Construction and Building Materials 94 (2015) 28–34

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Construction and Building Materials

journal homepage: www.elsevier .com/locate /conbui ldmat

A study on pozzolanic reaction of fly ash cement paste activatedby an injection of alkali solution

http://dx.doi.org/10.1016/j.conbuildmat.2015.06.0460950-0618/� 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author.E-mail addresses: d123252@hiroshima-u.ac.jp (P.T. Bui), ogaway@hiroshima-u.

ac.jp (Y. Ogawa), nakarai@hiroshima-u.ac.jp (K. Nakarai), kkawai@hiroshima-u.ac.jp(K. Kawai).

Phuong Trinh Bui, Yuko Ogawa, Kenichiro Nakarai, Kenji Kawai ⇑Department of Civil and Environmental Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima, 739-8527 Japan

h i g h l i g h t s

�We investigate the pozzolanic reaction of fly ash activated by alkali injection.� Alkali injection increases the CH consumption by pozzolanic reaction.� Alkali activation for 1 month decreases the volume fraction of 20–330 nm pores.� Alkali activation for 1 month increases the volume fraction of 3–20 nm pores.� Alkali injection accelerates the pozzolanic reaction of the fly ash cement paste.

a r t i c l e i n f o

Article history:Received 24 February 2015Received in revised form 15 June 2015Accepted 22 June 2015Available online 1 July 2015

Keywords:Alkali activationPozzolanic reactionFly ash cement pasteCa(OH)2 contentCa(OH)2 consumptionPorosity

a b s t r a c t

The purpose of this study is to investigate the pozzolanic reaction of the fly ash cement paste by an injec-tion of an alkali solution after hardening. Cement pastes with 0%, 20%, and 40% of fly ash replacementratios were used. An NaOH or saturated Ca(OH)2 solution was injected into the paste through a syringe1 month after casting at normal temperature. In addition to the reduction of the Ca(OH)2 content, theconsumptions of Ca(OH)2 in the paste with 40% replacement of fly ash activated by NaOH and Ca(OH)2

solution were observed to be 2.6 and 4.5 times as large as that with 20% replacement of fly ash. It indi-cates the alkali solution accelerates the pozzolanic reaction, together with the promotion of cementhydration. The results of pore structure analysis also confirmed this activation. As a result, it can beconcluded that the 1-month alkali solution injection was effective in accelerating the pozzolanic reactionof the cement paste with 40% replacement of fly ash.

� 2015 Elsevier Ltd. All rights reserved.

1. Introduction

Fly ash concrete has been applied extensively in the buildingconstruction because of taking the advantage of the enhanceddurability, cost saving, and environmental protection [1,2].However, it was reported that the early age strength of the flyash concrete is one of its disadvantages due to the high replace-ment of cement with fly ash [3]. In addition, the slow pozzolanicreaction of fly ash is considered as one of factors which result inthe low early age strength of the fly ash concrete [3–6].

Many researchers have focused on investigating the mechanismof the pozzolanic reaction and exploring the reasons of this slowreaction [7–11]. It is known that the pozzolanic reaction of thefly ash cement paste is the chemical reaction between reactive

silica or alumina in the fly ash particles and calcium hydroxide(Ca(OH)2–CH) formed from cement hydration in the presence ofwater at normal temperature [7]. Therefore, the cross-linkedsilica-tetrahedra (or cross-linked alumina-tetrahedra) in fly ashhave to be broken so that the silica or alumina become reactive[9,10]. It was found that the disruption of these links occurs at apH of 12.5 of pore solution in the paste at the room temperature[9] and more than 13 at 20 �C in NaOH solution [10]. This resultsin considering the addition of an alkali solution in order to increasethe alkalinity of pore solution in the fly ash cement paste whichowns the less alkalinity [10].

Hence, many methods of alkali activation have been investi-gated for accelerating the pozzolanic reaction of the fly ash cementpaste [11–15]. However, this alkali activation in almost of theseresearches has been mainly carried out by adding an alkali solutionor mixture of many alkali solutions in the water mixing of the flyash cement paste which is cured at high temperature.

The aim of this paper is to investigate an influence of activationon the pozzolanic reaction of the fly ash cement paste at normal

P.T. Bui et al. / Construction and Building Materials 94 (2015) 28–34 29

temperature by an injection of an alkali solution 1 month aftercasting through a syringe. In order to evaluate this influence, aquantitative analysis of the Ca(OH)2 content and porosity of thefly ash cement paste was carried out by thermal gravimetric anal-ysis and mercury intrusion porosimetry.

2. Experiments

2.1. Materials

The cementitious materials selected in this study werehigh-early-strength Portland cement, and low calcium fly ashwhich met the standard values of type II per JIS A 6201 (fly ashfor concrete). The chemical composition and physical propertiesof these materials are shown in Table 1.

2.2. Preparation of paste specimens

Fly ash was used to replace high-early-strength Portlandcement at ratios of 0%, 20% and 40% by mass (hereafter, abbrevi-ated as FA0, FA20, and FA40, respectively). A water to binder ratioof 0.30 was kept constant. The pastes were mixed in a mechanicalmixer and cast in 40 mm cube molds, and then sealed byaluminum tape to prevent water loss as well as carbonation.After that, a syringe with a capacity of 1 ml, of which the plungerwas disconnected, was inserted in the specimen center (as shownin Fig. 1). All specimens were demolded 24 h after casting andcured in sealed condition at 20 �C.

2.3. Activation method

An activation was conducted by supplying an alkali solution1 month after casting through a syringe, which was prepared inthe Section 2.2. Two alkali activators, which were an NaOH solu-tion (pH = 13.0) and a saturated Ca(OH)2 solution (pH = 12.6), wereinjected naturally through the permeability of the pastes by them-selves. Additionally, water was also injected instead of an alkalisolution for the comparison.

2.4. Measurement of CH content

The CH content of the pastes at the age of 2 months was deter-mined by thermal gravimetry analysis (TGA), using the half sam-ples as shown in Fig. 1. The CH content of the sample was testedby each 4 mm-section from the surface to the point of the needle(as shown in Fig. 1). These samples were obtained in the form ofa powder by a driller. Then, they were soaked in ethanol for 24 hto stop the further hydration, and dried in a vacuum desiccatorfor 24 h before measuring the CH content. The rate of the

Table 1Chemical composition and physical properties of cementitious materials.

Chemical composition and physicalproperties

Cement Low calcium flyash

SiO2 (%) 20.30 57.7Fe2O3 (%) 2.71 5.43Al2O3 (%) 4.96 27.54CaO (%) 65.49 1.26MgO (%) 1.21 1.06SO3 (%) 2.98 0.36Na2O (%) 0.22 0.44K2O (%) 0.35 0.76Loss on ignition (%) 1.19 2.8Density (g/cm3) 3.14 2.21Blaine specific surface area (cm2/g) 4590 3290

temperature increase of TGA equipment was installed at20 �C/min up to 100 �C and kept at 100 �C for 30 min to removeevaporable water completely, and then at 20 �C/min up to1000 �C. The CH content was calculated based on the ignited massof the sample and the mass loss due to the dehydration of calciumhydroxide. This mass loss was picked up from differential thermalgravimetric (DTG) curves between the initial and final tempera-tures of the corresponding DTG peaks [16].

2.5. Measurement of porosity

The porosity of the pastes at the age of 2 months was measuredby mercury intrusion porosimeter (MIP). The samples for test rang-ing 2.5–5 mm in size were obtained by crushing and choosing thearea, which was around the position of the needle for the injection(as shown in Fig. 1). After that, these samples were soaked in etha-nol for 24 h to stop the further hydration and dried in a vacuumdesiccator for 24 h before MIP measurement. The MIP is built onthe mercury intruded into the cylindrical pores under the strictlycontrolled pressure. The diameter of the cylindrical pore intowhich the mercury has been intruded is calculated according tothe following Washburn equation [17].

D ¼ �4c cos h=P ð1Þ

where D: the diameter of pores (lm), c: the surface tension of mer-cury (assuming a value of 480 mN/m), h: the contact angle of mer-cury on the paste (taken as 140�), P: the pressure at which mercuryis intruded into the pore (mN/m2).

The MIP equipment used in this study has the maximum pres-sure of 414 � 106 N/m2. The porosity of the pastes at the age of2 months was measured at a diameter range of 3 nm–300 lm.

3. Results and discussions

3.1. Effect of fly ash

3.1.1. CH contentThe CH content of the control samples, which were the samples

without the injection of water or alkali solution, at each4 mm-section of the samples is shown in Fig. 2. It can be seen thatthe higher the replacement ratio of cement with fly ash, the lessthe CH content. This can be attributed to the high cement replace-ment with fly ash [3] and the consumption of CH by the pozzolanicreaction [6].

The consumption of CH by the pozzolanic reaction was calcu-lated according to the following equation [18]:

CHcons: ¼ CHPC ðc=ðc þ f ÞÞ � CHFC; ð2Þ

where CHcons. is the consumption of CH by the pozzolanic reaction(%), CHPC is the CH content in the plain cement paste – FA0 (%), CHFC

is the CH content in the fly ash cement paste – FA20 or FA40 (%),(c/(c + f)) is the mass ratio of cement in the mixture of cementand fly ash.

The consumption of CH by the pozzolanic reaction at each4 mm-section of the samples is shown in Fig. 3 left. It shows theconsumption of CH was increased significantly with the increasein the fly ash content (40% replacement of fly ash). This phe-nomenon was considered to be the result of the large quantity offly ash used to replace the cement, resulting in the consumptionof CH by the pozzolanic reaction. Comparing the consumption ofCH between FA20 and FA40, the normalization is also shown inFig. 3 right. It can be seen that the consumption of CH for FA40,which had 2 times as high the fly ash content as FA20, was lessthan 2 times of that for FA20. This may be due to the fact thatthe smaller Ca(OH)2 content in FA40 than that in FA20 (as shown

Fig. 1. Activation method and preparation of the samples for analyzing the Ca(OH)2 content.

Fig. 2. The CH content of the control samples (FA0, FA20, and FA40) at each 4 mm-section of the samples at the age of 2 months.

Fig. 4. The porosity of the control samples (FA0, FA20, and FA40) at the age of2 months.

30 P.T. Bui et al. / Construction and Building Materials 94 (2015) 28–34

in Fig. 2) is attributed to the pozzolanic reaction of fly ash partially.It also results in a lot of unreacted fly ash particles in FA40 remain-ing, and playing mainly as the space filler at the age of 2 months[5,6]. Therefore, the injection of the alkali solution at the earlyage would be effective in accelerating the pozzolanic reaction ofthe fly ash cement paste.

3.1.2. PorosityThe total pore volumes and the volumes of pores ranging 20–

330 nm and 3–20 nm in diameter of the control samples are shownin Fig. 4. It is known that the porosity of the fly ash cement paste iscoarser at the early ages than that of the cement paste [10]. Similar

Fig. 3. The consumption of CH by pozzolanic reaction (left) and its normalization (right)

tendency was also observed in the case of FA40. However, the totalpore volume of FA20 was smaller than that of FA0. It can beexplained by the fact that the use of 20% replacement of fly ashat the same water to binder ratio increased the water to cementratio; resulting in promoting the hydration extent of cement andfly ash which forms more hydration products filling up the poresof the paste [5,13]. Furthermore, the decreasing and increasing ten-dency of the volume ratio of pores ranging 20–330 nm and 3–20 nm in diameter relative to the total pores respectively are alsoreported when the pozzolanic reaction of fly ash occurs [19].These tendencies are also observed in the cases of FA20 andFA40 when compared with FA0.

of the control samples at each 4 mm-section of the samples at the age of 2 months.

Fig. 5. The CH content of the control and 1-month-water-injected samples withoutfly ash – FA0 at each 4 mm-section of the samples at the age of 2 months.

Fig. 6. The CH content of the control and 1-month-water-injected samples with20% replacement of fly ash – FA20 (top) and 40% replacement of fly ash – FA40(bottom) at each 4 mm-section of the samples at the age of 2 months.

Fig. 7. The consumption of CH by pozzolanic reaction (left) and its normalization (righneedle at the age of 2 months.

P.T. Bui et al. / Construction and Building Materials 94 (2015) 28–34 31

3.2. Effect of water injection

3.2.1. CH contentThe CH contents of the pastes, into which water were injected

1 month after casting, and their comparisons with the control sam-ples at each 4 mm-section of the samples are shown in Figs. 5 and6. In general, it can be found that the injection of water increasedthe CH content in some positions of the samples. It is evident thatthe water injection promoted the cement hydration of the pastewith and without the fly ash replacement.

The consumption of CH by the pozzolanic reaction of thewater-injected samples at the point of the needle was only consid-ered for the comparison with that of the control sample. It was alsocalculated according to the Eq. (2). The consumption of CH by thepozzolanic reaction of the control and water-injected samples atthis position is shown in Fig. 7 left. In the case of thewater-injected sample with 20% replacement of fly ash, the con-sumption of CH was smaller than that of the control sample. Thismay be explained by the fact that water promoted the cementhydration more in FA20. This is also compatible with the largerCH content of the water-injected sample with 20% replacementof fly ash than that of the control sample (Fig. 6 top). Thus, theseadditional hydration products covered the surfaces of the fly ashparticles with more extent, resulting in the postpone of the poz-zolanic reaction of fly ash particles in FA20 [20].

In the case of the water-injected sample with 40% replacementof fly ash, the consumption of CH for the water injection was largerthan that for the control sample. It indicates the pozzolanic reac-tion of FA40 was accelerated by the addition of water.Comparing the consumption of CH between FA20 and FA40, thenormalization is also shown in Fig. 7 right. It can be found thatthe consumption of CH for FA40, which had 2 times as high thefly ash content as FA20, was 1.9 times as much as that for FA20in the case of the water injection, and were more than that ofthe control sample. It indicates the water injection was effectivein accelerating the pozzolanic reaction in the case of FA40.

3.2.2. PorosityThe total pore volumes and the volumes of pores ranging 20–

330 nm and 3–20 nm in diameter of the 1-month-water-injectedsamples and their comparison with the control samples are shownin Fig. 8. The total pore volume of the 1-month-water-injectedsamples was smaller than that of the control samples in the casesof FA0 and FA40, except for FA20. It indicates the water injectionwas effective in improving the porosity of the paste by promotingthe cement hydration more.

In the case of FA20 into which water was injected, the lessdecreasing and increasing tendencies of the volume ratio of pores

t) of the control samples and 1-month-water-injected samples at the point of the

Fig. 8. The porosity of the control and 1-month-water-injected samples at the age of 2 months.

Fig. 9. The CH content of the control and 1-month-NaOH-activated samples (top)and -saturated-Ca(OH)2-activated samples (bottom) without fly ash – FA0 at each4 mm-section of the samples at the age of 2 months.

Fig. 10. The CH content of the control and 1-month-NaOH-activated samples (top)and -saturated-Ca(OH)2-activated samples (bottom) with 20% replacement of flyash – FA20 at each 4 mm-section of the samples at the age of 2 months.

32 P.T. Bui et al. / Construction and Building Materials 94 (2015) 28–34

ranging 20–330 nm and 3–20 nm respectively than that in the con-trol sample also confirms the delay of the pozzolanic reaction.Therefore, the total pore volume of the water-injected samplewas larger than that of the control sample.

3.3. Effect of alkali activation

3.3.1. CH contentThe CH contents of the plain cement pastes into which the alka-

line solutions were injected 1 month after casting and their com-parisons with the control samples are shown in Fig. 9. It can be

said that the presence of NaOH solution (pH = 13.0) or saturatedCa(OH)2 solution (pH = 12.6) caused the CH content to decreaseslightly 1 month after the injection. It indicates the alkali solutioninjection for the sample without fly ash played a negative roletoward the production of CH. This may be explained by the factthat it was difficult for the cement paste to release Ca2+ ion tothe outside due to the high alkali concentration of pore solutionsupplied from the injected alkali solution, resulting in limitingthe production of CH in the case of the plain cement paste. Thus,the CH content of the activated samples was smaller than that ofcontrol samples.

Fig. 11. The CH content of the control and 1-month-NaOH-activated samples (top)and -saturated-Ca(OH)2-activated samples (bottom) with 40% replacement of flyash – FA40 at each 4 mm-section of the samples at the age of 2 months.

P.T. Bui et al. / Construction and Building Materials 94 (2015) 28–34 33

The CH contents of the fly ash cement pastes into which thealkaline solutions were injected 1 month after casting and theircomparisons with the control samples are shown in Figs. 10 and11. It is noticed that the CH contents of the activated samples werelarger than that of the control samples in both of FA20 and FA40. Inthe case of FA40, however, the CH content of the NaOH-activatedsample was approximately equal to and that of thesaturated-Ca(OH)2-activated sample was smaller than that of thecontrol sample at the point of the needle (as shown in Fig. 11). Itindicates that the alkali solutions were actually effective on theacceleration of the pozzolanic reaction after the activation for1 month in FA40.

Similar to the water injection, the consumption of CH of thealkali-activated samples at the point of the needle was also consid-ered for the comparison with that of the control sample. It was alsocalculated according to the Eq. (2). The consumption of CH of thecontrol and alkali-activated samples at this position is shown inFig. 12 left. In the case of FA20, the consumption of CH for the

Fig. 12. The consumption of CH by pozzolanic reaction (left) and its normalization (righneedle at the age of 2 months.

alkali-activated sample was smaller than that for the controlsample, whereas in the case of FA40, it was larger. This tendencywas similar to that in the case of the water-injected sample asshown in Section 3.2.1.

In the case where the alkali solution was injected into FA40,however, the consumption of CH for the NaOH activation wasslightly smaller than that for control sample. Comparing the con-sumption of CH between FA20 and FA40, the normalization is alsoshown in Fig. 12 right. It can be found that the consumption of CHfor FA40, which had 2 times as high the fly ash content as FA20,was 2.6 and 4.5 times as much as that for FA20 in the case of theNaOH activation and saturated Ca(OH)2 activation respectively,and these consumptions were more than that of the control sampleand water-injected sample with the values of 1.6 and 1.9 timesrespectively (as shown in Fig. 7 right). It indicates the alkali activa-tion was more effective in accelerating the pozzolanic reactionthan the water injection in the case of FA40.

3.3.2. PorosityThe total pore volumes and the volumes of pores ranging

20–330 nm and 3–20 nm in diameter of the saturated-Ca(OH)2-activated samples and their comparison with the controlsamples are shown in Fig. 13. In the case of FA0, the total porevolume of the saturated-Ca(OH)2-activated samples was smallerthan that of control samples because the cement hydration waspromoted by water supplied from the alkali solution. In thecases of FA20 and FA40, the total pore volume of thesaturated-Ca(OH)2-activated samples was almost the same as thatof the control samples. Compared with the control sample, how-ever, the decreasing and increasing tendency of the volume ratioof pores ranging 20–330 nm and 3–20 nm in diameter relative tothe total pores respectively were observed in the case of FA40 intowhich the saturated Ca(OH)2 solution was injected. It demon-strates that the saturated Ca(OH)2 activation was effective in accel-erating the pozzolanic reaction in FA40. This is also compatiblewith the decrease of the CH content in the case of thesaturated-Ca(OH)2-activated paste with 40% replacement of flyash at the point of the needle (as shown in Fig. 11 bottom).

4. Conclusions

An activation by the injection of an alkali solution was investi-gated on the fly ash cement paste 1 month after casting at normaltemperature. In addition, water was injected into the paste insteadof an alkali solution for the reference. A quantitative analysis of theCa(OH)2 content and porosity were examined in order to evaluateits effect on the pozzolanic reaction of fly ash. The following con-clusions can be drawn:

t) of the control samples and 1-month-alkali-activated samples at the point of the

Fig. 13. The porosity of the control and 1-month-saturated-Ca(OH)2-activated samples at the age of 2 months.

34 P.T. Bui et al. / Construction and Building Materials 94 (2015) 28–34

1. The 1-month water injection was effective in promoting thecement hydration of the paste with or without fly ash.

2. The 1-month water or alkali injection improved the porosity ofthe plain cement paste by promoting the cement hydration,resulting in reducing the total pore volume.

3. In the case of the cement paste without fly ash, the presence ofNaOH or Ca(OH)2 solution caused the Ca(OH)2 content in thecement paste to decrease slightly. It indicates the negative roleof the alkali injection for the production of Ca(OH)2 at the earlyage.

4. In the case of the cement paste with 40% replacement of fly ash,the 1-month alkali activation not only decreased the Ca(OH)2

content but also increased the consumption of Ca(OH)2 by thepozzolanic reaction. In addition, the volume ratio of pores rang-ing 20–330 nm in diameter relative to the total pores of thepaste was decreased, while the volume ratio of pores ranging3–20 nm in diameter relative to the total pores of the pastewas increased. It demonstrates that the 1-month alkali solutioninjection was effective in accelerating the pozzolanic reaction ofthe high volume fly ash cement paste.

Acknowledgement

The authors would like to thank Mr. Yuhei Ito, a master student,for his help in carrying out some of the experiments.

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