Hydration Kinetics of a Low Carbon Cementitious MaterialProduced by Physico-Chemical Activation of High Calcium Fly AshMonower Sadique, Hassan Al-NageimJournal of Advanced Concrete Technology, volume ( ), pp.10 2012 254-263

Early Age Stress Development, Relaxation, and Cracking in Restrained Low W/B Ultrafine Fly Ash MortarsAkhter B Hossain, Anushka Fonseka Herb Bullock,Journal of Advanced Concrete Technology, volume ( ), pp.6 2008 261-271

Experimental Investigation on Reaction Rate and Self-healing Ability in Fly Ash Blended Cement MixturesSeung Hyun Na, Yukio Hama Madoka Taniguchi, , Takahiro Sagawa Mohamed Zakaria,Journal of Advanced Concrete Technology, volume ( ), pp.10 2012 240-253

Effects of PFA and GGBS on Early-Ages Engineering Properties of Portland Cement SystemsXiangming Zhou , Joel R.Slater Stuart E. Wavell, , Olayinka OladiranJournal of Advanced Concrete Technology, volume ( ), pp.10 2012 74-85

Journal of Advanced Concrete Technology Vol. 10, 254-263, July 2012 / Copyright © 2012 Japan Concrete Institute  254 

 

Scientific paper

Hydration Kinetics of a Low Carbon Cementitious Material Produced by Physico-Chemical Activation of High Calcium Fly Ash Monower Sadique1 and Hassan Al-Nageim2

Received 15 September 2011, accepted 24 July 2012 doi:10.3151/jact.10.254

Abstract The hydration kinetics of a ternary system containing high lime fly ash activated with alkali sulphate rich fly ash has been studied. The effect of low intensive mechanical activation of fly ashes influencing the particle packing and water demand was undertaken. A ternary blend of fly ashes and silica fume was found to show improved bond strength be-tween the binder paste and aggregate rather than the binder paste matrix with progressive hydration after 90 days. Analogous physico-chemical properties of the developed ternary blend with the control cement were revealed. Quick dissolution of K+, Na+ and SO4

2- into the liquid phase as well as consumption of Ca2+ by silica fume in a high-pH envi-ronment were the predominant factors for producing a dense non-expansive hydration product by the ternary blend.

1. Introduction

The production of cement requires extensive quarrying and energy intensive procedures creating environmental impacts at all stages. In 2010, 3.8 million tonnes of CO2 were released by the UK cement industry (Department for energy and climate change 2012). The World Busi-ness Council for Sustainable Development refers to the commitment to social responsibility, environmental stewardship and economic prosperity in addition to fi-nancial performance as the ‘three bottom line’ elements for sustainable development (WBCSD 2002). In order to achieve a sustainable construction industry, in 2007, the UK government set out aims as part of a Sustainable Procurement Action Plan for the entire public sector. This action plan aims to reduce total UK CO2 emissions by at least 26% by 2020 and 60% by 2050 from 1990 levels. In the cement industry, the reduction in energy consumption and CO2 emissions was partly achieved by i) blending or inter-grinding cement at the burning stage with natural pozzolan, steel slag, and limestone and ii) by clinker substitution (WBCSD 2009). This substitu-tion is commonly expressed by the clinker to cement ratio (clinker/cement).

The British standard (BS-EN197 2007) describes 26 types of blended cement (with allowable composition) containing blast furnace slag (BFS), silica fume (SF), pulverised fuel ash (PFA), burnt shale (BS) and lime-stone as supplementary cementitious material (SCM). The SCM with lower reactivity in most cases were syn-thesised using conventional cement and as a result bulk

replacement of the cement clinker was hindered. Activa-tion is prerequisite for effective exploitation of a SCM. Three well-known basic activation techniques for en-hanced reactivity of a SCM are chemical activation, mechanical activation and physico-chemical activation. The activation technique for pozzolanic materials for accelerated hydration means offering less reactive mate-rials an improved capability to interact with other mate-rials in the presence of water, resulting in the production of reaction products that would otherwise form very slowly or not at all. Through the process of activation, new cementitious materials with similar cementing fea-tures can be achieved.

Physico-chemical activation of pulverised fuel ashes (PFA) using grinding and various alkalis has been stud-ied extensively (Juhasz 1998; Arjunan et al. 2001; Wang 2003; Qian et al. 2001; Blanco et al. 2006). Simultane-ous application of mechanical and chemical activation of PFA was reported to produce strength above that of reference mortar at 20% replacement level (Blanco et al. 2006) and the improvements were also greater when compared with the 20% replacement level of silica fume (SF). Extensive research has been conducted with bi-nary and ternary blends intended to induce physico-chemical activation using different additions with ce-ment (Tan and Pu 1998; Osborne 1999; Yazici 2008; O'Rourke et al. 2009; Winnefeld et al. 2010).

The present study presents an investigation for phys-ico-chemical activation of two types of fly ashes by grinding and blending in a cement free environment. The hydration kinetics of a ternary system of fly ashes, synthesised with silica fume leading to development of a new cementitious material was investigated.

2. Materials

2.1 Fly ashes Two types of fly ashes, FA1 and FA2, originating from two different industrial sources were collected and sub-

1PhD Researcher & Chartered Engineer, School of the Built Environment, Liverpool John Moores University, UK. E-mail: sadiquerhd@hotmail.com 2Professor of Structural Engineering, School of Built Environment, Liverpool John Moores University, UK.

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jected to mechanical activation at the laboratory scale. The source materials of said fly ashes were different and resulting from combustion between 700°C to 1200°C in power generation plants using fluidized bed combustion (FBC) systems. Fluidized beds use calcium hydroxide and activated carbon to capture sulphur and nitric oxide (NOx) released during combustion. The solid residue formed in the FBC processes may be considered to be a blend of the inorganic fraction of the employed fuel, the compound of sulphur produced in the desulfurization reaction, and the excess sorbent. Due to variation in the quality of the employed fuel, co-combustion with some biomasses, and the quantity and quality of the sorbent employed in FBC, the physical and chemical properties of the fly ashes varied widely from those of conven-tional fly ash. Moreover, the lower calcination tempera-ture also ensures more reactive fly ashes compared to conventional coal fly ashes, which burn at around 1600°C (Brandštetr et al. 1996).

Major crystal peaks identified in XRD were lime (CaOH)2, calcite (CaCO3), mayenite (Ca12Al14O33), merwinite (Ca3Mg[SiO4]), belite (2CaOSiO2) and

gehlenite (Ca2Al[Al,SiO7]), as shown in Fig. 1. The powder XRD of FA2 revealed mineralogy composed of crystalline calcite (CaCO3), arcanite (K2SO4), belite (2CaOSiO2), sylvite (KCl or NaCl-KCl) and orthoclase (KAlSi3O8), as shown in Fig. 2. It is expected that the content of arcanite in FA2 will provide an ambient envi-ronment for breaking the glass phase of fly ash particles (Poon et al. 2003) as well as playing a catalytic role during the early stage of hydration (Rodrigues et al., 1999). The chemical compositions of FA1 and FA2 are shown in Table 1.

2.2 Silica fume (SF) Commercially available silica fume containing more than 98% amorphous silica was used in this study. 2.3 Control cement (CEM-II) For analysis and comparison with the new blend, com-mercially available Portland composite cement type CEM-II/A/LL 42.5-N containing between approxi-mately 6% to 20% limestone has been used in this study.

C

G

M

B

L

CB

MrC

L

0

500

1000

1500

2000

2500

5 10 15 20 25 30 35 40 45 50 55 60

Intensitycps

Diffracti on angle 2 theta  Fig. 1 Diffractographs of untreated FA1 (lime-L, calcite-C, gehlenite-G, belite-B, mayenite-M, merwinite-Mr).

SC

K KP

S

B

O

0

200

400

600

800

1000

1200

1400

0 10 20 30 40 50 60

Intensitycps

Diffracti on angle 2 theta  Fig. 2 Diffractographs of untreated FA2 (calcite-C, belite-B, arcanite-K, pervoskite-P, sylvite-S, orthoclase-O).

Table 1 Chemical composition of untreated fly ashes.

pH CaO SiO2 Al2O3 MgO Fe2O3 SO3 K2O Cl Na2OFA1 12.48 57.0 28.0 3.7 3.7 0.2 0.35 - - 2.0 FA2 12.87 20.5 15.8 - 0.7 - 13.4 18.8 7.2 3.5

CEM-II 12.5 62.5 25.0 2.2 1.6 1.8 1.92 0.7 - 1.5

 

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2.4 Flue gas desulfurization (FGD) gypsum Flue gas desulfurization, the most widely used system in air pollution control equipment in coal fired power plants (for the removal of emitted sulphur) was used as a grinding aid (GA) in this study. The powder diffrac-tion pattern of FGD revealed that the major peaks were composed of gypsum hemi hydrate (CaSO4.5H2O). 3. Experimental method

Dry grinding using a mortar and pestle was utilised for physico-chemical activation of the fly ashes. Low en-ergy intensive agitation (1 horse power motor with 2.5 litre bowl capacity) with low duration of grinding was employed taking sustainability into account and to avoid agglomeration, which is detrimental to the quality and activity of the ground product (Juhasz et al. 1990). The fly ashes were ground for 15 minutes using 250 gm per batch. The initial mineralogical and physical properties of raw fly ashes were analysed and compared with the properties following physico-chemical activation to determine the most suitable grinding and blending tech-niques. The raw fly ashes were also ground with 5% FGD as grinding aid (GA) and their improvement in terms of physical and mineralogical properties were compared.

True density was determined using an automatic Quantachrome gas expansion multi-pycnometer purged with helium gas. Thus, avoiding solubility issues as might be encountered with liquid pycnometers. The elemental composition (major oxides and trace ele-ments) of materials were determined by the use of a Shimadzu EDX 720, energy dispersive X-ray fluores-cence (EDXRF) spectrometer, and the phase composi-tion was determined by x-ray diffraction (XRD) using a Rigaku Miniflex diffractometer (Miniflex gonimeter) with CuK X-ray radiation (30 kV voltage and 15 mA current at scanning speed of 2.0 deg./min in continuous scan mode). The particle size distributions (PSD) of untreated and ground fly ash particles were determined using a Beckman Coulter laser diffraction particle size

analyser in aqueous liquid module mode. A Quantasorb NOVA 2000 Brunauer, Emmett and Teller (BET) ana-lyser was also used to measure the specific surface area (SSA) of treated and untreated fly ashes. Morphological analysis of hydration products was performed using scanning electron microscopy (SEM) equipped with EDX. The EDX, containing Oxford Inca x-act detector (45 nA probe current and 100 sec counting time) and an FEI SEM model Inspect S (20 kV accelerating voltage) was used in this study. A Control Automax5 compres-sion tester featuring accurate load rate application (0.2 MPa/sec) was used for compression testing of mortar. pH measurement was done using aqueous solutions of the fly ashes or solids (1 part by mass with 5 part dis-tilled water) stirred for 30 minutes at 20°C.

The requirement for manual compaction with vibra-tion for full compaction during preparation of 50 mm cube specimens due to the high water demand as well as very low workability shown by the high volume of bio-mass fly ash mixture (50% biomass fly ash with 50% GGBS system) has been reported (Bai et al. 2003). Since the present study aimed to use fly ash in bulk for determining the compressive strength of binder, 100 mm mortar cubes were prepared and cured in water at 20°C per the designated period following the procedure listed in the British Standard (BS EN 12390-2 2009). CEN reference sand for mortar, specified by BS EN 196-1 (BS EN 196-1 2005) was used throughout the study. The paste and mortar specimens were mixed us-ing a Hobart mixer following the procedure specified in BS EN 196-1 for preparing 100 mm cubes and stripped after 24 hours. The average compressive strength value of four specimens prepared for each i) binder content ii) water content and iii) age, was utilised throughout this study.

4. Physico-chemical activation by low intensive grinding

The physical alteration upon mechanical activation of fly ashes is summarised in Table 2. The PSD generated

Table 2 Changes in physical properties upon grinding.

D50 µm

SSA(BET) m2/gm

Density (Pycnometer) gm/cm3

Standard Consistency

Reference cement undisturbed 13.3 6.78 3.05 28% Silica fume undisturbed 83.1 25.14 2.39 65%

FGD gypsum (GA) undisturbed 9.94 12.64 2.60 nd FA1 undisturbed 86.8 3.10 2.73 98%

FA1 undisturbed + 5% GA 9.86 3.70 2.70 nd Ground FA1 without GA 9.50 3.52 2.78 76%

FA1 Ground with 5% GA 7.02 4.41 2.72 72% FA2 undisturbed 62.8 5.70 2.48 38%

FA2 undisturbed +5%GA nd 6.65 nd nd FA2 Ground without GA 8.12 6.14 2.53 34% FA2 Ground with 5% GA 8.64 8.79 2.52 31%

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by ground and untreated FA1 particles is shown in Fig. 3. Due to the high fineness of FGD, the PSD and SSA of both undisturbed fly ashes containing 5% FGD were also measured to determine the actual contribution of FGD (as GA) in physico-chemical activation. It can be stated that grinding not only increases the specific sur-face area (SSA) by reducing the grain size but also brings about morphological modification and altered grain size distribution, which subsequently influences the rheology of the paste. A recent study (Schneider et al. 2011) reported that in the case of cement clinker, the water demand and associated strength development are dependent on particle size in addition to being depend-ent to a large degree on PSD. From Fig. 3, it can be stated that GA assisted grinding not only increases the number of fine particles in the size range of 0 µm to 10 µm but also creates five peaks in the 2-4 μm, 18-22 μm, 40–60 μm, 100–150 μm and 250–300 μm ranges, re-spectively, in their incremental PSD, thus creating a gap gradation which is expected to have a critical influence on water requirement, packing density and hydration behaviour of FA1 (Zhang et al. 2011). The reduced wa-ter demand for grinding was attributable to particle packing by fine ground particles that act as fillers for the void spaces and reduced inter-particle friction (Mehta 2004). An increase in density for grinding was identified for FA1 and FA2. Similar findings have been reported (Payá et al. 1995), (Rukzon et al. 2009) for grinding rice husk ash and coal fly ash. Higher SSA, increased density and lower water demand (as shown in Table 2)

for GA assisted grinding compared to non-GA grinding indicates the beneficial role of gypsum. 5. Physico-chemical activation by ternary blending

It is well documented that the influence of blending cement by supplementary cementitious materials (SCM) such as lime, GGBS, PFA or silica fume, results in up-graded performance of Portland cement concretes. It is obviously of importance to investigate how soon the incorporated pozzolanic material reacts with calcium hydroxide and, once reaction takes place, the rate at which activity is occurring. In the case of blending op-eration, homogeneous nucleation between various parti-cles sized fractions may occur, leading to the develop-ment of dense microstructure of the hardened product with higher durability. SF is widely used as a cement replacement for high strength concrete. However, there is strong disagreement regarding the dosage of SF. Pre-vious studies showed the optimum SF replacement should be in the range from 15% to 40% (Mohamed 2011; Rao 2001; Rasa et al. 2009). Rasa et al. also con-cluded that the optimum content of silica fume increases with increases in the value of the water-cementitious materials ratio, while the corresponding compressive strength decreases. As hundred percent cement replace-ment is the core objective of this study, initially 20% SF has been proposed for the new blend based on the pre-vious study (SF as OPC replacement) and for achieving

Fig. 3 Comparative change in PSD upon grinding FA1.

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a dense mix with upgraded pozzolanic reaction from FA1 and FA2.

Prior to optimisation of the constituent in ternary blend, the binary blends of ground FA1 and FA2 were examined and optimised based on compressive strength development. The strength development by binary blends of varying FA1 content (from 0% to 100%) with associated FA2 content (at binder to sand ratio of 1:3 and water/binder ratio of 0.60) has been illustrated in Fig. 4. The binary blend of 80% FA1 with 20% FA2 was found to show the highest strength after 28 day cur-ing.

When incorporating 20% silica fume into the ternary system, the binary ratio between FA1 and FA2 was re-quired to be changed to 64% and 16% respectively, to maintain the optimum proportion found earlier. How-ever, at the higher SCM replacement level, the drop in pH and as a consequence, the reduction of solubility of the amorphous silicates as well as the slow rate of reac-tion has been indicated (Lothenbach et al. 2011). Hence, to ensure sufficient alkalis (Na2O, K2O) and sulphates from FA2 in the ternary system containing silica fume and to provide higher alkalinity as well as advanced activation of the FA1-SF system, the ternary system was adjusted to FA1:FA2:SF = 60%:20%:20%. This ternary

blend has been designated as HSC-3 throughout this paper, in case of the blend prepared with GA assisted ground FA1and FA2.

The chemical and physical properties of the new blend and control cement are provided in Table 3. The soundness and setting time were determined per the British standard (BS EN 196-3 2009) and pozzolanicity was measured per the procedure listed in standard (BS EN 196-5 2011). Following the procedures outlined in these standards, the pozzolanicity of the HSC-3 (6.26 mmol/l, 2.09 mmol/l) and reference cement (40.71 mmol/l, 5.87 mmol/l) were both found to be positive after 15 days.

The SEM/EDX spectrum of the HSC-3 powder is displayed in Fig. 5 and its physico-chemical properties are shown in Table 3. Considering fineness, soundness, pozzolanicity and oxides contents, the new blend shows characteristics analogous to those of the control cement. The phase composition was found to consist of some crystal peaks that have been identified by XRD as cal-cite, gehlinite, alunite, lime (CaO), belite, arcanite, merwinite and mayenite. The silica fume within the blend was expected to reduce the K+ and OH- ion con-centration of the pore solution and lead to the formation of calcium silicate hydrate (C-S-H) by consuming

 Fig. 4 Optimisation of fly ashes in binary blend.

 Fig. 5 SEM/EDX analysis of HSC-3 powder.

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Ca(OH)2. It has also been suggested that the possibility of an alkali-silica reaction (ASR) is reduced in this situation (Durand et al. 1990).

6. Evaluating performance of HSC-3 upon hydration

Analysing the paste and mortar strength offered by any cementitious material is an important tool for character-ising its behaviour in the hydrated state. To analyse the paste and associated mortar strength development char-acteristics of the ternary blend, the mortar cube speci-mens of control (water/cement ratio of 0.35 and cement: sand of 1:2.25) and HSC-3 (water/binder ratio of 0.45 with 1.5% super plasticiser and binder: sand of 1:2.25) were prepared with a rich mix proportion. In the case of the control paste, the water/cement ratio (0.35) was used similar to its mortar, whereas for HSC-3, pastes speci-mens were prepared with water/binder ratio ranging from 0.4 to 0.5 using 1.5% (of binder) plasticiser. The

strength after 28 days of the water-cured sample is illus-trated in Fig. 6. The improved bond strength between the binder paste and aggregate rather than binder paste matrix in the ternary blend is evident from Fig. 6, which was in contrast to the control.

High water reducing admixtures (HWRA) are widely used in the construction industry to reduce the wa-ter/binder ratio for a given flow and to increase worka-bility, to improve the paste-to aggregate interfacial bond (Xu and Beaudoin 2000). In the case of the cement–silica fume system, formation of a densely compacted matrix at the interfacial transition zone (ITZ) is com-mented to provide high strength as a consequence of this improved bond (Toutanji and Elkorchi 1995). It has been concluded that silica fume generated higher strength concrete than that of the respective systems of paste (Bentur et al. 1987). In the present study, the higher strength offered by HSC-3 mortar compared with that of respective silica fume containing paste was at-tributed to the elimination of weak interfacial links and the formation of a less porous and more homogeneous microstructure in the interfacial region, where the ag-gregate particles act as reinforcing fillers rather than inert fillers (Bentur et al. 1987). A similar observation was made (Toutanji and Elkorchi 1995) in the case of a cement–silica fume system. On the other hand, in a pure cement mortar, the aggregate functions as an inert filler and due to the presence of weak interfacial zones, com-posite mortar is weaker than cement paste. It has been suggested that accumulation of bleed water underneath the aggregate particles creates voids in the transition zone and thus lower mortar or concrete strength (Hol-land and Detwiler 2000).

To analyse strength development, mortar specimens of HSC-3 and control cement were prepared and cured for 90 days in water without any admixture or further alkali activation. Details of the constituent matrix of the mortars and their strength development profile are shown in Table 4 and Fig. 7, respectively. The progres-sive strength development continued up to 90 days by HSC-3, as identified in control cement was evident from Fig. 7.

Furthermore, for analytical analysis of the hydration products, the paste specimens of HSC-3 at water/binder

Table 3 Comparative properties of new blend.

CEM-II HSC-3 Blend

D50 , μm 13.3 10.6 Fineness (BET), m2/gm 6.78 9.67

Density, gm/cm3 3.05 2.59 Soundness (mm)

[BS EN requirement<10mm] 1.3 2.0

Initial setting time(min) [BS EN requirement>60min] 150 70

Pozzolanicity (at 15 day) Positive Positive

Na2O 1.5 1.8 CaO 62.58 45.15 SiO2 25.06 31.19 Al2O3 2.26 3.49 SO3 1.92 3.5 Cl - 0.5

K2O 0.75 4.00 Na2O-equ [%] 2.0 4.2

 

 Fig. 6 Comparative strength development of mortar and paste.

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ratio of 0.4 was prepared and cured at 20°C for the des-ignated period. The phase composition of hydration products at designated ages were determined by XRD (as shown in Fig. 8) and microstructural analysis was performed by SEM (as shown in Fig. 9).

The blend contains a sufficient concentration of K2SO4, to activate calcium rich FA1 particles forming anhydrite, while potassium ions form solid solutions with dicalcium silicate. During hydration, soluble K+ will rapidly increase the pH and accelerate hydration, which will be expected to be balanced by SO4

2- (Katsioti et al. 2009). During hydration, immediately upon inter-action of HSC-3 with water, ionic species from alkali sulphate (K+, Na+ and SO4

2-) form FA2 dissolute in the liquid phase due to high solubility and form hydrates. Quick dissolution of K+ and Na+ ions into the liquid phase from their sulphates has also been suggested (Odler 2003). At the same time, CaSO4 (from GA and

FA1) dissolves and contribute Ca2+ and additional SO42-

ions until saturation. The aluminate containing oxides of FA1 dissolves at this stage and reacts with Ca2+ and SO4

2- and precipitates as ettringite or as the AFt phase. After a few hours of hydration, the concentration of Ca(OH)2 reaches its maximum and the concentration of SO4

2- ions remains constant as the fraction consumed during ettringite formation is replaced by the dissolution of additional CaSO4. The nucleation of the calcium sili-cate hydrate (C-S-H) phase is also initiated at this stage. Moreover, silica fume synthesis promotes pozzolanic reaction with calcium hydroxide and alkaline hydrox-ides, thereby forming C-S-H. The C-S-H phase contin-ues to be formed due to on-going hydration. After 12 hours of hydration, due to shortage of SO4

2- ions as the consequence of complete dissolution of CaSO4, previ-ously formed ettringite (AFt) phase converted to mono-sulphates (C3A.CaSO4.12H2O) (Odler 2003).

Though C-S-H is held to be amorphous, it may be considered to be gel-like and not necessarily amorphous. XRD Diffraction patterns from samples prepared with only C-S-H obtained either from CaO–SiO2 mix or from C3S hydration have been reported by Courault cited by Nonat (Nonat 2004). The C-S-H phase has also been found to be amorphous or semicrystalline and gives a powder pattern very similar to that of C3S paste Ramachandran (Ramachandran 2001). The phase com-

Table 4 Mortar mix matrix for blended mixtures.

HSC-3 (kg/m3)

Sand (kg/m3)

OPC (kg/cum)

Water/ Binder

HSC-3 Mortar 690 1552 - 0.45

Control Mortar - 1552 690 0.35

 

Fig. 8 XRD profile of the blend (HSC-3) at different ages of curing.

0

10

20

30

40

50

60

0 10 20 30 40 50 60 70 80 90 100

CompressiveStrength Mpa

Days of Curing

Control Cement HSC‐3

 Fig. 7 Strength development profile of HSC-3 and control mortar.

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position of the blend hydration products was identified in XRD (Fig. 8) as C-S-H, portlandite [Ca(OH)2], and ettringite [(Ca6Al2(SO4)3(OH)12•26H2O), which were found to increase with age. This indicates accelerated hydration as a consequence of progressive strength de-velopment. Generally the main hydrates of cementitious materials might be a combination of calcium so-dium/potassium silicate hydrate, C-Na/K-S-H, which is the main strength generating element. Formation of an alkali aluminosilicate network (with a general notation of Na/K-SiO2-Al-O.H2O) has also been stated (Guo et al. 2009) as the main stength generating agent of geopoly-meric gel. Progressive formation of a dense microstruc-ture continued up to 90 days as revealed by SEM analy-sis, which also confirms successful hydration (Fig. 9).

The presence of sulphate ions, supplied by alkali sul-phates of FA2, was expected to accelerate the dissolu-tion of FA1 by reducing the concentration of Ca+2 and Al+3 in the mix to form ettringite and provide early strength development and reduced initial setting time (Table 3). Similar hydration kinetics was reported (Konsta-Gdoutos and Shah 2003) during activating

GGBS with arcanite containing cement kiln dust (CKD). Like FA2, the CKD with high sulphate and alkali con-tent was found to be effective for dissolution of GGBS and show significant early and later strength develop-ment by formation of ettringite and C-S-H (Chaunsali and Peethamparan 2011).

Moreover, consumption of Na+, K+ and OH- ions as well as reduction of ion mobility in the pore solution, by silica fume, is expected to control the alkali silica reac-tion (ASR) (Durand et al. 1990). A study (Thomas et al. 1999) also found a blend of fly ash and silica fume with cement (ternary blend) to be very effective for the con-trol of ASR, replacing high concentrations of fly ash or silica fume. The content of SF is expected to consume free lime in a high pH environment, thus controlling the ASR (Fidjestøl and Lewis 2003). This will ensure the inhibition of recrystallization of ettringite in hardened concrete and ensure more strength/density producing C-S-H gel, thus minimising dissolution and expansion in the new cement free blend. A similar observation has been reported (Buhler 2008).

3  days

   90 days

  14 days

90  days

Fig. 9 SEM micrograph of HSC-3 paste at different ages.

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7. Conclusion

The study led to the following conclusions: A new cementitious material has been developed with

physical, chemical and pozzolanic properties analo-gous to conventional cement.

The characteristic of improved bond strength between binder paste and aggregate rather than binder paste matrix in the developed product is in contrast to con-ventional cement, i.e. at the selected water and binder content, the HSC-3 has more strength than its paste.

The highest rate strength development by the new blend between 7 and 14 days in association with non-expansive nature of ettringite formed after 90 days was promising.

The alkali sulphates supplied by fly ash FA2 consume the free lime and aluminium of high lime containing fly ash FA1 and solidify the system

The proposed techniques for advanced reactivity of the blended compounds with bulk usage of waste streams require no calcination process. The low en-ergy intensive grinding operation and optimized blending matrix between selected wastes streams (FA1 and FA2) synthesised with silica fume suggest a new cementitious material with minimum carbon emission through its production

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