (SiOAlO). A variety of aluminosilicate materials

such as kaolinite, feldspar and industrial solid residues such as

fly ash, metallurgical slag, mining wastes etc. have been used as

solid raw materials in the geopolymerization technology. The

reactivity of these aluminosilicate sources depends on their chemi-

cal make-up, mineralogical composition, morphology, fineness and

glassy phase content. The main criteria for developing stablegeopolymer are that the source materials should be highly

amorphous and possess sufficient reactive glassy content, low

water demand and be able to release aluminium easily. The alka-

line activators such as sodium hydroxide (NaOH), potassium

hydroxide (KOH), sodium silicate (Na2SiO3) and potassium silicate

(K2SiO3) are used to activate aluminosilicate materials. Compared

to NaOH, KOH showed a greater level of alkalinity. But in reality,it has been found that NaOH possesses greater capacity to liberate

silicate and aluminate monomers [4]. The properties of geopoly-

mers can be optimised by proper selection of raw materials, correct

mix and processing design to suit a particular application [4].

Viewing the importance of the subject, a collaborative project

sponsored by the European Commission BRITE was undertaken

jointly by France, Spain and Italy on development of Cost-effective

geopolymeric cement for innocuous stabilization of toxic elements

(GEOCISTEM). The project was aimed at manufacturing geopoly-

meric cement by replacing potassium silicate with cheaper alkaline

volcanic tuffs[9].

Geopolymers are synthesized by the reaction of a solid

aluminosilicate powder with alkali hydroxide/alkali silicate [8]. A

schematic representation on formation of fly ash-based geopoly-

mers/concrete is shown inFig. 1. Under highly alkaline conditions,

polymerisation takes place when reactive aluminosilicates are

rapidly dissolved and free [SiO4] and [AlO4]

tetrahedral unitsare released in solution. The tetrahedral units are alternatively

linked to polymeric precursor by sharing oxygen atom, thus form-

ing polymeric SiOAlO bonds. The following reactions occur dur-

ing geopolymerisation[7].

Si2O5Al2O2n H2O OH ! SiOH

4 AlOH

4 1

2

This process releases water that is normally consumed during

dissolution. The water, expelled from geopolymer during the reac-tion provides workability to the mixture during handling. This is in

contrast to the chemical reaction of water in Portland cement mix-

ture during the hydration process. It is reported that the hydration

products of metakaolin/fly ash activation are zeolite type: sodium

aluminosilicate hydrate gels with different Si/Al ratio whereas the

major phase produced in slag activation is calcium silicate hydrate

with a low Ca/Si ratio. Though many physical properties of

geopolymers prepared from various aluminosilicate sources may

appear to be similar, their microstructures and chemical properties

vary to a large extent. The metakaolin-based geopolymer has an

advantage that it can be manufactured consistently, with pre-

dictable properties both during the preparation and development.

However, its plate-shaped particles lead to rheological problems,

increasing the complexity of processing as well as the waterdemand of the system [6]. Contrary to this, the fly ash-based

B. Singh et al. / Construction and Building Materials 85 (2015) 7890 79
http://-/?-http://-/?-

7/17/2019 1-s2.0-S0950061815002834-main

3/13

geopolymer is generally more durable and stronger than that of

metakaolin-based geopolymer [4]. The slag-based geopolymer is

considered to have high early strength and greater acid resistance

than those of metakaolin and fly ash-based systems.

2.1. Constituents effect

The most important factors affecting the properties of geopoly-

mer pastes are: SiO2/Al2O3 ratio, R2O/Al2O3 ratio, SiO2/R2O ratio

(R = Na+ or K+) and liquidsolid ratio. The majority of research con-

cluded that an amorphous structure of geopolymers is preferable

in order to achieve desired mechanical strength [1015]. Therelationship between the compressive strength and SiO2/R2O ratio

showed that an increase in alkali content or decrease in silicate

content increases the compressive strength of geopolymers attri-

butable to the formation of aluminosilicate network structures

[10,11]. Geopolymer activated with NaOH alone with Si/Na of 4/4

or less formed the crystalline zeolite (Na96Al96Sr96O384216H2O)

but at a ratio >4/4, nanosized crystals of another zeolite

(Na6[AlSiO4]64H2O) were formed[12]. The addition of even smallamount of sodium silicate to the NaOH significantly reduces crys-

tallite formation due to templating function of silicate units. At low

activator dosage (18%), the pores developed in the fly ash-based

paste were larger and exhibited wider distributions (19.8

2342 A) whereas at higher activator dosage (30%), the pores were

smaller and showed a narrow distribution (19.81155 A) mainlydue to the pore refinement as a result of more dissolution of

particles and formation of reaction products (Fig. 2). The reduced

porosity enhanced the strength of geopolymer pastes [13].

Typically, the optimum geopolymer strength was reported with

SiO2/Al2O3 ratio in the range of 3.03.8 and Na2O/Al2O3 ratio of1 [14,15]. Changes in SiO2/Al2O3 ratio beyond this range havebeen found to result in low strength. The setting time of geopoly-

mer pastes increased with increasing SiO2/Al2O3ratio of the initial

mixture.

2.2. C-S-H phase effect

The effect of C-S-H phase on the geopolymerization of

aluminosilicates has been studied with a view to know its role in

early age strength [1622]. In metakaoin/slag blends, both C-S-H

phase and aluminosilicate gel (N-A-S-H) co-exist in the paste

[16] as similar to NaOH activated high calcium fly ash-based

geopolymer[17] which are responsible for the strength increase.

The little dissolution of calcium occurs in the case of adding naturalcalcium silicate minerals at lower alkalinity, resulting in less C-S-H

gel formation and subsequent strength reduction of geopolymer

pastes [18]. In the case of fly ash/slag blends, the reaction at

27 C is dominated by the slag activation, whereas the reaction at

60 C is due to combined activation of fly ash and slag. The

improvement in compressive strength of pastes with slag addition

is attributed to its compactness of the microstructure [19]. The

initiation of hardening in fly ash/slag geopolymer made with

potassium silicate and potassium hydroxide was due to C-S-H/C-

A-S-H formation and the hardening continues due to a rapid for-

mation of a C-A-S-H, K-A-S-H and (Ca, K)-A-S-H depending on

the availability of calcium ions and pH of the system. A slower dis-

solution rate of calcium ions effectively increased the compressive

strength as rapid geopolymerization continues for a longer dura-tion[20]. The low pH and limited calcium ion environment facili-

tate the polymerisation reaction between silicate and aluminate

species in high calcium fly ash-based geopolymers producing N-

A-S-H gel [21]. Guo et al. [22] reported 63.4 MPa compressive

strength of class C fly ash-based geopolymer paste showing the

role of calcium participation in the strength development.

2.3. Effect of admixtures

Kusbiantora et al.[23]reported from their studies that admix-

tures such as sucrose and citric acid which act as retarder in OPC

have different mechanism in fly ash-based geopolymers. Sucrose

acted as a retarder since it is absorbed by Ca, Al and Fe ions to form

insoluble metal complexes. On the other hand, citric acid acted asan accelerator reducing the setting time by 9 and 16 min

Fig. 1. Conversion of fly ash into geopolymers/concrete.

Fig. 2. Pore size distribution of fly ash-based geopolymer pastes at different

activator dosages[13].

80 B. Singh et al. / Construction and Building Materials 85 (2015) 7890

7/17/2019 1-s2.0-S0950061815002834-main

4/13

respectively. Amongst the commercial superplasticizers, the naph-

thalene based superplasticizer was effective when single activator

was used rendering 136% increase in relative slump without any

decrease in compressive strength. Modified polycarboxylate based

superplasticizer was efficient one when multi-compound activator

was used with a decrease in compressive strength of 29% [24].

However, retarding effect of polycarboxylate based super-

plasticizer was also reported in fly ash/slag blended system thoughthe improvement in workability was significant compared to naph-

thalene based superplasticizer[25].

2.4. Curing conditions

Several attempts[2631]have been made to study the effect of

different curing conditions on the properties of geopolymer pastes.

The curing temperatures were reported in the range between 40 C

and 85 C for complete geoplymerisation reactions. Palomo et al.

[26] studied curing of alkali activated fly ash (0.25 and 0.30 liq-

uid/solid ratio) at 65 C and 85C. They indicated that the com-

pressive strength of geopolymers (812 M) cured at 85C for

24 h was much higher than those cured at 65C. The rise of

strength was much smaller when curing time was extended after24 h. Perera et al. [27] studied the curing of metakaolin-based

geopolymers under ambient (2123 C) and heat conditions (40

60 C) with a controlled relative humidity (RH) for 24 h and found

that curing at 30% RH was preferable to that at 70% RH. Heah et al.

[28] concluded that the curing of metakaolin-based geopolymers

at ambient temperature was not feasible while increase in tem-

perature (40 C, 60 C, 80 C, 100C) favored the strength gain after

13 days. However, curing at higher temperature for a longer per-

iod of time caused failure of samples at a later age due to the ther-

molysis of SiOAlO bond. Rovnanik[29]reported that curing

of metakaolin based geopolymer at elevated temperature (40

80 C) accelerated the strength development but in 28 days, the

mechanical properties deteriorated in comparison with results

obtained for an ambient or slightly decreased temperature.Ebrahim and Ali [30] prepared three mixes with different for-

mulations and cured hydrothermally at different temperatures

(45, 65, 85C) and time (520 h) after 1 and 7 days of procuring.

Longer procuring at room temperature, before the application of

heat is beneficial for higher strength development. In general, ade-

quate curing of geopolymeric materials is required to achieve opti-

mal mechanical and durability performance to maintain their

structural integrity[31].

3. Geopolymer mortars

Various studies[3240]were conducted on flow and mechani-

cal properties of geopolymer mortars because of their more rele-

vant applications in building construction. The properties ofmortars were optimised with respect to initial flow, aggregate-bin-

der ratio, activator-binder ratio and activator molarity.

Chindaprasirt et al. [32] reported that the compressive strength

of class C fly ash-based geopolymer mortar was 52 MPa when

cured at 70 C for 3 days using sand-fly ash ratio of 2.75 at work-

able flow of 135 5%. Prolonged curing at high temperatures led

to the reduction in the compressive strength because of weakening

of microstructure and increased porosity due to the loss of mois-

ture. In another attempt [33], they produced geopolymer mortar

with a compressive strength of 86 MPa at 28 days with the help

of air classified class C fly ash (4500 cm 2/g fineness) activated with

sodium silicate and NaOH (10 M) at 1:1 mass ratio. The dimen-

sional change in terms of drying shrinkage (161 106 mm/

mm) was insignificant when compared with the Portland cementmortar (700850 106 mm/mm). The geopolymer mortars

(14 M activator solution) with 1030 wt% aggregate exhibited an

acceptable flowability, while the mortars containing 40 & 50 wt%

aggregate were stiff and difficult to pack in the mould. Increasing

aggregate content in the mortar mixes leads to insufficient activa-

tor for complete geopolymerization of fly ash/slag. The activator

may also be utilised for wetting of aggregate leaving less availabil-

ity for dissolution of these fly ash or slag particles. The compressive

strength of geopolymer mortars with high level of aggregate can beachieved by optimising the amount of activator dosage [34].

Khandelwal et al.[35]summarised that the compressive strength,

modulus of elasticity and Poissons ratio of fly ash-based geopoly-

mer mortars increased logarithmically with the increase of strain

rate. These engineering properties of geopolymer mortars com-

pared favourably with those predicted by Standards/Codes for con-

crete mixtures. When bottom ash was used, the geopolymer

mortars exhibited a low compressive strength (20 MPa). With10% replacement of sand by bottom ash, the mix exhibited a com-

parable compressive strength to those made with sand only. The

increase in strength (50100%) of bottom ash mortar was also

reported when the specimens were exposed at 800C probably

due to activation of bottom ash [36]. When lignite bottom ash

was ground to a mean particle size of 15.7 lm (3% retained on

sieve No. 325), the compressive strength of mortars activated with

sodium hydroxide/sodium silicate was 2458 MPa[37].

Brough and Atkinson[38]prepared geopolymer mortars using

slag, sand and activator in a ratio of 1:2.33:0.5. At water-to-total

solid ratio of 0.42, the mortar gained strength of40 MPa. Thesodium silicate activated mortars exhibited higher compressive

strength with low levels of porosity at the interface while KOH

activated mortars were highly porous in the interfacial zone giving

low compressive strength values. Yang et al. [39,40]found that the

flow of alkali-activated mortars increased with the increase of

water-binder ratio and decrease of aggregate-binder ratio. When

the aggregate-binder ratio was larger than 2.5, the flow of mortars

decreased sharply. They also found that slag-based geopolymer

mortars exhibited much higher compressive strength but exhibited

slightly less flow than the fly ash-based geopolymer mortars forthe same mixing condition. The poor compressive strength of fly

ash-based mortars cured at low temperatures is attributed to the

presence of unreacted fly ash particles and large number of voids.

As the aggregate-binder ratio increased, the compressive strength

increased up to a ratio of 2.5 which indicated that the threshold

of aggregates in geopolymer mortars were slightly lower than

OPC mortars. The shrinkage strain of alkali-activated mortars was

also found to be lower than the OPC mortars.

4. Geopolymer concrete

4.1. Fresh and hardened properties

Various mix proportioning of geopolymer concrete (GPC) were

reported with target strength up to 80 MPa. The typical properties

of geopolymer concrete mixes used by the various authors (41, 45,

48, 49, 53) are summarised in Table 1. The properties of mixes

were studied with respect to water-geopolymer solid ratio, activa-

tor strength, water/Na2O ratio, curing time, curing temperature,

and age hardening. The slump of mixes varied depending on the

molarity of activator, workability aids and extra water added to

the mix[41]. The rheological parameters such as yield stress and

plastic viscosity were attempted over slump test of concrete to

assess its workability loss and flow behaviour. Yield stress gives

initial resistance to flow arose from the friction among the solid

particles while plastic viscosity governs the flow after it is initiated

resulting from viscous dissipation due to the movement of water inthe sheared material. Laskar & Bhattacharjee [42] studied the

http://-/?-http://-/?-

7/17/2019 1-s2.0-S0950061815002834-main

5/13

rheology of fly ash-based geopolymer concrete with slump varying

from 25 mm to flowing concrete with 120 M activator strength.

They found that the yield stress and plastic viscosity were affected

by the molar strength of the sodium hydroxide solution and the

ratio of silicate to hydroxide solution. The setting time of geopoly-

mer concrete was reported up to 120 min. Like Portland cement

pastes and mortars, geopolymers also behave like Bingham fluid

and have a history dependent rheological profile, i.e., geopolymers

may be kept in a fluid form, if subjected to constant shearing for a

certain period of time before initial setting starts [43]. The settingcould be enhanced up to 180 min with the use of naphthalene

based admixture and extended mixing time especially in the case

of slag-based geopolymer which has the potential for a wide range

of technological applications[44].

Hardjito et al. [41]produced fly ash-based GPC with the com-

pressive strength ranging between 30 and 80 MPa with the slump

varied from 100 to 250 mm (activator strength: 814 M). The opti-

mum strength was obtained at 0.18 water-geopolymer solid ratio

cured at 90 C. As the water-geopolymer solids ratio increased,

the compressive strength of GPC decreased analogous to the well

known relationship between compressive strength and water-ce-

ment ratio for OPC concrete. The compressive strength of GPC

remained unchanged with the age when tested after 24 h curing

at elevated temperature. Fernandez-Jimenez et al. [45] made fly

ash-based geopolymer concrete with a compressive strength of

45 MPa at 0.55 liquid/solid ratio cured at 85 C for 20 h. The devel-opment of high early strength in GPC was explained by its compact

microstructure, formation of adequate reaction products, smaller

mean size of the pores and good aggregate-paste bond. They

observed that GPC has a much lower modulus of elasticity

(18.4 GPa) than the OPC concrete (30.3 GPa). Olivia and Nikraz

[46] proportioned fly ash-based geopolymer concrete mix with a

compressive strength of 55 MPa at 28 days and cured at different

temperatures in the range of 6075C. The hardened mix had

higher tensile and flexural strengths, produced less expansion

and showed modulus of elasticity that were 1529% lower than

that of OPC concrete mix. The drying shrinkage (0.025%) of GPC

was less than the OPC concrete (0.09%) after 12 weeks. The mini-

mal shrinkage of GPC may also be due to the significant resistance

offered by its zeolitic microstructure towards drying loss of thewater incorporated during casting[45].

Several attempts [4753] have also been made to establish

correlations within the mechanical properties of geopolymer con-

crete. It was reported that the experimental splitting tensile

strength of fly ash-based GPC was higher than the OPC concrete

(Fig. 3). The increased strength is accounted for a denser interfacial

zone established between the aggregate and geopolymer paste.

The modulus of elasticity increased as the compressive strength

of GPC increased. The modulus of elasticity of GPC was found to

be lower than the values predicted by ACI guidelines for OPC con-

crete. Sofi et al.[48]studied the engineering properties of fly ash/

slag-based GPC. The splitting tensile strength and flexural strength

of GPC were comparable to those models presented by the

Australian Standard (AS 3600) for OPC concrete. Although, the dif-ference between splitting tensile and flexural strength of GPC

mixes has been found to be approximately 2 MPa, similarities

between the strength gain was apparent. Diaz-Loya et al.[49]pro-

posed the equation fr= 0.69p

fc0MPa for correlation between the

flexural strength (fr) and compressive strength and the equation

Ec= 580p

fcMPa for correlation between elastic modulus (Ec)

and compressive strength of GPC (fc= compressive strength).

When compared with the typical Poissons ratio value of OPC con-

crete (0.150.22), the values of GPC appeared to reside toward the

low end of range (0.080.22). Ryu et al. [50]suggested a model for

relationship between compressive strength and splitting tensile

strength (fsp= 0.17 (f0c)

3/4) for fly ash-based GPC. Bondar et al.

[51] reported a relationship between ultrasonic pulse velocity

and compressive strength of GPC. They found that GPC showed a

lower ultrasonic pulse velocity than the OPC concrete even those

Table 1

Typical properties of geopolymer concrete mixes.

Density

(kg/m3)

Molarity

(M)

Slump

(mm)

CS

(MPa)

STS

(MPa)

FS

(MPa)

MOE

(GPa)

Poissons

ratio

Activator/

binder

ratio

Curing

temperature

and time

Hardjito et al. [41] 23302430 1016 60215 3080 3.746 512 2331 0.120.16 0.350.4 6080 C for 24 h

Jimenez et al.[45] NR 8 & 12.5 NR 2943.5 NR 6.86 10.718.4 NR 0.4 & 0.55 85C for 20 h

Sofi et al.[48] 21472408 NR NR 4756.5 2.84.1 4.96.2 2339 0.230.26 0.450.59 23C till testing

Diaz-loya et al.[49] 18902371 14 100150 1080 NR 2.246.41 1.942 0.080.22 0.40.94 60 C for 72 hPan et al.[53] 18762555 8 NR 65.177.9 2.85.1 NR 11.241.2 0.150.19 0.40.65 60 C for 24 h

CS: compressive strength; STS: splitting tensile strength; FS: flexural strength; MOE: modulus of elasticity; NR: not reported.

Fig. 3. Correlations within the mechanical properties of fly ash-based geopolymer

concrete. (a) Splitting tensile strength vs compressive strength. (b) Modulus ofelasticity vs compressive strength[47].


7/17/2019 1-s2.0-S0950061815002834-main

6/13

with the same or higher compressive strength. It was also reported

[52,53] that GPC was brittle as compared to its OPC counterpart

due to the highly cross-linked framework. The fracture energy of

GPC was also low because of its higher bond with aggregates as

compared to OPC concrete[54].

4.2. Interfacial transition zone

It is well known that the interfacial zone (ITZ) between aggre-

gate and matrix is the weakest link in OPC concrete at which

micro-cracks usually first develop under loads [55]. Investigation

of this zone is very crucial since it is known to have different

microstructure from the bulk of the hardened paste. The high

porosity of ITZ allows the easier penetration of external agents

such as chlorides, oxygen, sulphates, etc. into concrete structure.

Contrary to this, ITZ of GPC has been identified as being dense

and much less microstructurally distinct from the bulk of binder

region[56,57]. The stronger ITZ contributes to higher splitting ten-

sile strength, bond strength and durability of the GPC.

Lee and Deventer[56,57]discussed interface between the natu-

ral siliceous aggregates and paste in GPC using kaolin and albite as

precursors. The increase in concentration of the activating solutionincreased the binding capacity of the gel with natural aggregates.

The presence of chloride salts decreased the interfacial bonding

strength between the paste and aggregate probably by causing

gel crystallisation near the aggregate surfaces which resulted in

debonding. In another attempt, they found that the addition of

0.5 M soluble silicate into an activating solution (10 M NaOH and

2.5 M sodium silicate) facilitates the formation of an aluminium-

enriched aluminosilicate surface onto the aggregates through

accelerated Si-preferential dissolution of kaolin and albite. The sur-

face formed during albite leaching was found to possess a similar

Si/Al ratio to the real interface between a silicious aggregate and

fly ash/metakaolin geopolymer paste activated with 10 M NaOH

solution. Without soluble silicates, no deposited aluminosilicate

interface was observed. This suggested that both high concentra-tion of alkali and soluble silicate are essential for the formation

of a strong interface between silicious aggregates and geopolymer

pastes. Zhang et al.[58]reported that at the beginning, there were

many large voids in the fresh ITZ in potassium poly(sialate)

geopolymer concrete. As hydration proceeded, these voids were

completely filled with the hydration products. At this stage, the

difference in the microstructure between the ITZ and matrix was

hardly distinguishable. The contents of K/Al and Si/Al in the ITZ

were higher than those in the matrix. Demei et al. [59]presented

FESEM analysis of ITZ in the fly ash-based self compacting geopoly-

mer concrete with varying superplasticizer dosages. They reported

that relatively a loose and porous ITZ was found at low super-

plasticizer dosages (3%) whereas a dense ITZ was found between

the aggregate and geopolymer paste at higher dosage (7%). They

also found that the compressive strength increased with decrease

in the thickness of ITZ and this relationship depends on the super-

plasticizer dosage.

4.3. Bond between reinforcing bars and geopolymer concrete

The transfer of forces across the interface between concrete and

reinforcing steel bar is of fundamental importance in the structural

design[60]. Bond stresses in the reinforced concrete arise from two

distinct situations. The first is anchorage or development where

bars are terminated. The second is flexural bond or the change of

force along a bar due to a change in bending moment along the

member. The bond strength of reinforcing bars with concrete is

governed by several factors such as the strength of the concrete,

the thickness of the concrete surrounding the reinforcing bar, the

confinement of the concrete due to transverse reinforcement and

the bar geometry. Generally, the bond strength between the

reinforcing bar and matrix increases with increasing steel bar

diameter and compressive strength of GPC (Fig. 4). There is a

greater amount of slip for larger size rebars in GPC.

Sarker [61]found that the bond strength of fly ash-based GPC

increased with the increase of concrete cover-bar diameter ratio

(1.713.62) and the concrete compressive strength (2529 MPa).

He also observed that GPC has higher bond strength than the

OPC concrete because of higher splitting tensile strength and dense

interfacial transition zone between the aggregate and geopolymer

paste. Bond-slip behaviour[45]of GPC showed that the embedded

steel bar of 8 mm dia broke before slipping and concrete cracking

whereas the bar embedded in OPC concrete slipped. For

16 mm bar, GPC failed by matrix cracking while the bars in OPC

concrete were again observed to slip. Sofi et al. [62]reported that

the values of bond strength of steel bars in fly ash-based GPC werecomparable in both beam-end as well as direct pullout specimen

tests. The normalised bond strength increased with a reduction

in rebar size. The bond strength tested according to AS 3600, ACI

318-02 and EC2 recommendations showed that these Codes are

applicable and also safe to predict the developmental length for

GPC.

Attempts were also made to study behaviour of reinforced fly

ash-based GPC beams and columns with respect to longitudinal

tensile reinforcement ratio and concrete compressive strength as

test variables [6366]. Sumajouw et al. [63] reported that the

flexural capacity of beams increased with the increase in tensile

reinforcement (0.642.69%) but the effect of concrete compressive

strength was marginal. The ductility index increased significantly

for beams having longitudinal reinforcement ratio less than 2%.They also studied the strength of reinforced GPC slender columns

with respect to the compressive strength of concrete, longitudinal

reinforcement ratio and load eccentricity. The design provisions

mentioned in the Standards for OPC concrete can be used for

designing geopolymer concrete columns also. Dattatreya et al.

[64]found that the load carrying capacity of reinforced slag-based

GPC beams was 17.7% more than the Portland pozzolana cement

concrete beams at 2.68% tension reinforcement. Yost et al. [65]

indicated that loaddeflection behaviour of GPC beam was identi-

cal to OPC beam. The maximum strain obtained for under-rein-

forced beam was less than 3000 microstrains which is generally

assumed for design work. The predicted neutral axis depth was

15% less than the experimentally achieved value for GPC. The

Whitneys stress block for strength calculation was found applic-able for GPC also. Ng et al. [66]investigated potential use of steel

Fig. 4. Bond strength of fly ash-based geopolymer concrete as a function of steel bardiameter[47].

http://-/?-http://-/?-

7/17/2019 1-s2.0-S0950061815002834-main

7/13

fibres (up to 1.5 wt%) to replace conventional shear reinforcement

in GPC beams of 2250 mm span length. They found that the

increase in fibre volume led to an increase in the cracking load

and the ultimate shear strength. A good correlation of test data

was observed with the predictive fib Model Code 2010.

4.4. Fire behaviour

In general, concrete has good property with respect to fire resis-

tance. However, it is known that the residual strength of OPC con-

crete after firing between 800 C and 1000 C does not exceed 20

30% normally because of dehydration and destruction of C-S-H &

other crystalline hydrates, aggregate types, permeability etc. Fire

introduces high temperature gradient and as a result, the hot layer

tends to separate and spall from the cooler interior layer of the

body [67]. Contrary to this, geopolymers possess good fire resis-

tance at elevated temperature because of the existence of highly

distributed nano-pores in the ceramic like microstructure that

allows physically and chemically bonded water to migrate and

evaporate without damaging the aluminosilicate network [4].

During fire, several events such as evaporation of water adsorbed

by N-A-S-H gel, formation of anhydrous products, crystallizationof stable anhydrous phases and melting (sintering) leading to

destruction generally occurred. The phase transformation of

geopolymers during fire is depicted below.

Kong et al. [68] found that the residual strength of fly ash-based

geopolymer pastes increased by 6% after exposure to 800C,

whereas the strength of metakaolin-based geopolymer pastes was

reduced by 34%. During heating, the high permeability of fly ash-

based geopolymer provides the escape route for moisture in the

matrix, thereby decreasing the damage. The strength increase is

also partly attributed to the sintering reaction of unreacted fly ash

particles. Geopolymer pastes made with metakaolin and potassium

based activator showed an enhanced post-elevated temperature

performance compared to sodium based activator system. The

strength deterioration reduced with increasing Si/Al ratio (>1.5)

[69]. Aggregate size larger than 10 mm resulted in good strength

performance in both ambient and elevated temperature (800 C).

The strength loss in fly ash-based geopolymer concrete at elevated

temperatures is attributed to thermal mismatch between the

geopolymer paste and aggregate [70]. No spalling was reported in

the samples by Zhao and Sanjayan [71] when fly ash-based GPC

with compressive strength ranging from 40 to 100 MPa was

exposed to 850 C. They also found that at the same strength level,

GPC possessed higher spalling resistance under fire than the OPC

concrete due to its increased porosity.

5. Durability studies

One of the major problems associated with OPC concrete is its

long term durability which had always been an issue against

aggressive environments. The deterioration of concrete is usuallyassessed forsulphate attack,chloride induced corrosion, atmospheric

carbonation, alkali-silica reaction and freezethaw attack.In view of

this, several studies are being carried out to understand the beha-

viour of geopolymers exposed to these conditions.

5.1. Alkali-silica reaction

Alkali-silica reaction (ASR) causes gradual but severe deteri-

oration of hardened Portland cement concrete in terms of itsstrength loss, cracking, volume expansion etc. It involves the reac-

tion between the hydroxyl ion in the pore solution within the con-

crete matrix and reactive silica of the aggregate. In general terms,

the reactions will proceed in stages, with the first stage being the

hydrolysis of reactive silica by hydroxyl ions to form alkali-silica

gel and a later secondary overlapping stage being the absorption

of water by the gel, which will result in increase of volume[72].

(i) Acid-based reaction

H0:38SiO2:19 0:38NaOH!Na0:38SiO2:19 0:38H2O 3

(ii) Attack of the siloxane bridges and disintegration of the silica

Na0:38SiO2:19 1:62NaOH!2Na2 H2SiO

24

4

In geopolymer concrete, the un-utilised alkali after geopolymer-

ization of aluminosilicates is expected to react with the silica of the

aggregates causing disruption of their siloxane bridges. It is

reported that geopolymer mortars using aggregates of different

reactivities expanded less than the corresponding Portland cement

mortars[73]. The geopolymer mortars appeared to be sound with-

out any surface cracking. The cause of expansion in slag-based

geopolymer mortars is the formation of sodium calcium silicate

hydrate reaction product with rosette-type morphology [74].

Contrary to this, there was no significant expansion in fly ash-

based geopolymer mortars. The formation of crystalline zeolites

was very slow and since these minerals are usually found in thegaps of the matrix, the existence of stress that might generate

cracking is unlikely [75]. Geopolymer mortar bars made with fly

ash/slag blends expanded less than 0.1% limit prescribed in ASTM

C1260-07 after 16 days (Fig. 5). At 90 days exposure, these mortars

failed to meet the specified criteria. Increasing slag content in fly

ash/slag mix increased the expansion of resulting systems [76].

ASR has also been claimed to be helpful in providing a strong bond

at the paste-aggregate interface, thus enhancing the tensile

strength of GPC[8]. Patil et al.[73]indicated that sandstone, quartz

and limestone aggregates in geopolymer concrete were not prone

Fig. 5. Alkali-silica reaction in various geopolymer and OPC mortars under anaccelerated condition (1 M NaOH) at 80 C[76].

http://-/?-http://-/?-

7/17/2019 1-s2.0-S0950061815002834-main

8/13

to ASR. During accelerated mortar bar test, a slight expansion was

noticed because of re-initiation of the geopolymerization process

of unreacted fly ash particles leading to lower porosity and higher

strength. The lower sensitivity of reactive aggregates in GPC pro-

vides economic advantages in areas where high quality deposits

of aggregates have been depleted.

5.2. Effect of acid attack

The acid resistance of geopolymer pastes/concrete was studied

by several authors[7784]. The extent of degradation depends on

the concentration of acid solution and period of exposure.

Davidovits et al. [8] indicated that metakaolin-based geopolymer

pastes showed only 7% mass loss when sample was immersed in

5% H2SO4 for 30 days. It was also reported that fly ash-based

geopolymer pastes retained a dense microstructure after 3 months

exposure in HNO3. Temuujin et al. [77] concluded that acid and

alkaline resistance of fly ash-based geopolymer strongly depend

on its mineralogical composition. High solubility of Al, Si and Fe

ions was obtained in both strong alkali and acid solutions. The per-

formance of fly ash-based geopolymer pastes when exposed to 5%acetic acid and 5% H2SO4 solutions was superior to ordinary

Portland cement pastes. The deterioration in pastes was connected

to depolymerisation of the aluminosilicate network and formation

of zeolites[78].

5

Wallah and Rangan[41]found that the reduction in compres-

sive strength of fly ash-based GPC in 0.5% H2SO4 solution was

20% after 12 months exposure. This value was52% and65%respectively when samples exposed to 1% and 2% H2SO4 solution.

Pitting and erosion on the surface of the concrete were also

observed. The loss in strength of concrete is mainly due to the

degradation in the geopolymer matrix rather than the aggregate.

They concluded that the acid resistance of GPC was superior to

OPC concrete. Ariffin et al. [79] exposed GPC made with a blend

of pulverized fuel ash and palm oil fuel ash in 2% solution of sul-

phuric acid for 18 months. The weight loss in GPC was 8% whileOPC concrete exhibited 20% weight loss. The strength reduction

in GPC was 35% in 18 months as against 68% strength loss in OPC

concrete after 30 days and was severely deteriorated after

18 months. The C-S-H could have severe deleterious effect on

OPC concrete while N-A-S-H gel appeared to have little effect on

the structure of GPC. Sathia et al. [80]reported the weight loss in

concrete samples was less than 5% after 3 months exposure in 3%

H2SO4 solution. Bakharev et al. [81] found that slag-based GPC

(40 MPa) exhibited33% reduction in strength compared to 47%in OPC concrete when exposed in acetic acid solution (pH 4) for

12 months. The slag particles and low calcium C-S-H with average

Ca/Si ratio of 1 were more stable in the acid solution than the con-

stituents of the OPC pastes. During immersion in 2% H2SO4 solu-

tion, the strength loss was11% compared to 36.2% for OPCconcrete.

5.3. Effect of sulphate attack

Fly ash-based geopolymer pastes did not deteriorate signifi-

cantly, under the influence of water, sodium sulphate (4.4%)

and ASTM sea water [82]. Only some fluctuations in flexural

strength were observed between 7 days and 3 months exposures.

The least strength change was observed in the pastes exposed in

the 5% Na2

SO4

and 5% MgSO4

solutions while most significant

deterioration was observed in the 5% mixed sulphate solution

Fig. 6. Atomic force microscope images of fly ash-based geopolymer exposed under

sulphate after 4 months[86].

http://-/?-http://-/?-

7/17/2019 1-s2.0-S0950061815002834-main

9/13

(Na2SO4+ MgSO4) after 5 months exposure [83]. In fly ash/slag

system, the extensive physical deterioration of pastes was

observed during immersion in MgSO4 solution after 3 months

exposure but not in Na2SO4 solution. The calcium sulphate dihy-

drate formed in paste was identified as being particularly dama-

ging to the materials in MgSO4 [84]. Atomic force microscopic

images of fly ash-based geopolymer pastes exposed to sulphate

environment are shown inFig. 6. In the case of Na2SO4 solution,only exposition of grains was clearly visible while in MgSO4 solu-

tion, both exposition of grains and dissolved aluminosilicate

matrix were observed showing severity of MgSO4 attack [85].

The deterioration is considered mainly due to the destruction of

aluminosilicate skeleton, liberation of silicic acid, leaching of

sodium ion etc. [86]. These reactions seem to have significant

effect on the mechanical strength. The geopolymer prepared with

NaOH activator had the best performance over those made with a

synergistically used sodium silicate and NaOH/KOH activators,

which is attributed to its stable cross-linked aluminosilicate poly-

mer structure.

Several attempts [41,87] have been made to study sulphate

resistance of GPC. The deterioration in concrete was evaluated in

terms of its visual appearance, weight loss and change in compres-

sive strength. Hardjito et al. [41]observed that there was no sig-

nificant effect of 5% Na2SO4 solution in the compressive strength,

the weight loss and the dimension of fly ash-based GPC after

3 months exposure. Rajamane et al. [87] reported sulphate resis-

tance of fly ash-based GPC for 3 months in 5% Na2SO4 and 5%

MgSO4 solutions. The weight loss in samples was 2.4% only.

There was 229% loss of compressive strength as compared to 9

38% in the OPC concrete. The deterioration of OPC concrete can

be attributed to the formation of expansive gypsum and ettringite

which can cause expansion, cracking and spalling in the concrete.

Contrary to this, GPC in general do not contain Ca(OH) 2and mono-

sulphoaluminate in the matrix to cause expansion.

5.4. Carbonation and permeability

Bernal et al.[88]studied slag/metakaolin-based GPC (w/b ratio

0.47) under an accelerated carbonation test using CO2 concentra-

tion of 3.0 0.2% at 20 C for 28 days. They found that the com-

pressive strength decreased monotonically as the carbonation

proceeds. The relationship between the pore volume and extent

of carbonation was much more similar with samples with differ-

ent percentages of metakaolin contrary to the slag-based samples.

This suggested that porosity is not the only parameter controlling

the strength loss of the carbonated binder. There must be a

convoluting effect due to the binder gel chemistry, which deter-

mines the residual level of strength after an accelerated carbona-

tion. Olivia and Nikraz [46] reported lower water permeability

(2.464.67 1011

m/s) of GPC (activator-fly ash ratio, 0.300.40cured at 60 C for 24 h) than the OPC concrete due to its denser

paste and smaller pore inter-connectivity. They also reported that

the water-geopolymer solids ratio was the most influential

parameter that affects the properties of GPC. Bondar et al. [51]

studied the oxygen and chloride permeability of alkali-activated

concrete made with the Iranian natural pozzolan (Taftan andesite

and Shahindej dacite). They concluded that alkali-activated natu-

ral pozzalona concrete has 1035% lower oxygen permeability at

normal curing conditions for 90 days compared with the OPC con-

crete. The rapid chloride permeability test gave high values for

the alkali-activated concrete. This is probably due to the very

high alkali ion concentration in the pore solution promoting

higher electrical conductivity in the GPC. This effect seems to

reduce with age due to a change in the porosity of the GPCmicrostructure.

5.5. Corrosion of steel reinforcement

Corrosion potential is a technique used to detect the state of

reinforcement without disturbing the structures. This is important

because the intensity of corrosion of steel in concrete is generally

known only after the concrete has cracked or disrupted. Various

studies[46,80,89]were reported to estimate the corrosion poten-

tial of steel within the GPC as per ASTM C876. Olivia and Nikraz[46] reported that the half cell potential of GPC was lower than

the specified value of404 mV mentioned in the Standard for sev-ere corrosion after 91 days. Sathia et al. [80] also reported corro-

sion potential up to 300 mV which showed a probablecorrosion indication due to the lower pH of concrete during the

half-cell potential measurement. Accelerated corrosion results

showed that GPC mixes exhibited low level corrosion activity

and time to failure that were 3.865.70 times longer than those

of the OPC concrete. Under impressed voltage, a crack appeared

suddenly in the concrete when time to failure was reached and this

was followed immediately by high current reading. The large

amounts of fly ash and alkaline activators in the GPC mix increased

the availability of ions that can produce high electrical resistance at

high impressed voltage. This enhanced the cathodic reaction and

reduces the rate of corrosion, which in turn, reduces the tensile

stress of the specimens, thus decreasing the risk of cracking and

clearly extending the time to failure [46]. Reddy et al. [89]com-

pared the durability of GPC with that of OPC concrete exposed to

marine environment for a period of 21 days. The initial corrosion

current measured for GPC (7191 mA) was much lower than that

of OPC concrete (772 mA). The OPC specimens initially recorded

decrease in the current but later started increasing while the GPC

current never showed significant increase.

6. Research and development at CSIR-CBRI

A systematic R&D work is initiated at CSIR-Central Building

Research Institute, Roorkee on the development of heat and ambi-ent cured geopolymers using fly ash, slag and other aluminosili-

cates as precursors. In view of variability in the constituents of

fly ash, the property optimisation of geopolymeric pastes was car-

ried out as a function of activator concentration and its dosage,

water-geopolymer solid ratio, curing time and curing temperature

[13]. Geopolymerisation reaction, thermal stability, identification

of bond linkages and microstructural features were analysed by

various techniques such as quasi isothermal DSC, TGA, FTIR and

FESEM. The durability of geopolymer pastes/mortars was also

studied in terms of alkali-silica reaction and also in acidic and sul-

phate environments for 4 months [76,86]. The suitability of these

geopolymer pastes was assessed in making various geopolymeric

products such as mortars & concrete, bricks, solid & hollow blocks,

insulation concrete, foam, sandwich composites and temperatureresistant coatings (Fig. 7(ac)). Attempt was also made to utilise

lime sludge, a waste from paper industry with the geopolymeric

binders for making paving blocks.

Fly ash-based GPC mixes were made with the compressive

strength of 2555 MPa using absolute volume method adopted

for OPC concrete mixes. The strength of GPC increased with

decreasing water-geopolymer solid ratio as it is said analogous to

the water-cement ratio of the OPC concrete. The compressive

strength increased with increasing molarity of the activator (10

16 M) probably due to the formation of stable aluminosilicate net-

works following the dissolution of silica and alumina in the solu-

tion from the fly ash. It was found that the splitting tensile

strength of GPC was more than those of predicted values as per

ACI 318 guideline and other existing empirical equations. A trendline curve between the compressive strength and modulus of


7/17/2019 1-s2.0-S0950061815002834-main

10/13

elasticity showed that the elastic modulus was lower (17%) than

the one predicted by Ivan Diaz-Loya et al. for GPC and also the val-

ues obtained with ACI guidelines. As expected, the bond strength of

steel bar embedded in GPC increased with increasing steel bar

diameter and compressive strength of concrete. It was noted that

the bond strength between geopolymer paste and reinforcing bars

was found to be higher than the OPC concrete[47].

Light weight geopolymer concrete was proportioned with the

help of fly ash, activators, expanded polystyrene beads (EPS up

to 3 wt% or 91 vol%), admixtures and fine & coarse aggregates

(Fig. 7(a)). It was noted that a decrease in the strength was more

when larger size of EPS beads (

7/17/2019 1-s2.0-S0950061815002834-main

11/13

(15 MPa) as specified (13.1 MPa) in ASTM C 90. Regarding fireperformance, the samples were non-ignitable and exhibited Class

I-very low spread of flame as per BS EN-476 part 7 (Fig. 8). Itwas noted that the fire propagation index of the samples was

7/17/2019 1-s2.0-S0950061815002834-main

12/13

[4] Duxon P, Fernandez-Jiminez A, Provis JL, Luckey GC, Palomo A, Van DeventureJSJ. Geopolymer technology: the current state of the art. J Mater Sci2007;42:291733.

[5] Provis JL, VanDeventer JSJ, editors. Geopolymers, structure, processing,properties and application. UK: Woodhead Publishing Limited; 2009.

[6] Li C, Sun H, Li L. A review: the comparison between alkali-activated slag (Si+Ca)and metakaolin (Si+Al) cements. Cem Concr Res 2010;40:13419.

[7] Komnitsas KA. Potential of geopolymer technology towards green buildingsand sustainable cities. In: International conference on green buildings andsustainable cities, procedia engineering, vol. 21; 2011: p.102332.

[8] Davidovits J. Geopolymers: inorganic polymeric new materials. J Therm Anal1991;37:163356.

[9] Davidovits J, Buzzi L, Rocher P, Marini DGC, Tocco S. Geopolymeric cementbased on low cost geologic materials geocistem. In: second internationalconference geopolymer 99, France; 1999.

[10] Van Jaarsveld J, Van Deventure J. Effect of alkali metal activators on theproperties of fly ash-based geopolymer. Ind Eng Chem Res 1999;38:3932 .

[11] Xu H, Van Deventure JSV. Geopolymerisation of multiple minerals. Min. Eng.2002;15:11319.

[12] Zhang B, MacKenzie KJD, Brown IWM. Crystalline phase formation inmetakaolinite geopolymers activated with NaOH and sodium silicate. JMater Sci 2009;44:466876.

[13] Ishwarya G. Development of geopolymer concrete cured at ambienttemperature [M. Tech thesis]. Roorkee, India: CSIR-Central Building ResearchInstitute (AcSIR); 2013.

[14] Silva PD, Crenstil KS, Sirivivatnanon V. Kinetics of geopolymerization: role ofAl2O3 and SiO2. Cem Concr Res 2007;37:5128.

[15] de Vargas AS, DalMolin DCC, Vilela ACV, da Silva FJ, Pavo B, Veit H. The effectsof Na

2O/SiO

2 molar ratio, curing temperature and age on compressive

strength, morphology and microstructure of alkali-activated fly ash-basedgeopolymers. Cem Concr Comp 2011;33:65360.

[16] Yip CK, Luckey GC, Provis JL, van Deventure JSV. The coexistence ofgeopolymeric gel and calcium silicate hydrate at the early stage of alkalineactivation. Cem Concr Res 2005;35:168897.

[17] Somna K, Jaturapitakkul C, Kajitvichyanukul P, Chindaprasirt P. NaOH-activated ground fly ash geopolymer cured at ambient temperature. Fuel2011;90:211824.

[18] Yip CK, Luckey GC, Provis JL, van Deventure JSV. Effect of calcium silicatesources on geopolymerisation. Cem Concr Res 2008;38:55464.

[19] Kumar S, Kumar R, Melhotra SP. Influence of granulated blast furnace slag onthe reaction, structure and properties of fly ash-based geopolymer. J Mater Sci2010;45:60715.

[20] Puligilla S, Mondal P. Role of slag in microstructural development andhardening of fly ash-slag geopolymer. Cem Concr Res 2013;43:7080 .

[21] Chindaprasirt P, Silva PD, Crentsil KS, Hanjitsuwan S. Effect of SiO 2 and Al2O3on the setting and hardening of high calcium fly ash-based geopolymersystems. J Mater Sci 2012;47:487683.

[22] Guo X, Shi H, Dick WA. Compressive strength and microstructuralcharacteristics of class C fly geopolymer. Cem Concr Comp 2010;32:1427 .

[23] Kusbiantoro A, Ibrahim MS, Muthusamy K, Alias A. Development of sucroseand citric acid as natural based admixture for fly ash based geopolymer. ProcEnviron Sci 2013;17:596602.

[24] Nemotallahi B, Sanjayan JG. Effect of different superplasticizer and activatorcombinations on workability and strength of fly ash based geopolymer. MaterDes 2014;57. 667-67.

[25] Jang JG, Lee NK, Lee HK. Fresh and hardened properties of alkali-activatedfly ash/slag pastes with superplasticizers. Constr Build Mater2014;50:16976.

[26] Palomo A, Grutzeck MW, Blanco MT. Alkali-activated fly ashes: a cement forthe future. Cem Concr Res 1999;29:13239.

[27] Perera DS, Uchida O, Vance ER, Finnie KS. Influence of curing schedule on theintegrity of geopolymer. J Mater Sci 2007;42:3099106.

[28] Heah CY, Kamarudin H, Al Bakri AMM, Binhussain M, Luqman M, Nizar IK, et al.Effect of curing profile on Kaolin-based geopolymers. Phys Procedia2011;22:30511.

[29] Rovnanik P. Effect of curing temperature on the development of hard structure

of metakaolin-based geopolymer. Constr Build Mater 2010;24:117683.[30] Kani EN, Allahverdi A. Effects of curing time and temperature on strength

development of inorganic polymeric binder based on natural pozzolan. J MaterSci 2009;44:308897.

[31] Van Jaarsveld JGS, Van Deventer JSJ, Lukey GC. The effect of composition andtemperature on the properties of fly ash and kaolinite based geopolymers.Chem Eng J 2002;89:6373.

[32] Chindaprasirt P, Chareerat T, Sirivivatnanon V. Workability and strength ofcoarse high calcium fly ash geopolymer. Cem Concr Comp 2007;29:2249 .

[33] Chindaprasirt P, Chareerat T, Hatanaka S, Cao T. High strength geopolymerusing fine high-calcium fly ash. J Mater Civ Eng 2011;23:26470.

[34] Temuujin J, van Riessen A, Mackenzie. Preparation and characterisation of flyash-based geopolymer mortars. Constr Build Mater 2010;24:190610.

[35] Khandelwal M, Ranjith PG, Pan Z, Sanjayan JG. Effect of strain rate on strengthproperties of low-calcium fly ash-based geopolymer mortar under drycondition. Arabian J Geosci 2013;6:23839.

[36] Hardjito D, Fung SS. Fly ash-based geopolymer mortar incorporating bottomash. Mode Appl Sci 2010;4:4452.

[37] Sathonsaowaphak A, Chindaprasirt P, Pimraksa K. Workability and strength oflignite bottom ash geopolymer mortar. J Hazard Mater 2009;168:4450.

[38] BroughAR, Atkinson A. Sodium silicate-based alkali-activated slag mortars PartI. Strength, hydration and microstructure. Cem Concr Res 2002;32:86579.

[39] Yang KH, Song JK, Lee KS, Ashour AF. Flow and compressive strength of alkali-activated mortars. ACI Mater J 2009;106:508.

[40] Yang KH, Song JK. Workability loss and compressive strength development ofcementless mortars activated by combination of sodium silicate and sodiumhydroxide. J Mater Civ Eng ASCE 2009;21:11927.

[41] Hardjito D, Wallah SE, Sumajouw DMJ, Rangan BV. On the development of flyash-based geopolymer concrete. ACI Mater J 2004;101:46772.

[42] Laskar AI, Bhattacharjee R. Rheology of fly ash-based geopolymer concrete. ACI

Mater J 2011;108:53642.[43] Montes C, Allouche EN. Rheological behaviour of fly ash-based geopolymers.

STP 1566 on Geopolymer Binder Systems 2013, ASTM: p. 7284.[44] Palacios M, Banfill Phillip FG, Puertas F. Rheology and setting of alkali-

activated slag pastes and mortars: effect of organic admixture. ACI Mater J2008;105:1408.

[45] Fernandez-Jiminez AM, Palomo A, Lopez-Hombrados C. Engineering propertiesof alkali-activated fly ash concrete. ACI Mater J 2006;103:10612.

[46] Olivia M, Nikraz H. Properties of fly ash geopolymer concrete designed bytaguchi method. Mater Des 2012;36:1918.

[47] Singh A. Engineering properties of reinforced geopolymer concrete [B. Techproject report]. Roorkee, India. CSIR-Central Building Research Institute; 2012.

[48] Sofi M, Van Deventer JSJ, Mendis PA, Lukey GC. Engineering properties ofinorganic polymer concretes. Cem Concr Res 2007;37:2517.

[49] Diaz-Loya EI, Allouche EN, Vaidya S. Mechanical properties of fly ash-basedgeopolymer concrete. ACI Mater J 2011;108:3006.

[50] Ryu GS, Lee YB, Koh KT, Chung YS. The mechanical properties of fly ash-basedgeopolymer concrete with alkaline activators. Constr Build Mater2013;47:40918.

[51] Bondar D, Lynsdale CY, Milestone NB, Hassani N, Ramezanianpour AA.Engineering properties of alkali-activated natural pozzolan concrete. ACIMater J 2011;108:6472.

[52] Yost JR, Radlinska A, Ernst S, Salera M. Structural behavior of alkali activatedfly ash concrete. Part 1: mixture design, material properties and samplefabrication. Mater Struct 2012;46:43547.

[53] Pan Z, Sanjayan JG, Rangan BV. Fracture properties of geopolymer paste andconcrete. Mag Concr Res 2011;63:76377.

[54] Sarker PK, Haque R, Ramgolam KV. Fracture properties of heat cured fly ash-based geopolymer concrete. Mater Des 2013;44:5806.

[55] Mehta PK, Monteiro PJM. Concrete: microstructure, properties andmaterials. USA: The McGraw Hill companies, Inc.; 2006.

[56] Lee WKW, Van Deventure JSJ. The interface between natural siliceousaggregates and geopolymers. Cem Concr Res 2004;34:195206.

[57] Lee WKW, Van Deventure JSJ. Chemical interactions between siliceousaggregates and low-Ca alkali-activated cements. Cem Concr Res2007;37:84455.

[58] Zhang YS, Sun W, Li JZ. Hydration process of interfacial transition in potassium

polysialate (K-PSDS) geopolymer concrete. Mag Concr Res 2005;57:338.[59] Demie S, Nuruddin MF, Shafiq N. Effects of micro-structure characteristics of

interfacial transition zone on the compressive strength of self-compactinggeopolymer concrete. Constr Build Mater 2013;41:918.

[60] ACI 408R-03. Bond and development of straight reinforcing bars in tension.[61] Sarker PK. Bond strength of reinforcing steel embedded in fly ash-based

geopolymer concrete. Mater Struct 2011;44:102130.[62] Sofi M, Van Deventer JSJ, Mendis PA, Lukey GC. Bond performance of

reinforcing bars in inorganic polymer concrete (IPCs). J Mater Sci2007;42:310716.

[63] Sumajouw DMJ, Hardjito D, Wallah SE, Rangan BV. Fly ash-based geopolymerconcrete: study of slender reinforced columns. J Mater Sci 2007;42:312430 .

[64] Dattatreya JK, Rajamane NP, Ambily PS. Structural behaviour of reinforcedgeopolymer concrete beams and columns. CSIR-SERC Research Report RR-6;2009.

[65] Yost JR, Radlinska A, Ernst S, Salera M. Structural behaviour of alkali activatedfly ash concrete. Part 2: structural testing and experimental findings. MaterStruct 2012;46:44962.

[66] Ng TS, Amin A, Foster SJ. The behaviour of steel-fibre-reinforced geopolymer

concrete beams in shear. Mag Concr Res 2013;65:30818.[67] Neville AM. Properties of concrete. 4th ed. India: Dorling Kindersley

Publishing, Inc.; 1997.[68] Kong DLK, Sanjayan JG, Crentsil KS. Comparative performance of geopolymers

made with metakaolin and fly ash after exposure to elevated temperature.Cem Concr Res 2007;37:15839.

[69] Kong DLK, Sanjayan JG, Crentsil KS. Factors affecting the performance ofmetakaolin geopolymers exposed to elevated temperatures. J Mater Sci2008;43:82431.

[70] Kong DLY, Sanjayan JG. Effect of elevated temperatures on geopolymer paste,mortar and concrete. Cem Concr Res 2010;40:3349.

[71] Zhao R, Sanjayan JG. Geopolymer and Portland cement concretes in simulatedfire. Mag Concr Res 2011;63:16373.

[72] Swamy RN, editor. The alkali-silica reaction in concrete. UK: Blackie and SonsLimited; 1992.

[73] Patil KK, Allouche EN. Impact of alkali silica reaction on fly ash-basedgeopolymer concrete. J Mater Civ Eng ASCE 2013;25:1319.

[74] Fernandez-Jimenez A, Puertas F. The alkali-silica reaction in alkali-activated

granulated slag mortars with reactive aggregate. Cem Concr Res2002;32:101924.

http://refhub.elsevier.com/S0950-0618(15)00283-4/h0020http://refhub.elsevier.com/S0950-0618(15)00283-4/h0020http://refhub.elsevier.com/S0950-0618(15)00283-4/h0020http://refhub.elsevier.com/S0950-0618(15)00283-4/h0025http://refhub.elsevier.com/S0950-0618(15)00283-4/h0025http://refhub.elsevier.com/S0950-0618(15)00283-4/h0025http://refhub.elsevier.com/S0950-0618(15)00283-4/h0030http://refhub.elsevier.com/S0950-0618(15)00283-4/h0030http://refhub.elsevier.com/S0950-0618(15)00283-4/h0030http://refhub.elsevier.com/S0950-0618(15)00283-4/h0040http://refhub.elsevier.com/S0950-0618(15)00283-4/h0040http://refhub.elsevier.com/S0950-0618(15)00283-4/h0050http://refhub.elsevier.com/S0950-0618(15)00283-4/h0050http://refhub.elsevier.com/S0950-0618(15)00283-4/h0050http://refhub.elsevier.com/S0950-0618(15)00283-4/h0055http://refhub.elsevier.com/S0950-0618(15)00283-4/h0055http://refhub.elsevier.com/S0950-0618(15)00283-4/h0060http://refhub.elsevier.com/S0950-0618(15)00283-4/h0060http://refhub.elsevier.com/S0950-0618(15)00283-4/h0060http://refhub.elsevier.com/S0950-0618(15)00283-4/h0060http://refhub.elsevier.com/S0950-0618(15)00283-4/h0070http://refhub.elsevier.com/S0950-0618(15)00283-4/h0070http://refhub.elsevier.com/S0950-0618(15)00283-4/h0070http://refhub.elsevier.com/S0950-0618(15)00283-4/h0070http://refhub.elsevier.com/S0950-0618(15)00283-4/h0070http://refhub.elsevier.com/S0950-0618(15)00283-4/h0070http://refhub.elsevier.com/S0950-0618(15)00283-4/h0070http://refhub.elsevier.com/S0950-0618(15)00283-4/h0070http://refhub.elsevier.com/S0950-0618(15)00283-4/h0075http://refhub.elsevier.com/S0950-0618(15)00283-4/h0075http://refhub.elsevier.com/S0950-0618(15)00283-4/h0075http://refhub.elsevier.com/S0950-0618(15)00283-4/h0075http://refhub.elsevier.com/S0950-0618(15)00283-4/h0075http://refhub.elsevier.com/S0950-0618(15)00283-4/h0075http://refhub.elsevier.com/S0950-0618(15)00283-4/h0075http://refhub.elsevier.com/S0950-0618(15)00283-4/h0075http://refhub.elsevier.com/S0950-0618(15)00283-4/h0080http://refhub.elsevier.com/S0950-0618(15)00283-4/h0080http://refhub.elsevier.com/S0950-0618(15)00283-4/h0080http://refhub.elsevier.com/S0950-0618(15)00283-4/h0085http://refhub.elsevier.com/S0950-0618(15)00283-4/h0085http://refhub.elsevier.com/S0950-0618(15)00283-4/h0085http://refhub.elsevier.com/S0950-0618(15)00283-4/h0090http://refhub.elsevier.com/S0950-0618(15)00283-4/h0090http://refhub.elsevier.com/S0950-0618(15)00283-4/h0095http://refhub.elsevier.com/S0950-0618(15)00283-4/h0095http://refhub.elsevier.com/S0950-0618(15)00283-4/h0095http://refhub.elsevier.com/S0950-0618(15)00283-4/h0100http://refhub.elsevier.com/S0950-0618(15)00283-4/h0100http://refhub.elsevier.com/S0950-0618(15)00283-4/h0100http://refhub.elsevier.com/S0950-0618(15)00283-4/h0105http://refhub.elsevier.com/S0950-0618(15)00283-4/h0105http://refhub.elsevier.com/S0950-0618(15)00283-4/h0105http://refhub.elsevier.com/S0950-0618(15)00283-4/h0105http://refhub.elsevier.com/S0950-0618(15)00283-4/h0105http://refhub.elsevier.com/S0950-0618(15)00283-4/h0105http://refhub.elsevier.com/S0950-0618(15)00283-4/h0105http://refhub.elsevier.com/S0950-0618(15)00283-4/h0105http://refhub.elsevier.com/S0950-0618(15)00283-4/h0110http://refhub.elsevier.com/S0950-0618(15)00283-4/h0110http://refhub.elsevier.com/S0950-0618(15)00283-4/h0110http://refhub.elsevier.com/S0950-0618(15)00283-4/h0115http://refhub.elsevier.com/S0950-0618(15)00283-4/h0115http://refhub.elsevier.com/S0950-0618(15)00283-4/h0115http://refhub.elsevier.com/S0950-0618(15)00283-4/h0120http://refhub.elsevier.com/S0950-0618(15)00283-4/h0120http://refhub.elsevier.com/S0950-0618(15)00283-4/h0120http://refhub.elsevier.com/S0950-0618(15)00283-4/h0125http://refhub.elsevier.com/S0950-0618(15)00283-4/h0125http://refhub.elsevier.com/S0950-0618(15)00283-4/h0125http://refhub.elsevier.com/S0950-0618(15)00283-4/h0130http://refhub.elsevier.com/S0950-0618(15)00283-4/h0130http://refhub.elsevier.com/S0950-0618(15)00283-4/h0135http://refhub.elsevier.com/S0950-0618(15)00283-4/h0135http://refhub.elsevier.com/S0950-0618(15)00283-4/h0140http://refhub.elsevier.com/S0950-0618(15)00283-4/h0140http://refhub.elsevier.com/S0950-0618(15)00283-4/h0140http://refhub.elsevier.com/S0950-0618(15)00283-4/h0145http://refhub.elsevier.com/S0950-0618(15)00283-4/h0145http://refhub.elsevier.com/S0950-0618(15)00283-4/h0150http://refhub.elsevier.com/S0950-0618(15)00283-4/h0150http://refhub.elsevier.com/S0950-0618(15)00283-4/h0150http://refhub.elsevier.com/S0950-0618(15)00283-4/h0155http://refhub.elsevier.com/S0950-0618(15)00283-4/h0155http://refhub.elsevier.com/S0950-0618(15)00283-4/h0155http://refhub.elsevier.com/S0950-0618(15)00283-4/h0160http://refhub.elsevier.com/S0950-0618(15)00283-4/h0160http://refhub.elsevier.com/S0950-0618(15)00283-4/h0165http://refhub.elsevier.com/S0950-0618(15)00283-4/h0165http://refhub.elsevier.com/S0950-0618(15)00283-4/h0170http://refhub.elsevier.com/S0950-0618(15)00283-4/h0170http://refhub.elsevier.com/S0950-0618(15)00283-4/h0170http://refhub.elsevier.com/S0950-0618(15)00283-4/h0175http://refhub.elsevier.com/S0950-0618(15)00283-4/h0175http://refhub.elsevier.com/S0950-0618(15)00283-4/h0175http://refhub.elsevier.com/S0950-0618(15)00283-4/h0180http://refhub.elsevier.com/S0950-0618(15)00283-4/h0180http://refhub.elsevier.com/S0950-0618(15)00283-4/h0185http://refhub.elsevier.com/S0950-0618(15)00283-4/h0185http://refhub.elsevier.com/S0950-0618(15)00283-4/h0185http://refhub.elsevier.com/S0950-0618(15)00283-4/h0190http://refhub.elsevier.com/S0950-0618(15)00283-4/h0190http://refhub.elsevier.com/S0950-0618(15)00283-4/h0195http://refhub.elsevier.com/S0950-0618(15)00283-4/h0195http://refhub.elsevier.com/S0950-0618(15)00283-4/h0200http://refhub.elsevier.com/S0950-0618(15)00283-4/h0200http://refhub.elsevier.com/S0950-0618(15)00283-4/h0200http://refhub.elsevier.com/S0950-0618(15)00283-4/h0205http://refhub.elsevier.com/S0950-0618(15)00283-4/h0205http://refhub.elsevier.com/S0950-0618(15)00283-4/h0205http://refhub.elsevier.com/S0950-0618(15)00283-4/h0210http://refhub.elsevier.com/S0950-0618(15)00283-4/h0210http://refhub.elsevier.com/S0950-0618(15)00283-4/h0220http://refhub.elsevier.com/S0950-0618(15)00283-4/h0220http://refhub.elsevier.com/S0950-0618(15)00283-4/h0220http://refhub.elsevier.com/S0950-0618(15)00283-4/h0225http://refhub.elsevier.com/S0950-0618(15)00283-4/h0225http://refhub.elsevier.com/S0950-0618(15)00283-4/h0230http://refhub.elsevier.com/S0950-0618(15)00283-4/h0230http://refhub.elsevier.com/S0950-0618(15)00283-4/h0240http://refhub.elsevier.com/S0950-0618(15)00283-4/h0240http://refhub.elsevier.com/S0950-0618(15)00283-4/h0245http://refhub.elsevier.com/S0950-0618(15)00283-4/h0245http://refhub.elsevier.com/S0950-0618(15)00283-4/h0250http://refhub.elsevier.com/S0950-0618(15)00283-4/h0250http://refhub.elsevier.com/S0950-0618(15)00283-4/h0250http://refhub.elsevier.com/S0950-0618(15)00283-4/h0255http://refhub.elsevier.com/S0950-0618(15)00283-4/h0255http://refhub.elsevier.com/S0950-0618(15)00283-4/h0255http://refhub.elsevier.com/S0950-0618(15)00283-4/h0260http://refhub.elsevier.com/S0950-0618(15)00283-4/h0260http://refhub.elsevier.com/S0950-0618(15)00283-4/h0260http://refhub.elsevier.com/S0950-0618(15)00283-4/h0265http://refhub.elsevier.com/S0950-0618(15)00283-4/h0265http://refhub.elsevier.com/S0950-0618(15)00283-4/h0270http://refhub.elsevier.com/S0950-0618(15)00283-4/h0270http://refhub.elsevier.com/S0950-0618(15)00283-4/h0270http://refhub.elsevier.com/S0950-0618(15)00283-4/h0275http://refhub.elsevier.com/S0950-0618(15)00283-4/h0275http://refhub.elsevier.com/S0950-0618(15)00283-4/h0275http://refhub.elsevier.com/S0950-0618(15)00283-4/h0280http://refhub.elsevier.com/S0950-0618(15)00283-4/h0280http://refhub.elsevier.com/S0950-0618(15)00283-4/h0280http://refhub.elsevier.com/S0950-0618(15)00283-4/h0285http://refhub.elsevier.com/S0950-0618(15)00283-4/h0285http://refhub.elsevier.com/S0950-0618(15)00283-4/h0285http://refhub.elsevier.com/S0950-0618(15)00283-4/h0290http://refhub.elsevier.com/S0950-0618(15)00283-4/h0290http://refhub.elsevier.com/S0950-0618(15)00283-4/h0295http://refhub.elsevier.com/S0950-0618(15)00283-4/h0295http://refhub.elsevier.com/S0950-0618(15)00283-4/h0295http://refhub.elsevier.com/S0950-0618(15)00283-4/h0305http://refhub.elsevier.com/S0950-0618(15)00283-4/h0305http://refhub.elsevier.com/S0950-0618(15)00283-4/h0305http://refhub.elsevier.com/S0950-0618(15)00283-4/h0310http://refhub.elsevier.com/S0950-0618(15)00283-4/h0310http://refhub.elsevier.com/S0950-0618(15)00283-4/h0310http://refhub.elsevier.com/S0950-0618(15)00283-4/h0310http://refhub.elsevier.com/S0950-0618(15)00283-4/h0315http://refhub.elsevier.com/S0950-0618(15)00283-4/h0315http://refhub.elsevier.com/S0950-0618(15)00283-4/h0325http://refhub.elsevier.com/S0950-0618(15)00283-4/h0325http://refhub.elsevier.com/S0950-0618(15)00283-4/h0325http://refhub.elsevier.com/S0950-0618(15)00283-4/h0330http://refhub.elsevier.com/S0950-0618(15)00283-4/h0330http://refhub.elsevier.com/S0950-0618(15)00283-4/h0335http://refhub.elsevier.com/S0950-0618(15)00283-4/h0335http://refhub.elsevier.com/S0950-0618(15)00283-4/h0340http://refhub.elsevier.com/S0950-0618(15)00283-4/h0340http://refhub.elsevier.com/S0950-0618(15)00283-4/h0340http://refhub.elsevier.com/S0950-0618(15)00283-4/h0345http://refhub.elsevier.com/S0950-0618(15)00283-4/h0345http://refhub.elsevier.com/S0950-0618(15)00283-4/h0345http://refhub.elsevier.com/S0950-0618(15)00283-4/h0350http://refhub.elsevier.com/S0950-0618(15)00283-4/h0350http://refhub.elsevier.com/S0950-0618(15)00283-4/h0355http://refhub.elsevier.com/S0950-0618(15)00283-4/h0355http://refhub.elsevier.com/S0950-0618(15)00283-4/h0360http://refhub.elsevier.com/S0950-0618(15)00283-4/h0360http://refhub.elsevier.com/S0950-0618(15)00283-4/h0365http://refhub.elsevier.com/S0950-0618(15)00283-4/h0365http://refhub.elsevier.com/S0950-0618(15)00283-4/h0370http://refhub.elsevier.com/S0950-0618(15)00283-4/h0370http://refhub.elsevier.com/S0950-0618(15)00283-4/h0370http://refhub.elsevier.com/S0950-0618(15)00283-4/h0370http://refhub.elsevier.com/S0950-0618(15)00283-4/h0370http://refhub.elsevier.com/S0950-0618(15)00283-4/h0370http://refhub.elsevier.com/S0950-0618(15)00283-4/h0370http://refhub.elsevier.com/S0950-0618(15)00283-4/h0365http://refhub.elsevier.com/S0950-0618(15)00283-4/h0365http://refhub.elsevier.com/S0950-0618(15)00283-4/h0360http://refhub.elsevier.com/S0950-0618(15)00283-4/h0360http://refhub.elsevier.com/S0950-0618(15)00283-4/h0355http://refhub.elsevier.com/S0950-0618(15)00283-4/h0355http://refhub.elsevier.com/S0950-0618(15)00283-4/h0350http://refhub.elsevier.com/S0950-0618(15)00283-4/h0350http://refhub.elsevier.com/S0950-0618(15)00283-4/h0345http://refhub.elsevier.com/S0950-0618(15)00283-4/h0345http://refhub.elsevier.com/S0950-0618(15)00283-4/h0345http://refhub.elsevier.com/S0950-0618(15)00283-4/h0340http://refhub.elsevier.com/S0950-0618(15)00283-4/h0340http://refhub.elsevier.com/S0950-0618(15)00283-4/h0340http://refhub.elsevier.com/S0950-0618(15)00283-4/h0335http://refhub.elsevier.com/S0950-0618(15)00283-4/h0335http://refhub.elsevier.com/S0950-0618(15)00283-4/h0330http://refhub.elsevier.com/S0950-0618(15)00283-4/h0330http://refhub.elsevier.com/S0950-0618(15)00283-4/h0325http://refhub.elsevier.com/S0950-0618(15)00283-4/h0325http://refhub.elsevier.com/S0950-0618(15)00283-4/h0325http://refhub.elsevier.com/S0950-0618(15)00283-4/h0315http://refhub.elsevier.com/S0950-0618(15)00283-4/h0315http://refhub.elsevier.com/S0950-0618(15)00283-4/h0310http://refhub.elsevier.com/S0950-0618(15)00283-4/h0310http://refhub.elsevier.com/S0950-0618(15)00283-4/h0310http://refhub.elsevier.com/S0950-0618(15)00283-4/h0305http://refhub.elsevier.com/S0950-0618(15)00283-4/h0305http://refhub.elsevier.com/S0950-0618(15)00283-4/h0295http://refhub.elsevier.com/S0950-0618(15)00283-4/h0295http://refhub.elsevier.com/S0950-0618(15)00283-4/h0295http://refhub.elsevier.com/S0950-0618(15)00283-4/h0290http://refhub.elsevier.com/S0950-0618(15)00283-4/h0290http://refhub.elsevier.com/S0950-0618(15)00283-4/h0285http://refhub.elsevier.com/S0950-0618(15)00283-4/h0285http://refhub.elsevier.com/S0950-0618(15)00283-4/h0285http://refhub.elsevier.com/S0950-0618(15)00283-4/h0280http://refhub.elsevier.com/S0950-0618(15)00283-4/h0280http://refhub.elsevier.com/S0950-0618(15)00283-4/h0275http://refhub.elsevier.com/S0950-0618(15)00283-4/h0275http://refhub.elsevier.com/S0950-0618(15)00283-4/h0270http://refhub.elsevier.com/S0950-0618(15)00283-4/h0270http://refhub.elsevier.com/S0950-0618(15)00283-4/h0265http://refhub.elsevier.com/S0950-0618(15)00283-4/h0265http://refhub.elsevier.com/S0950-0618(15)00283-4/h0260http://refhub.elsevier.com/S0950-0618(15)00283-4/h0260http://refhub.elsevier.com/S0950-0618(15)00283-4/h0260http://refhub.elsevier.com/S0950-0618(15)00283-4/h0255http://refhub.elsevier.com/S0950-0618(15)00283-4/h0255http://refhub.elsevier.com/S0950-0618(15)00283-4/h0255http://refhub.elsevier.com/S0950-0618(15)00283-4/h0250http://refhub.elsevier.com/S0950-0618(15)00283-4/h0250http://refhub.elsevier.com/S0950-0618(15)00283-4/h0250http://refhub.elsevier.com/S0950-0618(15)00283-4/h0245http://refhub.elsevier.com/S0950-0618(15)00283-4/h0245http://refhub.elsevier.com/S0950-0618(15)00283-4/h0240http://refhub.elsevier.com/S0950-0618(15)00283-4/h0240http://refhub.elsevier.com/S0950-0618(15)00283-4/h0230http://refhub.elsevier.com/S0950-0618(15)00283-4/h0230http://refhub.elsevier.com/S0950-0618(15)00283-4/h0225http://refhub.elsevier.com/S0950-0618(15)00283-4/h0225http://refhub.elsevier.com/S0950-0618(15)00283-4/h0220http://refhub.elsevier.com/S0950-0618(15)00283-4/h0220http://refhub.elsevier.com/S0950-0618(15)00283-4/h0220http://refhub.elsevier.com/S0950-0618(15)00283-4/h0210http://refhub.elsevier.com/S0950-0618(15)00283-4/h0210http://refhub.elsevier.com/S0950-0618(15)00283-4/h0205http://refhub.elsevier.com/S0950-0618(15)00283-4/h0205http://refhub.elsevier.com/S0950-0618(15)00283-4/h0200http://refhub.elsevier.com/S0950-0618(15)00283-4/h0200http://refhub.elsevier.com/S0950-0618(15)00283-4/h0200http://refhub.elsevier.com/S0950-0618(15)00283-4/h0195http://refhub.elsevier.com/S0950-0618(15)00283-4/h0195http://refhub.elsevier.com/S0950-0618(15)00283-4/h0190http://refhub.elsevier.com/S0950-0618(15)00283-4/h0190http://refhub.elsevier.com/S0950-0618(15)00283-4/h0185http://refhub.elsevier.com/S0950-0618(15)00283-4/h0185http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0180http://refhub.elsevier.com/S0950-0618(15)00283-4/h0180http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0175http://refhub.elsevier.com/S0950-0618(15)00283-4/h0175http://refhub.elsevier.com/S0950-0618(15)00283-4/h0175http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0170http://refhub.elsevier.com/S0950-0618(15)00283-4/h0170http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0165http://refhub.elsevier.com/S0950-0618(15)00283-4/h0165http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0160http://refhub.elsevier.com/S0950-0618(15)00283-4/h0160http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0155http://refhub.elsevier.com/S0950-0618(15)00283-4/h0155http://refhub.elsevier.com/S0950-0618(15)00283-4/h0155http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0150http://refhub.elsevier.com/S0950-0618(15)00283-4/h0150http://refhub.elsevier.com/S0950-0618(15)00283-4/h0150http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0145http://refhub.elsevier.com/S0950-0618(15)00283-4/h0145http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0140http://refhub.elsevier.com/S0950-0618(15)00283-4/h0140http://refhub.elsevier.com/S0950-0618(15)00283-4/h0140http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0135http://refhub.elsevier.com/S0950-0618(15)00283-4/h0135http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0130http://refhub.elsevier.com/S0950-0618(15)00283-4/h0130http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0125http://refhub.elsevier.com/S0950-0618(15)00283-4/h0125http://refhub.elsevier.com/S0950-0618(15)00283-4/h0125http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0120http://refhub.elsevier.com/S0950-0618(15)00283-4/h0120http://refhub.elsevier.com/S0950-0618(15)00283-4/h0120http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0115http://refhub.elsevier.com/S0950-0618(15)00283-4/h0115http://refhub.elsevier.com/S0950-0618(15)00283-4/h0115http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0110http://refhub.elsevier.com/S0950-0618(15)00283-4/h0110http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0105http://refhub.elsevier.com/S0950-0618(15)00283-4/h0105http://refhub.elsevier.com/S0950-0618(15)00283-4/h0105http://refhub.elsevier.com/S0950-0618(15)00283-4/h0105http://refhub.elsevier.com/S0950-0618(15)00283-4/h0105http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0100http://refhub.elsevier.com/S0950-0618(15)00283-4/h0100http://-/?-http://refhub.elsevier.com/S0950-0618(15)00283-4/h0095http://refhub.elsevier.com/S0950-0618(15)00283-4/h0095http://refhub.elsevier.com/S0950-0618(15)00283-4/h0095http://-/?-http://refhub

Documents

1-s2.0-S0950061815002834-main