Hydration Process and Compressive Strength ofSlag-CFBC Fly Ash Materials without Portland Cement

Nguyen Tien Dung1; Ta-Peng Chang2; and Chun-Tao Chen3

Abstract: This study mixed ground granulated blast-furnace slag (S) and circulating fluidized bed combustion (CFBC) fly ash (CA) withoutany portland cement or alkaline activator to produce an eco-binder, abbreviated as SCA binder. The hydration process, microstructure, andcompressive strength of hydrated SCA materials were investigated. Although both the slag and CA had poor hydration with water, the SCAbinder produced satisfactory hydration products with sufficient cementitious properties. These hydration products detected by Fouriertransform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) were ettringite (AFt), calcium silicate hydrate (C-S-H), and calciumaluminosilicate hydrate (C-A-S-H). The scanning electron microscope (SEM) micrograph showed these hydration products formed densemicrostructure for SCA pastes. As a result, the SCA materials had sufficient compressive strength for practical applications in building ma-terials and civil engineering structures. The compressive strengths of the SCA paste and mortar reached 75MPa at 28 days. Moreover, the SCAconcretes met the requirements for structural concrete. An equation to predict the compressive strengths of the SCA concrete was proposed andagreed well with the experimental results. DOI: 10.1061/(ASCE)MT.1943-5533.0001177. © 2014 American Society of Civil Engineers.

Author keywords: Hydration; Slag; Fly ash; Microstructure; Compressive strength.

Introduction

Circulating fluidized bed combustion (CFBC) technology is anadvanced clean combustion technology in thermal power plantsand has many advantages, such as high combustion efficiency,low combustion temperatures, and low NOx emissions (Armestoand Merino 1999; Guo Li et al. 2012a, b). Such a combustion pro-cess has two major byproducts, CFBC fly ash collected by eitherfabric filters or electrostatic precipitators, and CFBC bottom ashobtained from the bottom of boiler (Ghafoori and Mora 1998;Glinicki and Zielinski 2009). The CFBC ashes are very differentfrom the conventional siliceous fly ashes from pulverized coalsin terms of mineralogical composition, chemical composition, andmorphology, because they are produced at the lower combustiontemperature and a higher amount of limestone addition is usedfor sulfur removal (Sheng et al. 2007). The implementation ofCFBC technology continuously increases due to its excellent envi-ronmental conservation and cost effectiveness so that the globalamount of CFBC ashes will increase accordingly. Therefore,how to properly utilize and dispose the CFBC ashes has becomea big challenge (Chi and Huang 2014).

Several attempts to use CFBC fly ash (CA) in construction werereported. The dominant CA particles are coarse and irregular with a

broad particle size range (Lecuyer et al. 1996) so that it has highdemand of water and admixture (Fu et al. 2008). In addition, thehigh content of anhydrite in CA can result in excessive expansionin cement-based materials due to abundant ettringite formations(Bernardo et al. 2004; Sheng et al. 2007). Moreover, CA has lesspozzolanic activity than the conventional fly ashes (Iribarne et al.2001) and does not meet the requirements for ASTM C 618(ASTM 2012) to be classified as either Class C or Class F flyash (Glinicki and Zielinski 2009; Marks et al. 2012). As little as5% CA is recommended to replace fine aggregates in roller-compacted concrete (Chi and Huang 2014). Thus, the utilizationof CA is restricted in many applications where the pulverizedfly ash often can be used (Sheng et al. 2012). As a result, it ismostly remained in landfills (Shon et al. 2010) which, however,also poses environmental problems, such as serious concerns overcontamination of groundwater and surface waters, and the decreas-ing availability of landfill site (Wang et al. 2005; Fu et al. 2008). Inaddition, its highly exothermic reactions with water (Montagnaroet al. 2008) and high pH leachates and excessive expansion ofsolidified materials (Guo Li et al. 2012a, b) often induce high coston the treatments and disposal.

Several papers reported that CA had a self-cementing property(Sheng et al. 2007, 2012) so that it could set and harden in thepresence of water. However, its self-cementitious strength was veryweak even when the hydration of CA was enhanced by increasedfineness (Sheng et al. 2007). Shon et al. (2010) also pointed out thatthe mixture of CA and pulverized fly ash could produce controlledlow-strength material for backfills. Dung et al. (2014) reported thatno-cement paste made by pure CA and pure ground granulatedblast-furnace (GGBF) slag could produce compressive strengthfrom about 40 to 75 MPa at the age of 28 days. This binder appa-rently had more advantages than other alternative binders to replaceordinary portland cement (OPC), such as geopolymers and super-sulfated cements with low cost and environmental impact. It usedraw industrial wastes, while the geopolymers used an alkaline ac-tivator to initiate the reaction, and the supersulfated cements gen-erally used around 5% portland cement to dissolve and react with80–85% GGBF slag and 10–15% anhydrite (hard burnt gypsum)

1Ph.D. Candidate, Dept. of Civil and Construction Engineering,National Taiwan Univ. of Science and Technology (NTUST) (TaiwanTech), Taipei 106, Taiwan (corresponding author). E-mail: tiendung568@gmail.com

2Professor, Dept. of Civil and Construction Engineering, NationalTaiwan Univ. of Science and Technology (NTUST) (Taiwan Tech),Taipei 106, Taiwan. E-mail: tpchang@mail.ntust.edu.tw

3Assistant Professor, Dept. of Civil and Construction Engineering,National Taiwan Univ. of Science and Technology (NTUST) (TaiwanTech), Taipei 106, Taiwan. E-mail: chuntaoc@gmail.com

Note. This manuscript was submitted on March 17, 2014; approved onAugust 7, 2014; published online on September 19, 2014. Discussion per-iod open until February 19, 2015; separate discussions must be submittedfor individual papers. This paper is part of the Journal of Materials in CivilEngineering, © ASCE, ISSN 0899-1561/04014213(9)/$25.00.

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(Mehrotra et al. 1982; Gruskovnjak et al. 2011; Juenger et al.2011). However, the hydration process of the non-portland cementbinder made of GGBF slag (S) and CA (abbreviated as SCA binderhereafter) and its application to make mortar and concrete have notbeen investigated yet.

The purposes of this research were to investigate the hydrationprocess of CA and SCA binders and to confirm the possibility ofapplying the SCA binder to make mortar and concrete. The hydra-tion products and hydration process of SCA paste were investigatedby Fourier transform infrared (FTIR) spectroscopy and X-ray dif-fraction (XRD). In addition, the microstructures of SCA pasteswere observed by scanning electron microscopy (SEM). The com-pressive strengths of SCA paste, mortar, and concrete were alsomeasured until the age of 150 days. The results indicated thatthe hydration of SCA binder produced ettringite (AFt), calciumsilicate hydrate (C-S-H), and calcium aluminosilicate hydrate(C-A-S-H), which induced dense microstructure and sufficientcementitious strength for SCA materials. Moreover, an equationto predict the compressive strengths of the SCA materials with ageswas proposed.

Materials and Testing Methods

Materials

Table 1 shows the fineness and chemical compositions of GGBFslag and CA used in this study. The XRD patterns of GGBF slagand CA are also shown in Fig. 1. The slag shows amorphous glasswithout clear peak of XRD pattern. On the other hand, majormineralogical compositions of CA detected by XRD are anhydrite(CaSO4), portlandite ½CaðOHÞ2�, lime (CaO), quartz (SiO2), andalumina (Al2O3). The presence of portlandite is attributed to thereaction between the free lime in CA and water vapor in the airduring the storage. Based on the preliminary study (Dung et al.2014), the SCA binder was produced by purely mixing GGBF slagand CA with the ratio of 4∶1 by mass.

The solutions filtered from the pure CA paste and the SCA pastewith a water/binder (W/B) of 0.40 were diluted by 100 times withdistilled water and then measured for pH. The pH values of thefiltered solutions from the pure CA paste and from the SCA pastewere calculated as high as 12.5 and 12.4, respectively.

The physical properties of crushed sand and crushed stone froma quarry used as fine and coarse aggregates to make mortar andconcrete are presented in Table 2. These fine and coarse aggregates

were air dried, and thus extra water was needed to add to reach asaturated surface dry (SSD) state as required by most mix designmethods. A polycarboxylate superplasticizer (SP) was used toproduce SCA pastes, mortars, and concretes with proper workabil-ity of flow larger than 195 mm and slump larger than 160 mm,respectively.

Test Methods

Fourier Transform Infrared (FTIR) SpectroscopyFTIR spectrophotometric studies were performed in a FTS-3500FTIR spectrophotometer to identify the hydration products of slag,CA, and SCA pastes. The samples for the FTIR test were obtainedafter the compressive strength tests at 7 days. They were immersedin acetone to stop the hydration process. Then, they were dried at60°C for 24 h and then ground into powders passing through the75 μm sieve. Powder samples were mixed with KBr and thenpressed at 40 MPa to produce a pellet for analysis. The analysiswas carried out in the frequency range of 400–4,000 cm−1 andthe samples were scanned at 128 times.

X-ray Diffraction Analysis (XRD)The samples for XRD test were obtained after the compressivestrength tests of CA paste at ages of 1, 3, and 7 days and SCA pasteat ages of 1, 3, 7, 28, and 91 days, respectively. The powder samplepreparation procedure was similar to that in FTIR test. The driedpowder sample then was used for XRD analysis by a D2-PhaserX-ray diffractometer with Cu-Kα radiation. A step scan was per-formed in the range of 5–55° (2θ) with a stepping interval of 0.02°and a duration of 0.1 s. Diffraction peaks on the XRD spectrumwere analyzed by software package EVA 2.0.

Fig. 1. XRD patterns of materials

Table 1. Physical and Chemical Compositions of GGBF Slag (S) andCFBC Fly Ash (CA)

Physical/chemical composition GGBF slag (S) CFBC fly ash (CA)

Specific gravity 2.83 2.47Blaine fineness (m2=kg) 600 300Loss of ignitiona (%) 4.72 7.83SiO2 (%) 34.9 5.22Al2O3 (%) 13.53 2.21CaO (%) 41.47 56.80K2O (%) N/A 0.53Na2O (%) N/A 0.26MgO (%) 7.18 2.06SO3 (%) 1.74 32.40S (%) 0.65 N/AFe2O3 (%) 0.52 0.581aThe loss on ignition at 950°C was performed after analyzing chemicalcompositions.

Table 2. Physical Properties of Fine and Coarse Aggregates

Property Fine aggregate Coarse aggregate

Dmax (mm) 4.75 19.5Specific gravity 2.65 2.67Fineness modulus 2.64 6.58Water absorption (%) 1.65 0.80

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Scanning Electron Microscopy (SEM) and EnergyDispersive X-ray Spectroscopy (EDX)The microstructure of slag, CA, and SCA pastes was observed by ascanning electron microscope (model: JSM-6390LV). The samplefor SEM observation was obtained after the compressive strengthtest at the age of 56 days and then immersed in acetone to stopthe hydration process. The specimens were then dried at 60°Cfor 24 h, prepared and coated by using K550 Sputter coater priorto SEM observations. The chemical composition of the hydrationproducts was determined by an energy-dispersive X-ray spectrom-eter (model: OXFORD INCAX-7060) that was equipped withJSM-6390LV.

Compressive Strength TestTable 3 shows mixture proportions for pastes with pure slag, pureCA and SCA, whereas Table 4 shows mixture proportions for theSCA mortars. After all constituents were mixed in a pan mixer, thefresh SCA pastes and mortars were then cast into cubic molds of50 × 50 × 50 mm for compressive strength test in accordance withASTM C 109 (ASTM 1999b). These specimens were demoldedafter 24 hours of casting and then were cured in air of 65� 5%

RH and temperature of 25� 2°C until the testing at the ages of7, 28, 56, 91, 120, and 150 days. Three specimens were tested ateach hydration time.

Table 5 shows mix proportions for SCA concretes with differentW/B ratios. These mixtures were designed based on ACI 211.1(ACI 1991) and supposed to provide medium compressive strengthfrom 25 to 40 MPa at the age of 28 days. They were cast intoϕ100 × 200 mm cylindrical molds for compressive test accordingto ASTM C 39 (ASTM 1999a). All SCA concrete specimens werestored in molds for the first day and then cured in air of 65� 5%

RH and temperature of 25� 2°C until the testing at the ages of 7,28, 56, 91, and 150 days, respectively. Three specimens were testedat each hydration time.

Results and Discussion

FTIR Study of the Hydrated Slag, CA, and SCA Binders

FTIR spectra of the hydrated slag, CA, and SCA at 7 days arepresented in Fig. 2. The FTIR spectrum frequencies and band as-signments of their hydration products also are given in Table 6. Theisolated OH stretching at 3,638 cm−1 (band a) is attributed to theinteraction of the water hydroxyl with the cations (García Lodeiroet al. 2009), associated with the CaðOHÞ2 due to the hydration offree lime of the CA. The weak OH band of SCA paste is related tothe consumption of CaðOHÞ2 to produce other hydration products.The other bands are attributed to the hydrogen bonding of watermolecule to surface oxygen (band b) and the bending mode ofwater (band c) (García Lodeiro et al. 2009). The signals appearingat 1,442 cm−1 (band d) and at around 876 cm−1 (band g) are attrib-uted to calcium carbonates (CaCO3) (Carmona Quiroga and BlancoVarela 2013) forming in specimens during the curing period. Thepresence of CaCO3 is due to the reaction between CaðOHÞ2 in spec-imens and CO2 during the air curing. The S-O stretching vibrationat around 1,122 cm−1 (b and e) and the Al-O-H bending at around679 cm−1 (b and h) are attributed to the formation of ettringite(AFt) (Carmona Quiroga and Blanco Varela 2013). The vibrationsin the region 964–980 cm−1 (b and f) are assigned to the presence

Table 3. Mixture Proportions of Pastes

Mixture Binders W/B GGBF slag (S) (kg=m3) CFBC fly ash (CA) (kg=m3) Water (kg=m3) SP (kg=m3)

P-W/S0.40 100%S 0.40 1,313 0.00 524 1.05P-W/CA0.40 100%CA 0.40 0.00 1,230 490 1.72P-W/SCA0.30 80%Sþ 20%CA 0.30 1,193 298 443 3.73P-W/SCA0.35 80%Sþ 20%CA 0.35 1,109 277 482 2.50P-W/SCA0.40 80%Sþ 20%CA 0.40 1,036 259 517 1.04

Table 4. Mix Proportions of SCA Mortars

Mixture W/B

GGBFslag (S)(kg=m3)

CFBC flyash (CA)(kg=m3)

Fineaggregatea

(kg=m3)Water(kg=m3)

SP(kg=m3)

M-W/SCA0.35 0.35 471 118 1,458 205 0.88M-W/SCA0.40 0.40 440 110 1,458 219 0.82aAggregate was in a saturated surface dry (SSD) state.

Table 5. Mix Proportions of SCA Concretes

Mixture W/BGGBF slag (S)

(kg=m3)CFBC fly ash (CA)

(kg=m3)Fine aggregatea

(kg=m3)Coarse aggregatea

(kg=m3)Water(kg=m3)

SP(kg=m3)

C-W/SCA0.42 0.422 362 90 628 1,024 190 0.68C-W/SCA0.47 0.472 323 81 674 1,024 190 0.61C-W/SCA0.54 0.543 281 70 725 1,024 190 0.53C-W/SCA0.61 0.612 249 62 764 1,024 190 0.47aBoth aggregates were in saturated surface dry (SSD) states.

Fig. 2. FTIR spectra for GGBF slag, CA, and SCA pastes at 7 days

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of Si-O stretching, which confirms the generation of calciumsilicate hydrate (C-S-H) (García Lodeiro et al. 2009). Band k ataround 594 cm−1 is assigned to S-O vibrations of SO2−

4 in gypsum(CaSO4 · 2H2O) (Lanzon and García-Ruiz 2012). The signals ap-pearing at around 490 cm−1 (b and l) and around 451 cm−1 (b andm), corresponding to the Si-O-Si and Si-O vibrations, could beattributed to the formation of C-S-H and calcium aluminosilicatehydrate (C-A-S-H), respectively (Payá et al. 2003; García Lodeiroet al. 2009).

Therefore, the FTIR results indicate that the hydration productsof slag are C-S-H, C-A-S-H, and little CaCO3 resulting from thecarbonation of the specimen during curing. The hydration productsof the CA are CaðOHÞ2, AFt, C-S-H, CaSO4 · H2O, and CaCO3.On the other hand, the main hydration products of SCA are AFt,C-S-H, C-A-S-H, CaðOHÞ2, and CaCO3.

Hydration Process of SCA Binder

Hydration Process of Pure CA PasteThe XRD patterns in Fig. 3 are consistent with the FTIR results.The hydration products of CA include AFt (3CaO · Al2O3 ·3CaSO4 · 32H2O), portlandite, and gypsum. The gypsum was justformed after 3 days of hydration while the portlandite was reducingwith age. The decrease of portlandite was attributed to the forma-tion of AFt and C-S-H. However, the C-S-H could not be found inthe XRD results because of its amorphous structure. Based on theFTIR analyses, XRD patterns and pH measurements, the hydrationprocess of CA is analyzed as follows:

The CA originally contains abundant limestone which is used asthe sorbent in CFBC for the SO2-removing process so that most CAparticles consist of an outer CaSO4 shell and unsulfated core madeup of many CaO grains (Wang et al. 2005; Li et al. 2007). Thus,during the hydration of CA, the outer inactivity CaSO4 shell pre-vents the water from contacting and reacting with the CaO coreso that it prevents CaO from slaking and causes the slow hydrationof CA. However, the CaSO4 shell is heterogeneous (Sheng et al.2012). Hence, in the hydration process of CA, the water can passthrough the thin CaSO4 shell to react with the lime or the water candirectly react with CA particles, which were incompletely coveredby CaSO4 shell, to form CaðOHÞ2. The reaction is shown in Eq. (1)

CaOþ H2O → CaðOHÞ2 ð1Þ

Because the CaðOHÞ2 has a molar volume nearly double that ofCaO (Ghafoori and Mora 1998; Sheng et al. 2012), the thin CaSO4

shell would crack, and thus the CaðOHÞ2 will be exposed tosolution. In addition to the initial CaðOHÞ2 existing in CA beforehydration, the formation of CaðOHÞ2 by the hydration of CaO rises

Table 6. Hydration Product Bands on Infrared Spectra

Band P-W/S0.4 (cm−1) P-W/CA0.4 (cm−1) P-W/SCA0.4 (cm−1) Assigned to

a — 3,638 3,638 O-H stretching [CaðOHÞ2] García Lodeiro et al. (2009)b 3,418 3,414 3,406 O-H stretching (H2O) García Lodeiro et al. (2009)c 1,627 1,631 1,627 H-O-H bending (H2O) García Lodeiro et al. (2009)d 1,442 1,442 1,442 C-O stretching (CO2−

3 ) García Lodeiro et al. (2009)e — 1,122 1,118 S-O stretching (AFt) Carmona Quiroga and Blanco Varela (2013)f 964 980 968 Si-O stretching (C-S-H) García Lodeiro et al. (2009)g 887 876 — C-O bending (CO2−

3 ) Carmona Quiroga and Blanco Varela (2013)h — 679 — AlO6 (AFt) Carmona Quiroga and Blanco Varela (2013)k — 594 — SO2−

4 bending Lanzon and García-Ruiz (2012)l 490 — 494 Si-O-Si García Lodeiro et al. (2009)m 451 — 447 Si-O (SiO4) García Lodeiro et al. (2009)

Fig. 3. XRD patterns of hydrated CA

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the pH of CA paste to 12.5 (as mentioned in the “Materials”section).

The active alumina (Al2O3) in CA can be dissolved in the strongalkaline solution with pH of 12.5 in the CA paste (Sheng et al.2012). Therefore, at early stages, the dissolved alumina reacts withthe Ca2þ and SO2−

4 ions dissolved from CaSO4 shell in the presenceof CaðOHÞ2 to form AFt, as seen in FTIR analyses (Fig. 2) andXRD patterns (Fig. 3), according to the reaction as shown inthe Eq. (2) (Álvarez-Ayuso and Nugteren 2005; Lothenbach 2010)

6Ca2þ þ 2AlðOHÞ−4 þ 3SO2−

4 þ 4OH− þ 26H2O → 3CaO

· Al2O3 · 3CaSO4 · 32H2OðAFtÞ ð2Þ

The AFt is very important for hardening of fresh pure CA pasteduring early hydration and the compressive strength development(Uchikawa and Uchida 1974; Dongxu et al. 2000; Sheng et al.2007; Renaudin et al. 2010) because it acts as skeletons and micro-steel fibers. The consumption of Ca2þ, SO2−

4 to form AFt causes theCaSO4 shell to dissolve more, and the reaction in Eq. (2) proceedscontinuously, resulting in more AFt.

Moreover, the C-S-H, as seen in FTIR analyses (Fig. 2), can begenerated by the reaction of active silica (SiO2) dissolved from CAand CaðOHÞ2 according to Eq. (3) (Dung et al. 2014)

xCaðOHÞ2 þ SiO2 þ ðy − xÞH2O → CxSHyðC − S − HÞ ð3Þ

The C-S-H is very important for the later strength developmentof CA pastes (Sheng et al. 2007). The generation of AFt and C-S-His considered the main reasons for the setting and hardening of CApaste (Sheng et al. 2007).

Furthermore, the nuclei of gypsum formed on the surface ofCaSO4 are associated with the growth of the Ca2þ and SO2−

4

ion layers adsorbed on the CaSO4 shell (Sievert et al. 2005). Thisprocess occurs very late, because the dissolution of anhydrite isvery slow. As a result, the gypsum has not been formed after1 day of hydration, as seen in the XRD patterns (Fig. 3).

Once the gypsum is formed, the active alumina dissolved fromCA, if any, can react with it in the presence of CaðOHÞ2 to generateAFt according to Eq. (4)

3Ca2þ þ 2AlðOHÞ−4 þ 3CaSO4 · 2H2Oþ 4OH− þ 20H2O

→ 3CaO · Al2O3 · 3CaSO4 · 32H2OðAFtÞ ð4Þ

The formation of AFt and C-S-H reduced pH and covered gyp-sum so that gypsum was stable. As a result, the hydration productsof CA paste include the generation of C-S-H, along with AFt, por-tlandite, gypsum, undissolved anhydrite, and unhydrated lime asshown in FTIR and XRD results. A large amount of anhydriteand portlandite left in the hydration products of the pure CA paste(Fig. 3) means that the amount of active silica and alumina in CA isinsufficient for the reactions forming AFt and C-S-H, as shown inEqs. (2)–(4). In other words, there are few hydration productsgenerated from the hydration of pure CA.

Hydration Process of SCA PasteWhen the slag is added to the aqueous solution with CA, the SCAhydration process is more complex due to the coexistence of theCA hydration, the dissolution of slag, and the chemical reactionbetween the hydrated CA and the active oxides dissolved from slag.The pH of SCA-water system produced from 80% GGBF slag (S)and 20% CA mixing with the W/B of 0.40 was as high as 12.4(mentioned in Materials section). Such a high pH value plays animportant role in dissolving the slag containing a massive glassyphase (Song and Jennings 1999; Wang et al. 2010). The dissolution

mechanism of slag in a high-pH environment was analyzed in(Duxson and Provis 2008; Li et al. 2010). The active aluminaand silica dissolved from the slag can react with CaðOHÞ2,Ca2þ, and SO2−

4 ions to generate AFt and C-S-H, as in Eqs. (2)and (3). However, dramatic changes were found in the hydrationprocess and the resulting hydration products of SCA, as comparedin FTIR analyses (Fig. 2) and XRD patterns (Figs. 3 and 4). Themain hydration products of the SCA paste consist of C-A-S-H andAFt. Fig. 4 shows the increases of C-A-S-H and AFt with age. Suchincreases were associated with the formation of C-A-S-H and AFtby consumption of portlandite and anhydrite and resulted in theweak peaks of lime and anhydrite. The hydration process of theSCA paste is analyzed as follows:

The slag is rich in silica and alumina so that the active silica andalumina dissolved from the slag outnumber those from CA. Theypreferentially combine with CaðOHÞ2, Ca2þ, and SO2−

4 ions fromthe anhydrite solution, as shown in Eqs. (2) and (3), and then pre-cipitate AFt and C-S-H. The Ca2þ and SO2−

4 ions released fromCaSO4 shell of CA adsorb on the surface of CA particles so thatthe AFt is formed on the surface of CA particles and then hindersthe hydration of CA particles. However, these two reactionsreduce the concentration of Ca2þ and SO2−

4 , encouraging thedissolution of anhydrite into Ca2þ and SO2−

4 ions. This processenables the hydration of CA to proceed. In addition, the formationof the AFt and C-S-H also consumes water. This increases thealkalinity of the system and further encourages the dissolutionof CA particles.

The larger consumption of Ca2þ and SO2−4 ions by abundant

active silica and alumina dissolved from slag reduces the numberof Ca2þ and SO2−

4 ions absorbed on unhydrated anhydrite shell sothat the gypsum nuclei could not be formed, and thus the gypsum isnot shown in the FTIR (Fig. 2) and XRD (Fig. 4) results. On theother hand, the feeble peaks of portlandite in the XRD results arerelated to the generation of the AFt and C-S-H, as shown in thereactions of Eqs. (2) and (3). Moreover, the C-A-S-H observed

Fig. 4. XRD patterns of SCA pastes (P-W/SCA0.35)

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in FTIR and XRD patterns is produced from the crystallizationprocess of the dissolved alumina and the silica in portlandite.

Therefore, the abundant active silica and alumina dissolvedfrom slag enhance the hydration of CA particles by reacting withthe portlandite, Ca2þ and SO2−

4 ions from CA-water system togenerate plenty of phases which contribute to self-hardening andstrength development of the SCA paste. In contrast, the CAwithoutslag poorly produces hydration products because its inappropriatecompositions could not initiate chemical reactions effectivelyto generate a large amount of phases, producing a good self-cementing property.

SEM Observation

As seen in Fig. 5, SEM micrographs show poor microstructure ofpure slag paste (P-W/S0.40) and pure CA paste (P-W/CA0.40).It appears that there is very little dissolution of the slag andCA particles. The microstructure of slag and CA pastes seemsto resemble a fractured sample of densely packed slag or CA par-ticles without any apparent interconnectivity. By contrast, Fig. 6clearly shows dense microstructure of SCA pastes with massivesolid phases well connected. It is evident that there are plenty ofhydration products formed from the hydration of SCA binder.It confirms that the presence of slag produces active oxides toenhance the hydration of CA and to chemically react with thehydrated CA-water products to create massive solid phases. Suchphases properly provide the self-cementitious property and suffi-cient strength for SCA materials.

The SCA paste with W/B of 0.30 (P-W/SCA0.30) exhibits themost compact matrix among three SCA pastes. As the W/Bincreases from 0.30 to 0.40, the hydration products of P-W/SCA0.40 paste are divided into smaller solid phases and thus resultin voids [Fig. 6(c)]. More voids in P-W/SCA0.40 paste are attrib-uted to a larger amount of free water from the mixture with W/B of0.40. However, more free water is beneficial to AFt formation sothat many AFt crystals with about 1–2 μm long and 0.1–0.2 μmwide are observed in P-W/SCA0.40 paste [Fig. 6(c)]. As seen inFig. 6(c), AFt crystals fill the pores and connect massive solidphases and thus they can contribute the strengths of SCA materials.On the other hand, the EDX result shows that the location 1

Fig. 5. SEM micrographs at 56 days: (a) GGBF slag paste; (b) CApaste

Fig. 6. SEM micrographs of SCA pastes at 56 days: (a) W/B of 0.30;(b) W/B of 0.35; (c) W/B of 0.40

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(Si/Ca of 0.55 and Al=Ca of 0.13) is identified as C-S-H phase(Taylor 1986, 1993; Iribarne et al. 2001).

Cementitious Strength of SCA Materials

The preliminary studies reported that the pure slag paste and thepure CA paste produced poor compressive strengths (less than6 MPa at 91 days) (Dung et al. 2014). This is related to the factthat there were little and sparse hydration products of the slag andCA pastes (as observed in Fig. 5) such that these scarce and

scattered hydration products could not be bonded to each other,and thus they just produced a very little strength. By contrast,as the slag was used to enhance the CA hydration, such SCA binderproduced sufficient compressive strength for SCA materials, asseen in Fig. 7. Particularly, the compressive strengths at 28 daysof SCA pastes with W/B of 0.30 (P-W/SCA0.30), W/B of 0.35(P-W/SCA0.35), and W/B of 0.40 (P-W/SCA0.40) were about75, 70, and 45 MPa, respectively [Fig. 7(a)]. In addition, thoseof SCA mortars with W/B of 0.35 (M-W/SCA0.35) and W/B of0.40 (M-W/SCA0.40) were about 75 and 55 MPa, respectively[Fig. 7(b)], suitable for practical applications. Moreover, the SCAconcretes with W/B of 0.42 (C-W/SCA0.42), W/B of 0.47(C-WSCA0.47), W/B of 0.52 (C-W/SCA0.52), and W/B of 0.61(C-W/SCA0.61) had the compressive strengths slightly over 40,35, 30, and 25 MPa; respectively [Fig. 7(c)], which satisfy thedesign strength criterion of structural concrete as specified in theACI 211.1 design method. Furthermore, the compressive strengthsof SCA materials were shown to be higher at later ages.

In general, the compressive strengths of SCA materials increasewith age for all W/B values. It is consistent with the fact that theintensities of diffraction peaks of hydrated SCA products increasewith age, as seen in XRD results (Fig. 4). The rate of strength gainis high at early ages and gradually slows down at later ages. This isbecause that the formation of hydrated product is fast at early agesand slow at later ages, as seen in the XRD results in Fig. 4. Thedeposit of hydration products on the CA grain makes the diffusionof water into the unhydrated nucleus become more and moredifficult, so that the rate of hydration and strength gain reduces withtime.

The test data demonstrate that higher W/B results in lower com-pressive strength of SCA materials. This is related to the fact thathigher water content increases the free water in SCA paste and thusresults in more porous microstructure (as seen in Fig. 6) and lowerstrength. In addition, the free water is beneficial to the hydration ofanhydrite to cause earlier AFt formation. The AFt at early agescauses microcracks, because at that time the strength of SCA ma-terials is still to small to resist the expansive of AFt. As a result,microcracks lead to a noticeable strength loss.

It is worthy of noting that the compressive strengths of SCAmortars (M-W/SCA0.35 and M-W/SCA0.40) are comparable toor even higher than those of SCA pastes (P-W/SCA0.35 andP-W/SCA0.40) with same W/B. Therefore, a quite satisfactorybonding between the aggregates and the SCA paste for SCA mortarcan be thus deduced.

Prediction of Compressive Strength of SCA Concretes

The compressive strengths of SCA concretes at 28 days are shownin Fig. 8. Thus, it is reasonable to use the following empiricalregression Eq. (5) to relate the values of W/B and the correspondingcompressive strengths of these SCA concretes with a correlationcoefficient (R2) of about 0.99, as shown in Fig. 8

f 0cð28Þ ¼

fcrðW=BÞ1.29 ð5Þ

where f 0cð28Þ = compressive strength of SCA concretes at 28 days,

fcr = reference strength of 13.54 MPa.In order to predict the compressive strengths of these SCA con-

cretes at other ages, the following Eq. (6), taken from ACI 209R(ACI 1992), is adopted:

fcðtÞ ¼t

αþ βtf 0cð28Þ ð6ÞFig. 7. Compressive strength of (a) SCA pastes; (b) SCA mortar;

(c) SCA concrete

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where fcðtÞ is the compressive strength of SCA concretes at theage t (days), and α and β are constants of 4.0 and 0.85 forOPC concrete cured in air. The constant α controls the increasingrate of strength with time t after 28 days; i.e., at same age, the lagerthe α is, the higher the strength will be. On the other hand, theconstant β controls the ultimate strength as time t approaches toinfinitive, i.e., the smaller the β is, the higher the ultimate strengthwill be. By using Eq. (6) and test data in Fig. 7, Fig. 9 shows thatmost experimental data of compressive strength at various ages arelocated within the �10% of those predicted from Eq. (6).

By combining both Eqs. (5) and (6), a proposed equation forpredicting the compressive strength of SCA concretes in termsof ages and values of W/B is expressed as follows:

fcðtÞ ¼13.54

ðW=BÞ1.29t

αþ βtð7Þ

Fig. 10 shows the predicted compressive strength of SCA con-cretes in good consistent tendency with the test results. Particularly,the strength gain is higher at early ages and then becomes lower atlater ages. Also, lower W/B results in higher compressive strengthfor SCA concretes.

Conclusions

This study investigated the hydration products of CFBC fly ash(CA) and the SCA eco-binder made of ground granulated blast-furnace slag (S) and CA. The microstructure of SCA paste andthe compressive strengths of SCA paste, mortar and concrete werealso studied. Through the results in this study, the followingconclusions can be drawn:1. Due to very little alumina and silica, the hydration of CA just

produces few hydration products and induces poor microstruc-ture. However, the dissolved alumina and silica from slag canaccelerate the hydration of CA and react with the hydrationproducts of CA to produce AFt, C-S-H and C-A-S-H, whichform dense microstructure and provide sufficient compressivestrengths for all SCA materials.

2. The compressive strengths of SCA concrete in this study beinghigher than 25 MPa, which satisfies the requirement ofdesigned strength for structural concrete, as requested in theACI 211.1 standard, can be used for practical application.Similar conclusion is also applied to the SCA paste and mortar.

3. Similar to OPC-based materials, the compressive strength ofSCA materials increases with time but decreases with water/binder ratio. The strength gain of SCA materials is fast at earlyages and slow at later ages.

4. This study proposes an equation in terms of various ages andW/B values to satisfactorily predict the compressive strengthof SCA materials, which could be applied as an estimationmethod of strength for practical application.

Acknowledgments

The author Nguyen Tien Dung gratefully acknowledges thefinancial support of this work by National Taiwan University ofScience and Technology (NTUST) (Taiwan Tech) through thescholarships. The chemical tests were carried out at the Departmentof Chemical Engineering of NTUST by valuable assistance ofMr. Vo Duc Thang.

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