Evaluation of alkalinity of pore solution based on the phasecomposition of cement hydrates with supplementarycementitious materials and its relation to suppressing ASRexpansionYuichiro Kawabata, Kazuo YamadaJournal of Advanced Concrete Technology, volume ( ), pp.13 2015 538-553

Effect of electrochemical penetration of lithium ions on concrete expansion due to ASRTakao Ueda , Yuuta Baba Akira Nanasawa,Journal of Advanced Concrete Technology, volume ( ), pp.9 2011 31-39

Elastoplastic-damage compression constitutive model for cementitious material subjected to alkali-silicareactionYingdi Liao, Da Chen Zihan Liu, , Feng Ouyang, Lijun HouJournal of Advanced Concrete Technology, volume ( ), pp.12 2014 158-166

Assessment of blended cements effectiveness against ASR by mortar and concrete expansion tests.Umberto Costa,Teresa Mangialardi Antonio Evangelista Paolini,Journal of Advanced Concrete Technology, volume ( ), pp.12 2014 266-278

Journal of Advanced Concrete Technology Vol. 13, 538-553, November 2015 / Copyright © 2015 Japan Concrete Institute 538

Scientific paper

Evaluation of Alkalinity of Pore Solution Based on the Phase Composition of Cement Hydrates with Supplementary Cementitious Materials and its Relation to Suppressing ASR Expansion Yuichiro Kawabata1* and Kazuo Yamada2

Received 27 July 2015, accepted 20 November 2015 doi:10.3151/jact.13.538

Abstract A model to evaluate quantitatively the alkalinity of pore solution based on phase composition of cement hydrates with SCMs was proposed and was compared with suppressing effect of ASR expansion. The model is devised from the per-spective of alkali sorption by C-S-H gel, and the parameters for calculation can be evaluated thanks to phase composition analysis such as XRD/Rietveld analysis and selective dissolution. The experimental results have shown that ASR ex-pansion is strongly correlated to the alkalinity of the pore solution, which can be calculated with the proposed model. Based on the results, the ASR suppressing effects of SCMs are converted to the reduction in total alkali content as available alkali content. Finally, the required replacement level of SCM with the proposed model was compared to the CSA A23.2-27A standard based on numerous experiments and field experiences in Canada. The calculated result was well consistent with the minimum replacement level of SCMs specified in CSA A23.2-27A. A subsequent interpretation of this study supports that the dominant mechanism of SCMs for ASR suppression is a reduction of alkalinity of pore solution.

1. Introduction

It is globally accepted that the replacement of cement by an appropriate amount of supplementary cementitious materials (SCMs), such as fly ash (FA) or blast furnace slag (BFS), effectively suppresses expansion due to al-kali-silica reaction (ASR) (e.g., Thomas 2011). The mechanism of SCMs on ASR suppression, however, has yet to be identified and remains controversial. According to the previous studies, the mechanism has been consid-ered to be related to the following factors: reduction in alkalinity of pore solution (Diamond 1981; Canham et al. 1987; Durand et al. 1990; Duchesne and Bérubé 1994a; Thomas and Innis 1998; Ramlochan et al. 2000; Boddy et al. 2003; consumption of calcium hydroxide (Chatterji 1979); low permeability derived from densification of hydrates (Kawamura and Takemoto 1988; Bakker 1981; Xu et al. 1995); reduction of silica dissolution of the aggregate in the presence of aluminium (Chappex and Scrivener 2012a). Among these mechanisms, a reduction in alkalinity is believed to be the most effective. The addition of a sufficient amount of SCMs leads to forma-tion of calcium-silicate-hydrate (C-S-H) gel with a low Ca/Si ratio, resulting in a low alkalinity of the pore so-lution (Hong and Glasser 1999). When Al-including

SCMs are added in the mix, Al substitutes the bridging site of Si in C-S-H gel (Richardson and Groves 2003). The influence of aluminium on alkalinity of pore solution remains controversial. Though Hong and Glasser (2002) reported that C-A-S-H gel has large capacity for alkali sorption, the recent publication reported that the presence of aluminium in C-S-H gel has little significant influence on the alkali uptake (L’Hôpital 2014). Chappex and Scrivener (2012a) also indicated that this effect could be limited. The influence of aluminium on alkali sorption, hereafter, was not considered in this paper.

The composition of SCMs significantly affects the suppressing effect of ASR expansion. This is attributable to the composition of cement hydrates formed by cement and SCMs themselves. For C-S-H gel, its amount and composition, especially mean Ca/Si ratio of C-S-H gel, also greatly depends on the type and replacement level of SCMs. Some chemical indices using chemical composi-tion of SCMs have been devised (Thomas 2011; Malvar and Lenke 2006; Kawabata et al. 2008). Fineness of FA also affects the suppressing effect of SCMs on ASR expansion. Obla et al. (2003) revealed that ultra-fine FA, of which average particle size is 3 μm, is very effective despite slightly higher calcium content. Ravina (1980) also pointed out that the fineness is the most important physical property for pozzolanic reactivity. For low-Ca siliceous FA, silica content in glass phase of fly ash and the specific surface area (SSA) are the main influent parameters related to suppressing of the ASR expansion (Nagataki et al. 1990; Kawabata et al. 2008).

The suppressing effect of SCMs on ASR expansion is generally correlated to their effect on alkalinity of pore solution. Hydroxide ion concentration of pore solution is generally balanced with alkali metal cations (Na+, K+) so that the alkalinity expresses the hydroxide ion concen-

1Senior Researcher, Port and Airport Research Institute, Yokosuka, Kanagawa Pref. 239-0826 Japan and Université Paris-Est, IFSTTAR, Marne-la-Vallée Cedex 2, France. *Corresponding author, E-mail: kawabata-y@pari.go.jp2Senior Researcher, National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki, 305-8506, Japan.

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tration. Regarding the alkalinity of pore solution, pore solution extraction was carried out to measure its alka-linity in many experiments (e.g., Diamond 1981; Can-ham et al. 1987; Kawamura and Takemoto 1988; Duchesne and Bérubé 1994b; Shehata et al. 1999). In this method, however, the composition of pore solution is inevitably affected by the applied pressure (Bérubé and Tremblay 2004; Lothenbach 2010; Chappex and Scriv-ener 2012a). Because the microstructure of hardened cement paste differs significantly when cement is re-placed by SCMs, there has been no direct validation that the “real composition” of the pore solution can be evaluated by pore solution extraction.

The previous studies using SEM-EDS analysis also revealed that Ca/Si molar ratio of C-S-H gel decreases when a sufficient amount of SCMs replaces cement added in the mix. It is proved that SEM-EDS analysis is a promising tool which gives useful local information of C-S-H gel. However, due to the heterogeneous micro-structure of cement paste, especially when cement is partly replaced by SCMs, Ca/Si molar ratio of C-S-H gel varies widely from place to place. For example, around FA, Ca/Si molar ratio is lower compared to other parts (Katayama and Sakai 1998). Thus it is difficult to evaluate the effect of reduced Ca/Si molar ratio of C-S-H gel on the alkalinity of pore solution quantitatively.

Consequently there is a missing link between compo-sition of cement hydrates and alkalinity of pore solution and furthermore suppressing effect of ASR expansion. Therefore, some different approaches will be beneficial to further investigate the correlation between the alka-linity of pore solution and ASR expansion. One possible method is the evaluation of the alkalinity of pore solution from the perspective of the phase composition of cement hydrates and its equilibrium with alkalis concentration in the solution (Kawabata et al. 2012, 2013; Kawabata and Yamada 2015). By this method, although it is difficult to evaluate the local variation of Ca/Si molar ratio of C-S-H, its mean value can be obtained. The mean value should correspond to the alkalinity of pore solution.

In the present study, a model for alkalinity of the pore solution was devised based on phase composition of cement hydrates with SCMs. Firstly, a model to calculate the alkalinity of the pore solution was proposed based on the phase composition of cement hydrates and its equi-librium with alkali concentration in solution. To charac-terize the phase composition of cement hydrates, a combination of X-Ray diffraction (XRD)/Rietveld analysis and selective dissolution method was used to measure the degree of hydration of cement and SCMs. XRD-based technology has enabled evaluation of phase composition of cement hydrates (Scrivener et al. 2004; Snellings et al. 2014a, 2014b; Maruyama and Igarashi 2014). Moreover, the combination of XRD/Rietveld analysis and selective dissolution is a promising method when SCMs are added to the mix (Hoshino et al. 2006; Brunet et al. 2010). In parallel, accelerated mortar bar tests and concrete tests were also performed to evaluate

the suppressing effect of SCMs on ASR expansion. The test results were compared, and the relationship between the alkalinity of the pore solution calculated by the phase composition of cement hydrates and the suppressing effect on ASR expansion was discussed. Further, apart from the experimental data, the concept of “available alkali conversion” was proposed and was applied to ASR preventive measures specified in CSA A23.2-27A and its validity was shown.

2. Model for alkalinity of pore solution

The replacement of cement by SCMs drastically reduces the alkalinity of the pore solution. For FA, Shehata et al. (1999) carried out numerous experiments to evaluate the relationship between the alkalinity of pore solution and ASR expansion of concrete. There was a positive linear relationship between them except for alkali-rich FA. This tendency has been known in other studies (Kawamura and Takemoto 1988). The mechanism of reduction in alkalinity of pore solution can be attributed to the dilu-tion effect caused by the reduction of cement clinker and by sorption of alkalis by C-S-H gel. In the present study, sorption of alkalis by C-S-H gel is modeled from the perspective of the phase composition of cement hydrates and its equilibrium with alkali concentration in the solu-tion.

A closed system in which there is no external supply of alkalis is considered. Total alkali content per volume unit of cement paste, Calkali, can be expressed by equation (1).

alkali s CSH l fwC R C R C= + (1)

where, Calkali is the total alkali content per volume unit of cement paste (mmol/m3) CCSH is the C-S-H amount per volume unit of cement paste (g/m3), Cfw is the free water per volume unit of cement paste (ml/m3), Rs is the alkali content in solid C-S-H (mmol/g) and Rl is the alkali concentration in solution (mmol/ml).

The relationship between Ca/Si molar ratio and alkali sorption of C-S-H gel was modeled in order to evaluate the alkalinity of pore solution. There are some references related to alkali sorption of C-S-H gel (Stade 1989; Hong and Glasser 1999; Bach et al. 2013). Among them Hong and Glasser (1999) performed an experiment for C-S-H gel with different Ca/Si molar ratios from 0.85 to 1.8 and different concentrations of alkaline solution (1 – 300 mmol/l). The distribution ratio, Rd, can be written as equation (2).

/d s lR R R= (2)

where, Rd is the distribution ratio (ml/g). The test result in the case of 300 mmol/l of NaOH

solution by Hong and Glasser (1999) is shown in Fig. 1. Equation (3) was fitted to the test data by the least squares method to formulate the distribution ratio with Ca/Si molar ratio of C-S-H gel.

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( / )dR Ca Si βα= (3)

From the regression curve shown in Fig. 1, α and β can be calculated as 2.5 and -3.1, respectively. For simplicity, the physical properties such as specific surface area (SSA) and density of C-S-H gel formed in the paste were assumed to be the same as the synthesized one.

When the interaction of alkalis between solids and liquids achieves equilibrium, the alkalinity in the pore solution, Rl , can be obtained from equations (1) - (3). Here, a simply calculated result is shown in Fig. 2. The parameters for the calculation are as follows: W/C = 0.50, density of cement = 3.16 g/cm3, Ca/Si molar ratio = 2.21, CCSH = 0.79×106 g/m3 and Cfw = 0.27×106 ml/m3. It was assumed that the phase composition does not change when changing the alkali content of cement. The calcu-lated line from Helmuth (1993) is also described for comparison. Helmuth (1993) formulated the relation between alkali content of cement and OH- concentration in pore solution in the following equation (4).

2[ ] 0.34 /( / ) 0.02 0.06eqOH Na O W C− = + ± (4)

where, [OH-] is the hydroxide ion concentration in the

pore solution (mol/l), Na2Oeq is the equivalent alkali content of the cement and W/C is the water-to-cement ratio.

According to Fig. 2, the calculated value in this study was 20 % larger than that of Helmuth. The discrepancy could be attributed to the difference in experimental conditions such as solid-liquid ratio, temperature and the texture of C-S-H gel. The regression line for this study gives following equation (5).

2[ ] 0.89 eqOH Na O− = (5)

As described above, in order to calculate the alkalinity of the pore solution based on the phase composition of cement hydrates, it is necessary to estimate three essen-tial parameters at a given total alkali content: (1) Ca/Si molar ratio; (2) C-S-H amount and (3) free water amount. These parameters can be measured by using XRD/Rietveld analysis combined with the selective dissolution method. The details are described in the fol-lowing chapter.

3. Experiments

In order to validate the proposed quantitative method, the experimental program shown in Fig. 3 was carried out. In order to evaluate the phase composition of cement hy-drates, phase composition analysis based on XRD/Rietveld analysis and selective dissolution was performed. From the results, OH- concentration in pore solution was estimated. In parallel, accelerated mortar bars tests and concrete expansion tests were performed and the suppressing effect of SCM was compared with the estimated OH- concentration. The details are de-scribed in the following. 3.1 Materials The chemical composition and physical properties (den-sity, Blaine SSA) of the materials used in the experiments are shown in Table 1. Ordinary Portland cement (OPC) with an alkali content of 0.62 wt% was used as the ref-erence material. The mineral composition of the cement analyzed with XRD/Rietveld analysis is shown in Table 2.

Four types of fly ashes (FA) and one blast furnace slag (BFS) were prepared as SCMs; they were collected in Japan. The mineral composition of FAs based on XRD/Rietveld analysis according to Yamashita et al. (2007) is shown in Table 3. Japanese BFS contains over 95% glass phase so that the BFS used was assumed to consist of glass phase only. The glass content of the FA was calculated from the determined mineral composition. The chemical composition of the glass phase was cal-culated by subtracting the chemical composition of the crystalline phase from the bulk chemical composition determined by X-ray fluorescence (XRF). SO3 was as-sumed to exist in the form of anhydrite. Unburnt carbon was also calculated by subtracting the bound water of the crystalline phase and the decarbonation of calcite from loss on ignition (LOI). The chemical composition of the

0

0.5

1

1.5

2

0.5 1 1.5 2

OH

- con

cent

ratio

n (m

ol/l)

alkali content of cement (wt%)

this studyHelmuth

W/C=0.50

[OH-]=0.89Na2Oeq

[OH-]=0.34Na2Oeq/(W/C)+0.02±0.06

Fig. 2 Alkali content of cement vs. calculated OH- con-centration (W/C=0.50).

0

1

2

3

4

0.5 1 1.5 2 2.5

dist

ribut

ion

ratio

R d (m

l/g)

Ca/Si molar ratio of C-S-H gel

Experimentaldata byHong& Glasser (1999)(300mmol/l)

Rd=2.5(Ca/Si)-3.1

Fig. 1 Ca/Si molar ratio of C-S-H gel vs. distribution ratio (adapted from Hong and Glasser (1999)).

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glass phase in FA is shown in Table 4. The chemical composition of glass was normalized to 100 %. In the XRD/Rietveld analysis, alkali sulfate could not be iden-tified. To confirm the phase of alkalis, water-soluble alkali content in FA was measured according to JCAS I-04 (Japan Cement Association Standard 2004, solid/liquid = 10, duration time = 10 min. with shaking). The result showed that water-soluble alkali could not be detected (< 0.01%) in each FA so that the alkalis can be assumed to exist in the glass phase of FA.

The results of fly ash characterization are as follows: (1) The glass content and the alumina in the glass of “FA(B)” are significantly less than those in the other types of FA due to much mullite precipitation; (2) “FA(C)” contains approximately 14 % CaO in the glass phase though the glass content is high; (3) The chemi-cal/physical properties of “FA(A)” and “FA(D)” are almost the same while the glass content of “FA(A)” is 10 % higher than that of “FA(D)”; (4) As for FAs used in this study the Blaine SSA is almost equal.

Two types of reactive aggregate were used for accel-erated mortar bar test and concrete test. For the acceler-ated mortar bars test, highly reactive artificial glass was used. For the concrete test, highly reactive andesite was

used, which is collected from the Hokkaido area con-taining reactive cristobalite.

3.2 Mixture proportions 3.2.1 Cement paste Cement paste was prepared with a water-to-cement ratio of 0.50. Sodium hydroxide solution was added to the cement so that the Na2Oeq of the cement was boosted up to 1.20 wt%. Replacement levels of FA were 10, 20 and 30 % by volume of cement whereas those of BFS were 40, 50 and 60 % by volume. Replacement of SCMs by volume of cement was intended not to change the binder volume.

In SCM-replaced paste, sodium hydroxide solution was added to boost the total alkali content in SCM-replaced paste equal to that of the paste without SCM (control specimen), with a consideration of de-creased total alkali content in the paste due to cement reduction. Therefore the dilution effect of cement can be negligible because the total alkali content was kept con-stant in all mixtures. However, in the mixture design of the total alkali content in the paste, the alkali content in SCMs was not included, because the amount of released alkali from SCMs was not clear at this step. After per-forming phase composition analysis, the amount of alkali

Table 3 Mineralogy of fly ash. FA(A) FA(B) FA(C) FA(D)

Quartz 7.6 18.6 5.9 10.2Mullite 15.6 37.9 9.0 21.2Magnetite 0.3 0.3 0.5 0.6Hematite 0.3 0.9 0.5 0.4Lime 0.1 0.0 0.4 0.3Portlandite 0.1 0.0 2.0 0.3Calcite 0.2 0.0 0.0 0.0Periclase 0.2 0.0 0.3 0.3Rutile 0.0 0.5 0.0 0.1Glass 74.1 40.2 80.3 64.4Unburnt carbon & moisuture 1.4 1.6 1.1 2.2Total 100.0 100.0 100.0 100.0

Table 1 Chemical composition and physical properties of materials. FA OPC FA(A) FA(B) FA(C) FA(D) BFS

LOI 0.60 1.50 1.58 1.56 2.25 0.11 SiO2 21.79 60.17 61.16 51.96 59.05 33.64 Al2O3 4.98 22.24 29.69 20.09 23.72 14.97 Fe2O3 2.91 4.29 2.70 5.85 4.33 0.17 CaO 65.23 5.80 0.75 13.78 5.12 42.83 MgO 1.21 1.66 0.59 2.17 1.36 5.71 Na2O 0.31 0.48 0.18 0.78 0.44 0.31 K2O 0.47 0.97 0.34 1.22 0.87 0.24 SO3 1.72 0.47 0.00 0.75 0.30 0.05 P2O5 - 0.81 0.42 0.59 0.72 0.01

Chemical composition (%)

Total 99.22 98.39 97.41 98.75 98.16 98.04density (g/cm3) 3.16 2.3 2.26 2.41 2.24 2.91 Blaine SSA (cm2/g) 3280 3910 4510 4100 3860 3850

Table 4 Chemical compositions in glass phase of fly ash (normalized to 100%).

FA(A) FA(B) FA(C) FA(D) SiO2 65.59 79.65 55.11 66.99 TiO2 2.34 4.63 1.58 2.80 Al2O3 15.07 6.27 17.25 13.24 Fe2O3 4.97 3.74 6.13 5.18 MgO 1.93 1.48 2.33 1.68 CaO 7.01 1.88 14.32 6.94 Na2O 0.65 0.45 0.99 0.69 K2O 1.32 0.85 1.54 1.36 P2O5 1.10 1.05 0.75 1.13 Total 100.0 100.0 100.0 100.0

Table 2 Mineralogy of cement. C3S C2S C3A C4AF Bassanite Gypsum Calcite Periclase Total 57.2 17.6 6.6 10.9 1.9 1.4 4.2 0.3 100.0

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released from SCM can be calculated, as described in section 4.3.

Hereafter, the mixture proportion of paste was ex-pressed as “SCM type - replacement level”.

3.2.2 Mortar Mortar without SCMs (Control specimen) was mixed so that the water-to-cement ratio and sand-to-cement ratio were 0.50 and 2.25, respectively. A sodium hydroxide solution was added so that the Na2Oeq of the cement was boosted up to 1.2 wt%. The replacement level of SCM and calculation of total alkali content were set to be the same as for the cement paste. Therefore the amounts of aggregate and water per unit mortar were kept constant in all mix proportions. Also, the total alkali content per unit mortar was the same in all the mixture proportions of mortar. The expression of the mixture proportion of mortar was same as the cement paste (“SCM type - re-placement level”). 3.2.3 Concrete The water-to-cement ratio was set to be 0.50. Three types of fly ash (FA(A), FA(B) and FA(D)) replaced 15 wt% of the cement. The total alkali content of the concrete mix was boosted up to 6.0 and 8.0 kg/m3 with sodium chlo-ride solution. Although the alkalinity of pore solution boosted with sodium chloride would be relatively lower than that with sodium hydroxide (85 - 90 %) (Kawabata et al. 2007), the comparison of the effect of SCMs is possible. Hereafter, the mixture proportion of concrete was expressed as “SCM type - replacement level-total alkali content”.

3.3 Phase composition analysis of cement paste 3.3.1 Cement paste mixing Cement paste was mixed for 3 min in a planetary mixer. After one day of hydration, the mixed paste was stored in sealed containers at 40 °C for 91 days. 3.3.2 Sample preparation for analysis At each age, hydration was stopped by immersing the crushed paste (under 20mm) in acetone and storing the specimen under relative humidity (R.H.) of 11 % at am-bient temperature in N2 gas purged chamber for 7 days before being analyzed. After drying, the sample was ground so as to obtain a powder passing through a 150 μm sieve. The powder was used for the following analysis. 3.3.3 Loss on ignition (LOI) LOI was quantified to measure the weight loss after heating at 1000 °C for 1 hour. In the BFS-replaced sys-tem, maximum temperature was set to 850 °C in order to avoid devitrification of unhydrated BFS. 3.3.4 XRD/Rietveld analysis XRD measurement and Rietveld analysis were carried out according to Hoshino et al. (2006). The sample measured by XRD was prepared by mixing a paste for which hydration was stopped by immersing it in a large volume of acetone and 10 wt% α-Al2O3 (corundum) with an average particle size of 0.4 μm. D8 ADVANCE X-ray diffractometer (Bruker AXS) was used for analysis. The measurement conditions for XRD were a tube voltage of 50 kV, tube current of 250 mA, scan range of 2θ = 5–65 °, step width of 0.02 ° and scan speed of 2 °/min. TOPAS

Cement paste casting(Na2Oeq=1.2%

of cement)

Mortar casting(Na2Oeq=1.2%

of cement)

Concrete casting(Na2Oeq=6.0, 8.0kg/m3)

Acecelerated curing(40 °C , R.H. >95%)

Acecelerated mortarbars test

(40 °C , R.H. >95%, wrapping)

Phase compositionanalysis

Expansion ratio

1day:demolding

Pre-curing(20 °C , R.H. >95%,

wrapping)

Concrete test(40 °C , R.H. >95%,

wrapping)

1day: demolding

1day:demolding

Estimated alkalinity of pore solution

Asymptotic ultimate expansion(fitting by Larive ’s law)

Cement paste Mortar Concrete

Validation of the proposed model with expansion tests

91days

91days

28days

600-650 days

Fig. 3 Experimental program.

Y. Kawabata and K. Yamada / Journal of Advanced Concrete Technology Vol. 13, 538-553, 2015 543

software (Bruker AXS) was used for Rietveld analysis. The quantified phases were calcite (CC), gypsum (Gy), bassanite (Ba), portlandite (CH), ettringite (Ett), mono-sulfate (Ms), monocarbonate (Mc), and hemicarbonate (Hc) depending on the sample, as well as basic minerals such as alite (C3S), belite (C2S), cubic C3A, orthorhom-bic C3A, and C4AF. The amount of C3A was calculated as the sum of both phases detected by Rietveld analysis.

Structural models for clinker minerals (C3S, C2S, cu-bic C3A, orthorhombic C3A and C4AF) were taken from Stutzman and Leigh (2002). Those for CC, Gy, Ba, CH, Ett, Ms, Mc and corundum were taken from the ICSD database (Inorganic Crystal Structure Database 2006). Since there is no refined structural model for Hc, the data of a previous study were used (Hoshino et al. 2006).

The amount of amorphous phase was calculated by equation (6):

{ }100 ( ) / (100 ) /100G A R A R= × − × − (6)

where G is the amount of amorphous phase (wt%), R is the added ratio of corundum (wt%), and A is the meas-ured value of corundum (wt%). As the SCMs contain a large amount of amorphous phase, the amount of C-S-H gel was obtained by subtracting the unhydrated glass amount in SCMs from the calculated amorphous phase amount. The unhydrated glass amount in SCMs was measured with selective dissolution described in the next section.

The hydration ratio of each mineral, Rx , was calcu-lated by equation (7):

{ }100 /[ (100 ) /100] 100x x xR H C LOI= − × − × (7)

where Cx is the initial amount in unhydrated cement (wt%), Hx is the quantified amount of mineral after hy-dration (wt%), and LOI is the loss on ignition (wt%).

In this study, the difference in the mean Ca/Si molar ratio for each mix proportion was compared. In future, the absolute value should be confirmed by comparing some appropriate direct measurement methods such as TEM/EDS or SEM/EDS. The Ca/Si molar ratio of C-S-H gel is thought to vary within a sample in μm order. Es-pecially, around siliceous SCM particles, although a lower Ca/Si molar ratio is expected at least in 1 μm order (Hashimoto and Torii 2013), SEM/EDS has a limitation for the spatial resolution to evaluate this kind of transi-tion of chemical composition. For the purpose of this study, the estimation of solution chemistry is based on the dissolution equilibrium and the mean Ca/Si molar ratio of C-S-H gel is important to know. Therefore, al-though the accuracy of absolute values of Ca/Si molar ratio of C-S-H gel has a certain inaccuracy, it is mean-ingful to adopt this method described above for the comparative discussion of the effect of SCMs on alka-linity of the pore solution. Judging from the continuous change of mean Ca/Si molar ratio when the amount of SCM was changed, it is reasonable to assume that the method has appropriate repeatability.

3.3.5 Selective dissolution In order to measure the hydrationdegree of FA, the

selective dissolution method using 2 mmol/l HCl and 5% Na2CO3 solution was used (Ohsawa et al. 1999). For measurement of the hydrationdegree of BFS, the sali-cylic acid – acetone – methanol method was used to quantify the amount of BFS (Kondo and Ohsawa 1968; Luke and Glasser 1987).

3.4 Accelerated mortar bars test 24 h after casting, specimens were demolded and the initial length was measured immediately and carefully. Thereafter, specimens were stored in a humid chamber at 40 °C and R.H. > 95 %. Change in length of mortar was measured using a dial gauge.

The dimensions of the mortar specimens were 40×40×160 mm. The mortar specimens were small so that alkali leaching is a concern (Rivard et al. 2007). It remains controversial how to minimize alkali leaching (Lindgård et al. 2012). When concrete specimens are stored over water in a sealed container, a significant amount of alkalis is leached from the specimens (Rivard et al. 2003). This is because water condensation is gen-erated on the inner wall of container and water drops on dried concrete by a temperature difference between container and concrete specimens when the container is cooled before measurement. When the container is heated after measurement, water condensation is gener-ated on the concrete surface reducing alkali content. Consequently, the alkalis in the concrete specimens dif-fuse out of the specimens. To avoid alkali leaching, mortar and concrete specimens sometimes have been wrapped in a moist cotton cloth (Bérubé and Fournier 1993). Though a cloth-wrapped concrete specimen is also affected by alkali leaching, the amount of alkalis leached is limited compared to concrete specimens ex-posed to humid atmosphere without wrapping within a container (Yamada et al. 2014). This is because wrapping significantly reduces the extent of evaporation during cooling and measuring. Lindgård et al. (2013) also found that wrapping specimens in a cotton cloth increases early-age alkali leaching, but the amount of leached alkali is less than that without wrapping over the long term.

3.5 Concrete test At 24 h after casting, specimens were demolded and cured in a moist cloth at 20 °C for 28 days. Then the initial length was measured immediately and carefully. Thereafter, the specimens were stored in the accelerated conditions (40 °C, R.H. > 95%). Additionally, the specimens were wrapped in a moist cloth. Change in length of concrete was measured using a contact gauge. Cylinder specimens with 100 mm in diameter and 200 mm in length were used for the test. Expansion tests were carried out for approximately 600 - 650 days.

In the concrete test additional alkali leaching in the pre-curing can also be a concern. Diffusion of alkali from

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concrete specimens into a moist cloth can occur. How-ever there was no water condensation during pre-curing. Therefore, the amount of leached alkalis was thought to be low although it was not measured.

4. Results

4.1 Hydration degree of cement and SCMs The hydration degrees of cement minerals and SCMs at 91 days are shown in Fig. 4. In the FA(A)-replaced sys-tem, the replacement of FA decreased the hydration de-gree of C2S while other minerals were almost the same as non-replaced control specimen. Sakai et al. (2005) re-ported that the hydration of C2S is retarded at long-term though it is accelerated at the early age when cement is replaced by FA. The result of this study well agreed with Sakai et al. (2005) though the temperature is different. High replacement of cement by FA(A) also decreased the hydration of FA(A). This tendency was in accordance with Hanehara et al. (2001). This mechanism would be attributed to reduced alkalinity of pore solution, resulting in decreased dissolution of glass phase of FA. Hydration degree of FA was accelerated by high temperature and additional NaOH.

At a given replacement level of FA, C3S, C3A and C4AF showed almost the same hydration degrees in all the systems, while the hydration degrees of C2S and FA differed. FA(C), which has a moderate CaO content (13.8 wt%), showed a low hydration degree. According to Sakai et al. (2005), the effect of glass content and ba-sicity of glass in FA does not significantly affect the hydration of FA. The results showing that the hydration degrees of FA were almost the same in low-Ca FAs

(FA(A), FA(B), FA(D)), supports Sakai et al. (2005), though it is not applicable to moderate-Ca FA. The pre-vious studies pointed out the importance of fineness of FA when cement is replaced by low to moderate-Ca FA (Obla et al. 2003; Nagataki et al. 1990; Kawabata et al. 2008). As four kinds of FAs were used in this study, with almost equal fineness, the effect of fineness on ASR expansion could hardly be distinguished.

The hydration degree of each mineral in the BFS-replaced system at 91 days of the accelerated period is also shown in Fig. 4. The higher the BFS replacement ratio, the lower the C2S and BFS hydration degrees. This tendency is also reported by Sagawa and Nawa (2006) and the mechanism would be almost the same as that of FA. It is interesting that C3A and C4AF showed a higher hydration degree with higher BFS replacement ratios, consistent with the results reported by other researches (Hoshino et al. 2006; Escalante-Garcia and Sharp 1998).

4.2 Phase composition of cement hydrates with SCMs Based on the combination of XRD/Rietveld analysis and the selective dissolution method, the phase composition of the hydrated systems were calculated. In the calcula-tion, the data in the previous literature was used to con-vert the mass fraction of each phase to the volume (Taylor 1997; Hoshino et al. 2006). Since the density of C-S-H gel, ρCSH (g/cm3), depends on its Ca/Si molar ratio, the following equation (8) was used for calculation, in which the density of C-S-H gel is derived from various Portland cements (Suda et al. 2010).

0.24 / 1.72CSH Ca Siρ = + (8)

Because of the mass balance between the reacted components and produced hydrates, Ca/Si molar ratio of C-S-H gel can be calculated. Chemical bound water of C-S-H gel is assumed to be 20%, independent of the Ca/Si molar ratio of C-S-H gel.

The phase composition of cement hydrates in all the systems is shown in Fig. 5. Figure 6 also shows the relationship between replacement level of SCM and Ca/Si molar ratio of C-S-H gel. The Ca/Si molar ratio of C-S-H gel of non-replaced control specimen was larger than that of the published data (e.g., Taylor 1997). The data of Hoshino et al. (2006), in which the procedures were the same as those of this paper, showed Ca/Si molar ratio of 1.83 at 28 days of standard curing (20 °C). The differences between the data of Hoshino et al. (2006) and this paper are mainly the addition of sodium hydroxide solution and curing temperature. High temperature in-creases the Ca/Si molar ratio of C-S-H gel (Lothenbach et al. 2007). Additionally, the presence of amorphous Ca(OH)2 can be pointed out (Bentur and Berger 1979).

In the FA(A)-replaced system, the amount of C-S-H gel slightly decreased with an increase in the replacement ratio of FA(A), while the CH content was significantly decreased. Pore volume also decreased with an increase in the replacement ratio of FA(A). HC can be detected at

0 20 40 60 80 100

BFS-60

BFS-50

BFS-40

FA(D)-20

FA(C)-20

FA(B)-20

FA(A)-30

FA(A)-20

FA(A)-10

OPC

hydration degree (%)

C3S C2S C3A C4AF FA BFS

Fig. 4 Hydration degrees of cement and SCMs (91days).

Y. Kawabata and K. Yamada / Journal of Advanced Concrete Technology Vol. 13, 538-553, 2015 545

the replacement level of more than 20 vol%. It should be noted that there was a drastic decrease in the Ca/Si molar ratio when cement was replaced by FA(A), as shown in Fig. 6.

At a given FA replacement level of 20 vol%, the effect of FA type on phase composition of cement hydrates can be compared. FA(A), which contains high glass and high silica in the glass phase, showed the lowest Ca/Si molar ratio of C-S-H gel. On the other hand, FA(B) showed the largest CH content and higher Ca/Si molar ratio than FA(A) and FA(D). The reason can be linked to the lower glass content of FA(B): that is, at a given hydration de-gree, the dissolved silica from low glass content FA is less than that of high glass content FA. Moderate-Ca FA, FA(C), showed the highest Ca/Si molar ratio but almost the same CH content as FA(A) and FA(D). This result shows that moderate-Ca FA forms C-S-H gel with higher Ca/Si molar ratio than low-Ca FA by consuming CH and calcium dissolved from the glass phase. The amount of C-S-H gel of FA(B)-20% was slightly less than that of other FAs because the hydration degree of FA was less.

In the BFS-replaced system, the amount of C-S-H gel significantly increased with increase in the replacement level of BFS. The volume increase with replacement levels of 40, 50 and 60 vol% was 19, 22 and 36 %, re-

spectively. The decrease in Ca/Si molar ratio, however, was less compared to that for FA since BFS contains a higher CaO content. The Ca/Si molar ratio of C-S-H gel for a BFS replacement level of 60 vol% was almost the same as that of a FA replacement level of 20 vol% though the amount of C-S-H gel of the former was significantly larger than the one of the latter.

4.3 Calculation of pore solution alkalinity As described above, in order to calculate the alkalinity of the pore solution, it is necessary to know the degree of hydration of all reactive components and to estimate three important parameters at a given total alkali content: (1) mean Ca/Si molar ratio; (2) C-S-H amount and (3) free water amount. Thanks to XRD/Rietveld analysis combined with the selective dissolution method, these parameters can be determined. Cfw can be calculated by subtracting the combined water amount in the crystalline phase from the initial free water amount.

The calculation procedures for alkalinity of pore so-lution are described in Fig. 7. In the calculation, the dilution effect due to a reduction in cement has not to be taken into account because the initial alkali content of cement paste was kept constant in all the mixtures at the

0% 20% 40% 60% 80% 100%

BFS-60%

BFS-50%

BFS-40%

FA(D)-20%

FA(C)-20%

FA(B)-20%

FA(A)-30%

FA(A)-20%

FA(A)-10%

OPC

volume fraction (%)

pore

C-S-H

AFt

Mc

Hc

CH

cement

FA

BFS

Fig. 5 Cement hydrates with and without SCMs.

(I) Initial alkali content calculation[alkali content of cement + NaOH solution]

(II) Determination of parameters- mean Ca/Si molar ratio- C-S-H amount- free water amount

(III) Released alkali content calculation[Hydration degree×alkali content of SCM]

(IV) Updating total alkali content[(I) + (III)]

(V) Estimation of OH- concentration of pore solution(II) & (IV) with Eqs. (1) - (3)

Dilution effect due to cement reductioncan be neglected

Fig. 7 Calculation procedures for alkalinity of pore solution.

1

1.2

1.4

1.6

1.8

2

2.2

2.4

0 20 40 60

Ca/

Si m

olar

ratio

SCM replacement level (vol%)

OPC FA(A)FA(B) FA(C)FA(D) BFS

BFS

FA(A)

Fig. 6 SCM replacement level vs. Ca/Si molar ratio of C-S-H gel.

Y. Kawabata and K. Yamada / Journal of Advanced Concrete Technology Vol. 13, 538-553, 2015 546

step of mixture designs (step I). Then the parameters for calculation was determined by phase composition (step II, Figs. 5 and 6). In contrast, alkalis are supplied from SCM during an accelerated period (step III, described in 3.2.1). The supplied alkali content was calculated by multiply-ing the hydration degree and alkali content in the glass phase. The individual alkali content released from SCM was calculated and added to the initial alkali content for each mixture proportion (step IV). As a result, the cal-culated alkali concentration released from SCM was 33 ± 13 mmol/l. Base on the steps II and IV, alkalinity of pore solution was estimated by using eqs. (1) – (3) (step V).

It should be noted that there are some calculation er-rors related to the assumption of density of C-S-H gel and free water amount. For the density of C-S-H gel, when density of C-S-H gel was assumed to be constant as 2.41 g/cm3 (Feldman and Ramachandran 1982), the calculated alkalinity of pore solution equals to 93.0 ± 5.0 % of the one calculated with equation (8). The more recent equa-tion (9) by Suda et al. (2015), in which various SCMs are covered, gives variations less than 98.5 ± 1.5% of ones with equation (8).

0.459 / 1.392CSH Ca Siρ = + (9)

As for free water amount, it can be also determined from the thermogravimetric (TG) analysis. The calcu-lated alkalinity using TG was 105 ± 5 % of the one cal-culated by the applied method. These data showed that the calculation errors derived from the assumptions for density of C-S-H gel and free water amount are small and the estimated alkalinity can be comparable.

The relation of the Ca/Si molar ratio of C-S-H gel to the calculated hydroxide ion concentration of pore solu-tion is shown in Fig. 8, as a positively linear curve, in-dependently of SCM type. This result indicates the im-portance of the Ca/Si molar ratio of C-S-H gel on the alkalinity of pore solution. According to the experiments by Hong and Glasser (1999) (see Fig. 1), Rd of C-S-H gel with a Ca/Si molar ratio of 1.2 is 4.2 times higher than that with a Ca/Si molar ratio of 1.8. Therefore, the de-crease in the Ca/Si molar ratio of C-S-H gel strongly affects the alkalinity of the pore solution more than the amount of C-S-H gel when SCMs replace cement.

The calculated alkalinity shown in Fig. 8 was re-plotted as the relationship between replacement level of SCMs and ratio of alkalinity reduction as shown in Fig. 9. The replacement level of SCMs is converted to weight percentages for the following discussions. For FA(A) and BFS, a linear relationship between the replacement level and the ratio of alkalinity reduction can be observed. Assuming that other FAs show similar tendency, the linear relationship gives the following equation (10).

100 ( ) /OH OPC SCM OPCR RL C C C× = × − (10)

where, ROH is the ratio of alkalinity reduction per unit replacement level of SCMs (%/wt%), RL is the re-placement level of SCMs (wt%), COPC is the alkalinity of non-replaced control specimen (mmol/l), and CSCM is the alkalinity of SCM-replaced paste (mmol/l).

The regression lines for each SCM are shown in the Fig. 9. A slope of the regression line, ROH, indicates an efficiency of SCM on a reduction in alkalinity by alkali sorption of C-S-H gel (Table 5). Comparing FA and BFS, a higher replacement level is required in the case of BFS to reduce alkalinity to a certain concentration. Moder-ate-Ca fly ash (FA(C)) and low-glass fly ash (FA(B)) also require a higher replacement level compared to siliceous and glassy fly ash (FA(A) and FA(D)).

4.4 Relation of phase composition to ASR ex-pansion of mortar The relationship between the replacement level of SCMs and expansion ratio of mortar at 91 days is shown in Fig. 10. Expansion ratio is defined as the ratio of the expan-sion of the mix to non-replaced control specimen. Ex-

0.2

0.4

0.6

0.8

1

1.2

1 1.5 2 2.5

calc

ulat

ed [O

H] (

mol

/l)

Ca/Si molar ratio of C-S-H gel

OPCFA(A)-10%FA(A)-20%FA(A)-30%FA(B)-20%FA(C)-20%FA(D)-20%BFS-40%BFS-50%BFS-60%

[OH-]=0.64Ca/Si-0.33

Fig. 8 Ca/Si molar ratio of C-S-H gel vs. calculated OH- concentration of pore solution.

Table 5 Ratio of alkalinity reduction per unit replacement level of SCM.

FA(A) FA(B) FA(C) FA(D) BFS

ROH 2.49 2.16 1.47 2.49 0.89

0

20

40

60

80

100

0 20 40 60

ratio

of a

lkal

inity

redu

ctio

n (%

)

SCM replacement level (wt%)

FA(A)FA(B)FA(C)FA(D)BFS

FA(A)&FA(D)

FA(C)

BFS

FA(B)

Fig. 9 SCM replacement level vs. ratio of alkalinity re-duction.

Y. Kawabata and K. Yamada / Journal of Advanced Concrete Technology Vol. 13, 538-553, 2015 547

pansion of non-replaced control specimen was 0.53% even at 91 days. The range of the three individual specimen measurements was less than 10% of the aver-age. In the FA(A)-replaced system, the expansion ratio significantly decreased with an increase in the replace-ment level by FA(A). Focusing on the influence of FA type at a replacement level of 20 vol%, the difference between tested FA types was 16%. FA(A) and FA(D), glassy and rich in silica, showed less expansion than low-glass FA (FA(B)) and moderate-Ca FA (FA(C)). It is known that the higher the CaO content of FA, the lower is the reduction in alkalinity of the pore solution (Shehata et al. 1999). Consequently, the suppressing effect of high CaO content FA decreases (Shehata and Thomas 2000). In this experiment, all the FAs had almost the same Blaine SSA, so the effect of SSA is insignificant, al-though the previous studies pointed out the importance of SSA and the silica content of glass in FA (Nagataki et al. 1990; Kawabata et al. 2008; Takahashi and Yamada 2012). Replacement by BFS also reduced the expansion ratio of the mortar. This was well consistent with the previous result (e.g., Thomas and Innis 1998). In Fig. 10, two regression lines for FA(A) and BFS were described. The gradient of the regression line for FA(A) was 2.8 times larger than that for BFS.

Assuming that the hydration of cementitious materials in mortar is the same as the one in cement paste, the expansion ratio of mortar and the calculated alkalinity of pore solution in cement paste can be compared. The relationship between calculated alkalinity of the pore solution and expansion ratio of mortars at 91 days is shown in Fig. 11. There was a strong and positive linear relationship irrespective of SCM types and replacement levels, suggesting the reduction in alkalinity of the pore solution by replacing cement by SCMs to be the main contributive mechanism to the suppression of ASR ex-pansion.

4.5 Relation of phase composition to ASR ex-pansion of concrete The expansion behaviors of concrete are shown in Fig. 12.

The range of the three individual specimen measure-ments was less than 20 % of the average. Because of the addition of excess alkalis, a significant expansion of concrete can be found even when SCMs replace cement. The regression curves are also described in Fig. 12. Equation (11) is the mathematical model proposed by the previous research (Larive 1998; Multon 2003; Poyet 2003) and the regression curve was fitted to the experi-mental results by the least square method.

{ } { }1 ( / ) 1 [ ( ) / ]t C L cexp t exp tε ε τ τ τ∞= − − + − − (11)

where, t : time (day) , tε : expansion at time t (%), ε∞ : asymptotic ultimate expansion of concrete (%), Cτ : characteristic time (day), Lτ : latency time (day).

The regression curve well fitted the experimental ex-pansion behavior. Fitted parameters are summarized in Table 6. The concrete test results also showed that the replacement of FA(A) led to lower ASR expansion. At a given FA replacement level, FA(A) and FA(B) showed the highest and the lowest reduced ASR expansion re-spectively. The test results for concrete expansion test were well consistent with the results of the mortars de-scribed above.

Here, the influence of phase composition on concrete expansion due to ASR was investigated. The replace-ment level of 20 vol% with FA(A) corresponds to ap-

Table 6 Fitted parameter by equation (10).

τ∞ Cτ Lτ (%) (day) (day) OPC-6.0 0.34 27 113 FA(A)-15-6.0 0.06 49 373 FA(B)-15-6.0 0.11 27 275 FA(C)-15-6.0 0.22 60 280 FA(D)-15-6.0 0.17 35 251 OPC-8.0 0.50 28 74 FA(A)-15-8.0 0.22 36 189 FA(B)-15-8.0 0.32 31 149 FA(C)-15-8.0 0.30 44 171 FA(D)-15-8.0 0.28 45 173

0

0.5

1

1.5

0.2 0.4 0.6 0.8 1 1.2

expa

nsio

n ra

tio

calculated [OH] (mol/l)

OPC FA(A)-10%FA(A)-20% FA(A)-30%FA(B)-20% FA(C)-20%FA(D)-20% BFS-40%BFS-50% BFS-60%

Fig. 11 Calculated OH- concentration of pore solution vs. expansion ratio of mortar.

0

0.2

0.4

0.6

0.8

1

1.2

0 20 40 60 80

expa

nsio

n ra

tio

SCM replacement level (vol%)

OPCFA(A)FA(B)FA(C)FA(D)BFSBFS

FA(A)

Fig. 10 SCM replacement level vs. expansion ratio of mortar at 91 days.

Y. Kawabata and K. Yamada / Journal of Advanced Concrete Technology Vol. 13, 538-553, 2015 548

proximately 15 wt% which depends on the density of FA. Therefore, the calculation was carried out by using the phase composition of cement paste described in the pre-vious section as it is, but total alkali content was changed. It should be noted that water-to-cement ratios were dif-ferent.

There was a positive linear relationship between cal-culated alkalinity of the pore solution in concrete at 91 days and asymptotic ultimate expansion of concrete as shown in Fig. 13. The higher the alkalinity of the pore solution, the higher was the asymptotic ultimate expan-sion of the concrete. The expansion of concrete mixed with FA(D) was larger than that with FA(A) though the suppressing effect of FA(D) was the same as the one of FA(A) when they were used in the mortar test. There is a possibility that the pre-curing might influence the poz-zolanic activity of FA, resulting in the suppressing effect. The expansion test result for concrete agreed with that of the accelerated mortar bar test in which reduced alkalin-ity due to alkali sorption by C-S-H gel with low Ca/Si molar ratio significantly decreased ASR expansion of mortar and concrete.

5. Discussion

The results of the accelerated mortar bar tests and con-crete tests strongly support that the suppressing mecha-nism of SCM in regard to ASR expansion is attributed to the reduction in alkalinity of the pore solution. Though the accuracy of estimation of alkalinity and Ca/Si molar ratio remains controversial, the results agreed well with previous studies (e.g., Thomas 2011). As mentioned above, it is well accepted that replacement of cement with SCMs decreases the alkalinity of the pore solution. Numerous experiments support this estimation (e.g., Diamond 1981; Canham et al. 1987; Kawamura and Takemoto 1988; Duchesne and Bérubé 1994b; Shehata et al. 1999). Therefore, it is believed that formation of C-S-H gel with a low Ca/Si molar ratio contributes to the reduction in alkalinity due to alkali sorption of C-S-H gel, which strongly affects ASR expansion. It has been dif-ficult, however, to evaluate these effects quantitatively. This is an important point of this study to link ASR ex-pansion and phase composition of cement hydrates. In the phase composition, the Ca/Si molar ratio and the amount of C-S-H gel are supposed to be the important factors in evaluating the suppressing effect of SCMs on ASR expansion. The results of this study theoretically supported our empirical understanding.

It is necessary to validate this model for varying SCMs, especially for high-alkali SCMs, because only five SCMs were selected in this study. According to Shehata and Thomas (2000), the high-alkali fly ashes containing above 3.0 wt% show significant increases in the alkalin-ity of pore solution but they do not increase ASR ex-pansion. Although controversy remains in the under-standing of the mechanisms, the results in this study would be applicable to low alkali and low-to-moderate

Ca FA. The results of this study also put forward the possibil-

ity of an “available alkali conversion concept” in which the replacement level of SCMs can be converted to the equivalent total alkali content in concrete without SCMs for ASR expansion. Alkalinity of pore solution of con-

0

0.2

0.4

0.6

0.8

0.5 1 1.5 2 2.5

expa

nsio

n (%

)

calculated [OH] at 91 days (mol/l)

OPC-6.0 OPC-8.0FA(A)-15-6.0 FA(A)-15-8.0FA(B)-15-6.0 FA(B)-15-8.0FA(C)-15-6.0 FA(C)-15-8.0FA(D)-15-6.0 FA(D)-15-8.0

Fig. 13 Calculated OH- concentration of pore solution vs. asymptotic ultimate expansion of concrete.

0

0.1

0.2

0.3

0.4

0.5

0 150 300 450 600 750

expa

nsio

n (%

)

accelerated period (days)

OPC-6.0 FA(A)-15-6.0FA(B)-15-6.0 FA(C)-15-6.0FA(D)-15-6.0

(a) Total alkali content = 6.0 kg/m3

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 150 300 450 600 750

expa

nsio

n (%

)

accelerated period (days)

OPC-8.0 FA(A)-15-8.0FA(B)-15-8.0 FA(C)-15-8.0FA(D)-15-8.0

(b) Total alkali content = 8.0 kg/m3

Fig. 12 Expansion behavior of concrete.

Y. Kawabata and K. Yamada / Journal of Advanced Concrete Technology Vol. 13, 538-553, 2015 549

crete calculated with the proposed method has a roughly linear correlation with the total alkali content in concrete as described before. Therefore the reduction in alkalinity by replacing cement with SCMs can be thought to be equal to a decrease in the total alkali content in concrete. Considering that the suppressing effect of SCMs on ASR expansion is attributable to the reduction of the alkalinity of the pore solution, the amount of reduction can be calculated with the model proposed in this study. Taking into account the dilution effect of cement content will further reduce alkalinity.

When the ratio of alkalinity reduction linearly depends on the replacement level of SCMs (Fig. 9), the available total alkali content in concrete, with considering the alkali sorption and dilution effect, can be expressed in the following equation (12).

(100 )(100 ) /10000SCM OPC OHT T RL RL R= × − − × (12)

where, TSCM is the converted available total alkali content of SCM-replaced concrete (kg/m3), TOPC is the total al-kali content of control concrete specimen (kg/m3).

The replacement level of SCMs used in this study and the available total alkali content in concrete in the case of a total alkali content of 5.25 kg/m3 is shown in Fig. 14. The available total alkali content differs by type of SCM. When FA(A) replaces 20 wt% of cement in concrete having 5.25 kg/m3, the available total alkali content of concrete can be equal to the concrete with total alkali content of 2.1 kg/m3. In the case of BFS, in contrast, the replacement level of 40 wt% is necessary to attain such available total alkali content of concrete.

Assuming that the degree of hydration is the same, though only the data for 91 days of accelerated period are available, the calculated values were validated with CSA A23.2-27A (2009). CSA A23.2-27A has 6 levels of pre-vention against ASR, and the possible measures for re-quired level of prevention should be identified. Among the 6 levels of prevention, 4 levels (W, X, Y, and Z) re-quire either limiting the total alkali content up to a certain value or the use of a sufficient amount of SCMs, except for rejection of the aggregate. These threshold values in CSA were decided from the numerous experiments and field experiences in Canada and North America

(Fournier and Bérubé 2000). As the proposed concept can convert the replacement level of SCMs to the total alkali content, the minimum replacement level of SCMs used in this study to satisfy each prevention level can be calculated (Fig. 14). The minimum replacement level for CSA A23.2-27A and the calculated result are summa-rized in Table 7. For the prevention level X, the mini-mum replacement level calculated by the proposed concept was 18-20 wt% for low-Ca FA, 26 wt % for moderate-Ca FA and 35 wt% for BFS whereas CSA A23.2-27A requires 20 wt% for low-Ca FA, 25 wt % for moderate-Ca FA and 35 wt% for BFS. Similarly, for the prevention levels X, Y and Z, the calculated minimum replacement level for each SCM can be compared with the requirement by CSA.

The calculated minimum replacement levels of SCMs used in this study are well consistent with those de-scribed in CSA A23.2-27A. The results supported the specifications in CSA A23.2-27A which are determined based on numerous field experiences and laboratory experiments. The agreement between the proposed model and the CSA specifications indicates that the available alkali conversion concept would be proven to be a promising way to estimate a sufficient replacement

0

1

2

3

4

5

6

0 20 40 60avai

labl

e to

tal a

lkal

i con

tent

(kg/

m3 )

SCM replacement level (wt%)

FA(A) & FA(D)FA(B)FA(C)BFS

BFSFA(A)&FA(D)

FA(C)

-alkalibinding & dilution effect-

FA(B)

Fig. 14 SCM replacement level vs. available total alkali content.

Table 7 Minimum replacement level of SCMs to satisfy the required prevention level. Prevention level

W X Y Z Total alkali content in concrete (kg/m3) 3.0 2.4 1.8 1.2 Minimum replacement level of SCMs

Na2Oeq<3.0%, CaO<8% FA 15 20 25 35 Na2Oeq<3.0%, CaO=8-20% FA 20 25 30 40

CSA A23.2-27A

Na2Oeq<1.0% BFS 25 35 50 60 Calculated minimum replacement level of SCMs

FA(A) 14 18 23 28 FA(B) 16 20 25 31

Na2Oeq<3.0 %, CaO<8%

FA(D) 14 18 23 28 Na2Oeq<3.0%, CaO=8-20% FA(C) 20 26 33 41

this study

Na2Oeq<1.0% BFS 26 35 44 56

Y. Kawabata and K. Yamada / Journal of Advanced Concrete Technology Vol. 13, 538-553, 2015 550

level of SCMs. This concept has an advantage that it enables to adopt both of preventive measures, limiting the total alkali content and the use of SCMs, in order to satisfy the required prevention level. When the preven-tion level Y is required for concrete, the required pre-vention level can be satisfied when the total alkali con-tent in concrete is limited to less than 3.0kg/m3 and the cement is replaced by SCMs as following replacement levels: 13-14 wt% for low-Ca FA; 19 wt% for moder-ate-Ca FA; and 24 wt% for BFS.

One concern to be raised, however, is the long-term behavior of alkalis fixed by C-S-H gel. The previous papers reported that the hydroxide ion concentration starts to increase slowly with time beyond 3 months in the case of cement paste with silica fume (Shehata and Thomas 2002; Duchesne and Bérubé 1994c; Lothenbach et al. 2011). This indicates the possibility that alkalis would be released from C-S-H gel in the long term.

Further work should be performed to evaluate the long-term behavior of alkalis in C-S-H gel and interac-tion between cement hydrates and alkalis supplied from external sources; those could provide more reliable re-placement levels with SCMs. This study assumed that there was no supply of alkalis from external sources such as aggregate (Bérubé et al. 2002; Constantiner and Diamond 2003; Bérubé and Fournier 2004), seawater (Kawamura et al. 1996; Shayan et al. 2010) and deicing salts (Katayama et al. 2004; Rangaraju et al. 2005; Diamond et al. 2006). The long-term expansion of ex-isting structures such as dams is attributed to release of alkalis from the aggregates in concrete (Sims et al. 2012). When alkalis are released by aggregates, it should be considered in the calculation, which will increase the required replacement level of SCMs. The amount of alkalis from external sources, however, remains contro-versial (Katayama et al. 2008; Laboratoire Central des Ponts et Chaussées 1994; Federal Highway Administra-tion 2012). The alkali recycling is also one of the con-cerns (Poyet et al. 2007; Katayama 2012).

6. Conclusion

A model to evaluate the alkalinity of the pore solution based on phase composition of cement hydrates with SCMs was proposed. Phase composition analysis was carried out by using XRD/Rietveld analysis and selective dissolution.

The ASR expansions of mortar and concrete having various kinds and replacement levels of SCMs had a strong correlation with the alkalinity of the pore solution calculated by the proposed model. The results indicated that the reduction in alkalinity of pore solution is the major suppressing effect of SCMs on ASR expansion. The Ca/Si molar ratio and amount of C-S-H gel are im-portant factors for alkali sorption which results in ASR suppression. When cement is replaced by FA at a suffi-cient replacement level, FA contributes to a decreased Ca/Si molar ratio of C-S-H gel while the C-S-H amount

is almost the same as that without SCMs. Replacement with BFS increases the C-S-H amount formed, although the Ca/Si molar ratio hardly decreases compared with that of the FA. Finally, the available alkali conversion concept was proposed and was validated by a compari-son with CSA A23.2-27A which is based on numerous field experiences in Canada. The calculated results were well consistent with the minimum replacement level of SCMs specified in CSA A23.2-27A. From the results, the proposed model was proven to be a promising way to estimate a sufficient replacement level of SCMs with respect to ASR prevention.

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