The purpose of this study is to quantitatively evaluate the effects of steam curing and internal curing on contribution of fly ash to strength development of mortar by using cementing efficiency factor (k-value) that represents strength development performance as a binder of fly ash. In addition, the pozzolanic reaction of fly ash was evaluated from the viewpoint of calcium hydroxide consumption by using thermogravimetry and differential thermal analysis as well as the degree of fly ash reaction by using selective dissolution method. The result indicated that steam curing improved early compressive strength and internal curing improved compressive strength and k-value at all ages. Also, a linearrelationship between the degree of fly ash reaction and the k-value was shown regardless of the age and the replacement ratio of fly ash

Huynh, Phat Tan

Insako, Ryo

Kawai, Kenji

Miyoshi, Yuki

Ogawa, Yuko

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

https://doi.org/10.14311/APP.2022.33.0232Acta Polytechnica CTU Proceedings 33:232–237, 2022 © 2022 The Author(s). Licensed under a CC-BY 4.0 licencePublished by the Czech Technical University in PragueCONTRIBUTION OF FLY ASH TO MORTAR STRENGTHDEVELOPMENT UNDER STEAM AND INTERNAL CURINGRyo Insakoa, Yuki Miyoshia, Phat Tan Huynha,b,c, Yuko Ogawad,∗,Kenji Kawaida Hiroshima University, Graduate School of Engineering, 1-4-1 Kagamiyama, Higashi-Hiroshima, Hiroshima739-8527, Japanb Ho Chi Minh City University of Technology, Faculty of Civil Engineering, 268 Ly Thuong Kiet Street,District 10, Ho Chi Minh City, Vietnamc Vietnam National University Ho Chi Minh City, 2 Linh Trung Ward, Thu Duc City, Ho Chi Minh City,Vietnamd Hiroshima University, Graduate School of Advanced Science and Engineering, 1-4-1 Kagamiyama,Higashi-Hiroshima, Hiroshima 739-8527, Japan∗ corresponding author: ogaway@hiroshima-u.ac.jpAbstract.The purpose of this study is to quantitatively evaluate the effects of steam curing and internalcuring on contribution of fly ash to strength development of mortar by using cementing efficiency factor(k-value) that represents strength development performance as a binder of fly ash. In addition, thepozzolanic reaction of fly ash was evaluated from the viewpoint of calcium hydroxide consumption byusing thermogravimetry and differential thermal analysis as well as the degree of fly ash reaction byusing selective dissolution method. The result indicated that steam curing improved early compressivestrength and internal curing improved compressive strength and k-value at all ages. Also, a linearrelationship between the degree of fly ash reaction and the k-value was shown regardless of the ageand the replacement ratio of fly ash.Keywords: Cementing efficiency factor (k-value), fly ash, internal curing, steam curing, strengthdevelopment of mortar.1. IntroductionFly ash, a by-product of coal-fired power generation,has been commonly utilized as a supplementary ce-mentitious material in concrete to obtain advantagessuch as long-term strength enhancement and densi-fication of the microstructure. Although coal-firedpower plant slowly has been shut down in westernEurope concerning the environmental impact and thefly ash is decreasing, a certain amount of fly ash pro-duction is still produced and desired to be utilized.However, there is a concern that the early strengthdecreases due to replacing a part of cement with flyash, and most of fly ash is currently used as a cementraw material in Japan [1].Steam curing is one of the useful methods for im-proving the early strength of fly ash concrete, andsome properties of steam-cured fly ash concretes havebeen investigated. Yazici et al. reported that whensteam curing was performed, the long-term strengthof fly ash concrete was low compared with sealed cur-ing at 20 ◦C [2]. Meanwhile, pozzolanic reaction offly ash could continue when the steam-cured fly ashconcrete was cured in water after steam curing [3].On the other hand, there is an internal curingmethod as a long-term water supply method. Inter-nal curing promotes hydration of cement by supplyingwater from the inside by using internal curing mate-rials with high water absorption. Water-absorbingpolymer and artificial lightweight aggregate are usedas internal curing materials [4, 5]. In addition to thesetwo conventional internal curing materials, roof-tilewaste aggregate has been also investigated as an in-ternal curing material. Roof-tile is Japanese tradi-tional roofing material and sintered clay at high tem-perature. Roof-tile waste aggregate is produced fromthe roof-tile in its production due to cracking whilesintering at high temperature. Sekishu Kawara, oneof the Japanese traditional roof tiles, was firstly usedas an internal curing material by Suzuki et al. [6],and it was reported that the compressive strength ofultra-high strength concrete was improved and theautogenous shrinkage was reduced by partially re-placing the coarse aggregate with the roof-tile waste.It was also reported that the strength of steam curedfly ash concrete was improved by using internal cur-ing material [7]. However, the effects of steam curingas well as internal curing on the reaction of fly ashhave not been clearly understood.The purpose of this study is to evaluate the effectsof steam curing and internal curing on contributionof fly ash to strength development of mortar by usingcementing efficiency factor (k-value). Furthermore,232vol. 33/2022 Fly Ash for Mortar Strength DevelopmentCement Fly ashCaO (%) 65.09 3.59SiO2 (%) 20.43 56.74Al2O3 (%) 4.95 28.06Fe2O3 (%) 2.52 5.75MgO (%) 1.62 0.92Na2O (%) 0.25 0.79SO3 (%) 2.86 1.07Loss on ignition (%) 0.76 2.08Density (g/cm3) 3.14 2.23Blaine fineness (cm2/g) 4560 3530Table 1. Chemical compositions and physical properties of cement and fly ash.Crushed sand Roof-tile waste fine aggregateDensity (g/cm3) 2.58 2.28Water absorption (%) 2.05 9.75Table 2. Density and water absorption of crushed sand and roof-tile waste fine aggregate.Unit content (kg/m3)Water Cement Fly ash Crushed sand Roof-tile waste fine aggregateF0S0 278 696 0 1290 0F20S0 269 537 134 1290 0F40S0 260 389 260 1290 0F0S20 278 696 0 1032 228F20S20 269 537 134 1032 228F40S20 260 389 260 1032 228Table 3. Mixture proportion.the pozzolanic reaction of fly ash was assessed, andthe relationship between the fly ash reaction and thek-value of fly ash was discussed.2. Experimental Method2.1. Materials, mixture proportion andcuring methodsPaste and mortar specimens were prepared in thisstudy. High-early-strength Portland cement and typeII fly ash were used as cementitious materials. Thesecement and fly ash met JIS R 5210 and JIS A 6201,respectively. The chemical compositions and physicalproperties of cement and fly ash are shown in Table 1.The water to binder ratio was 40% by mass, andfine aggregate to mortar ratio was 50% by volume.Roof-tile used in this study was derived from theSekishu Kawara, which composed of approximately70% SiO2, and 20% Al2O3. The density in saturatedand surface-dry condition and the water absorptionof crushed sand and roof-tile waste fine aggregate areshown in Table 2. The fly ash replacement ratioswere 0% (F0), 20% (F20), and 40% (F40) by mass.The roof-tile waste fine aggregate was used for mortarspecimens as an internal curing material by replacingcrushed sand at 0% (S0) and 20% (S20) by volume.The mixture proportions are shown in Table 3.Two curing methods were applied in this study.One was sealed curing at 20 ◦C at all ages (20 ◦Csealed curing), while the other was steam curing ata maximum temperature of 50 ◦C for 6 hours aftercasting. The steam-cured specimens were demoldedat the age of 1 day and stored at 20 ◦C and 60%relative humidity.2.2. Compressive strength and k-valueMortar specimens, which were cast into cylindricalmolds of 50-mm diameter and 100-mm height, wereused for the compressive strength test. The compres-sive strength test was conducted according to JIS A1108 at the ages of 1, 7, and 28 days. The equiva-lent cement-to-water ratio ((C/W )eq) of the fly-ashmortar was calculated from the compressive strengthand the relationship between the cement-to-water ra-tio (C/W ) and the compressive strength of mortarwithout fly ash. The k-value was obtained from the(C/W )eq, the C/W and the replacement ratio of flyash (r) as shown in Eq. 1 [8].k =!(C/W )eqC/W− 1"· 1 − rr(1)233R. Insako, Y. Miyoshi, P. T. Huynh et al. Acta Polytechnica CTU Proceedings2.3. Calcium hydroxide (CH) contentand its consumption per unit flyash contentPaste specimens cast into 16-mL polypropylene bot-tles were used for examining the CH content by ther-mogravimetry and differential thermal analysis. TheCH content in the sample was measured at the agesof 1, 7, and 28 days. Specimens were cut and samplesin the size of 2.5 − 5.0 mm range were obtained. Thesamples were soaked in acetone to stop hydration anddried in a vacuum desiccator for 24 hours. After thatthe samples were ground into a powder less than 150µm before the measurements of the degree of fly ashreaction and the CH content.Figure 1. Compressive strength of mortar understeam curing.The CH consumption per unit fly ash content canbe calculated using the following Eq. 2.CHconsumption =CH (1 − r) − CHF Ar(2)• CHconsumption: CH consumption per unit fly ashcontent (%)• CH: CH content in paste without fly ash (%)• CHF A: CH content in paste with fly ash (%)2.4. Degree of fly ash reactionThe sample used for measuring the degree of fly ashreaction was prepared similarly to that for CH con-tent. The degree of fly ash reaction in paste spec-imens was measured at 1, 7, and 28 days by usingselective dissolution method (SDM). To evaluate thereaction ratio of fly ash, Ohsawa et al. proposeda method using 2-M hydrochloric acid (HCl) and5% sodium carbonate (Na2CO3) aqueous solution [9].The procedure of SDM is as follows:A 1-g powder sample was put in a pre-weighed cen-trifuge tube and weighed precisely, and 30 mL of 2-MHCl solution was added in the tube. Then, the tubeFigure 2. Compressive strength of mortar under20 ◦C-sealed curing.was placed in a water bath at 60 ◦C for 15 min topromote the reaction. The solution was stirred occa-sionally with a glass rod during this step. The tubewas removed from the bath and centrifuged at 4000rpm (revolutions per minute) for 30 sec to separatesolid and liquid phases.The liquid phase was decanted as much as possiblewithout disturbing the solid residue in the bottom.After that, the tube was filled with hot water, cen-trifuged again and decanted. This operation was re-peated 3 times. The tube was then filled with 30 mLof Na2CO3 solution, placed in a water bath at 80 ◦Cfor 20 min and stirred continuously.The tube was then centrifuged at 4000 rpm for 1min. The liquid phase was decanted and the residuewas washed with hot water and centrifuged. Thiswashing was repeated 3 times. The tube with residuewas dried at 105 ◦C for 24 h and then weighed.The degree of fly ash reaction is calculated usingthe following Eq. 3.α = 1 − X (1 − Ig)(1 − Ig) k1k2 (3)• α : degree of fly ash reaction• k1 : original fraction of fly ash in ignited base• k2 : residue extracted of a 1-g fly ash• X : residue extracted of the hydrated fly ash in a1-g hydrated sample• Ig : loss on ignition of the hydrated sample• Ig′ : loss on ignition of the residue3. Results and Discussion3.1. Compressive strengthThe compressive strengths of mortar specimens understeam curing and 20◦C-sealed curing are presented in234vol. 33/2022 Fly Ash for Mortar Strength DevelopmentFigure 3. k-value of fly ash with age.Figure 4. CH content with age.Figures 1 and 2, respectively. At the age of 1 day, thecompressive strength of steam-cured mortar was 10%to 35% higher than that of 20 ◦C-sealed cured mor-tar. The roof-tile waste fine aggregate replacementincreased the compressive strength of mortar by 15%to 20% at all ages, regardless of curing methods. Atthe age of 28 days, the mortar specimens with 20% flyash and 20% roof-tile waste fine aggregate (F20S20)had the same compressive strength as that with 0%fly ash and 0% roof-tile waste fine aggregate (F0) inthe case of 20 ◦C-sealed curing. Meanwhile, the com-pressive strength of F20S20 was lower than that of F0in the case of steam curing. This could indicate thatsteam curing mainly promoted the initial hydrationof cement and slightly promoted the pozzolanic re-action of fly ash which can occur at later ages, whileinternal curing by roof-tile waste fine aggregate couldenhance the cement and fly ash reaction at early agesas well as the later ages.-5-4-3-2-1012340 7 14 21 28CH consumptionper unit fly ash content (%, g / fly ash-100g)Age(days)F20 20ºC-sealed F20steamF40 20ºC-sealed F40steamFigure 5. CH consumption per unit fly ash contentwith age.3.2. Cementing efficiency factor(k-value)Figure 3 shows the k-value of fly ash under steam cur-ing and 20 ◦C-sealed curing with time. The steamcuring had a lower k-value than the 20 ◦C-sealed cur-ing for all mixture proportions. This result also sug-gests that steam curing mainly promoted the hydra-tion of cement. In addition, the k-value in the caseof 20% roof-tile waste replacement (S20) was higherthan that in the case of no roof-tile waste. It indi-cated that the internal curing by using the roof-tilewaste fine aggregate promoted the fly ash reaction,and the fly ash contributed to the strength develop-ment of mortar. The F40S20 in both curing condi-tions had a significant increase in the k-value from 7to 28 days when compared with the F40. This showsthat the internal curing continuously promoted thepozzolanic reaction of fly ash for a long term whenthe replacement ratio of fly ash was high.235R. Insako, Y. Miyoshi, P. T. Huynh et al. Acta Polytechnica CTU ProceedingsFigure 6. Degree of fly ash reaction with age.3.3. Calcium hydroxide (CH) contentand its consumption per unit flyash contentFigure 4 shows the CH content under 20 ◦C-sealedcuring and steam curing. At the age of 1 day, the CHcontent of steam-cured specimens was higher thanthat of 20 ◦C-sealed cured specimens. It could im-ply that steam curing accelerated cement hydrationand CH was produced more than that under 20 ◦C-sealed curing. At the age of 7 days, then, the CHcontent for each mixture proportion was almost thesame, and increased up to the age of 28 days slightly.Figure 5 shows CH consumption per unit fly ashcontent by fly ash reaction under 20 ◦C-sealed curingand steam curing. At the age of 28 days, the CHconsumption per unit fly ash content in the case ofF20 with 20◦C-sealed curing was greater than that inthe case of F20 with steam curing. Besides, the unitCH consumption at the ages of 1 day and 7 days wasnegative. It may indicate that the cement hydrationcould be promoted by the incorporation of fly ashand more CH was produced in fly ash cement pastecompared to the plain cement paste.3.4. Degree of fly ash reactionFigure 6 shows the degree of fly ash reaction understeam curing and 20◦C-sealed curing. At all ages, thedegree of fly ash reaction in the case of 20 ◦C-sealedcuring was higher than that in the case of steam cur-ing. The longer the material age, the greater thedifference between steam curing and 20◦C-sealed cur-ing. It could imply that drying after the end of thesteam-supply hindered the fly ash reaction.Figure 7 shows the relationship between the k-valueand the degree of fly ash reaction. The k-value in-creases as the degree of fly ash reaction increases. Therelationship between the degree of fly ash reactionand the k-value can be represented by one straightline regardless of the age and the replacement ratioof fly ash.Figure 7. Relationship between k-value and degreeof fly ash reaction.4. ConclusionsThe present study investigated the k-value of flyash regarding compressive strength development, CHconsumption as well as the degree of fly ash reactionin order to evaluate the effects of steam curing andinternal curing on contribution of fly ash to strengthdevelopment of mortar. The following conclusionscan be drawn within the limits of this study.Steam curing improved early compressive strengthof mortar regardless of the fly ash or roof-tile replace-ment.The 20% roof-tile waste fine aggregate replacementincreased the compressive strength by 15% to 20% forboth 20 ◦C-sealed curing and steam curing. The in-ternal curing could contribute not only to the cementhydration but also to the fly ash reaction.The k-value in the case of 40% fly ash replacementfor both steam and 20 ◦C-sealed curing conditions in-creased from 7 to 28 days by using roof-tile waste fineaggregate. It was suggested that when the replace-ment ratio of fly ash is high, the effect of internalcuring on the pozzolanic reaction may continue for along time.The k-value in the case of steam curing was lowerthan that in the case of the 20 ◦C-sealed curing, andit implies that steam curing significantly promotedthe cement hydration whereas it slightly promotedthe pozzolanic reaction of fly ash. The relationshipbetween the degree of fly ash reaction and the k-valuecan be represented by one straight line regardless ofthe age and the replacement ratio of fly ash.References[1] K. Ogawa, H. Uchikawa, K. Takemoto, et al. Themechanism of the hydration in the systemC3S-pozzolana. Cement and Concrete Research10(5):683-96, 1980.https://doi.org/10.1016/0008-8846(80)90032-0.[2] H. Yazici, S. Aydin, H. Yi ‘giter, et al. Effect of steamcuring on class C high-volume fly ash concretemixtures. Cement and Concrete Research35(6):1122-7, 2005. https://doi.org/10.1016/j.cemconres.2004.08.011.236vol. 33/2022 Fly Ash for Mortar Strength Development[3] J. Tomiyama, Y. Suda, T. Saeki, et al. Basic Studyon Strength Characteristic of Fly Ash Concrete underSteam Curing. Cement Science and ConcreteTechnology 66(1):359-66, 2012.https://doi.org/10.14250/cement.66.359.[4] K. Yokota, S.-i. Igarashi. Effects of AggregateParticles on Spatial Distribution of SuperabsorbentPolymers for Internal Curing. Cement Science andConcrete Technology 67(1):179-86, 2013.https://doi.org/10.14250/cement.67.179.[5] D. Tachibana, K. Kimura, K. Naitoh. Effect ofHigh-Performance Artificial Lightweight Aggregate onSeveral Properties of Lightweight Concrete. DobokuGakkai Ronbunshu 1994(496):89-98, 1994.https://doi.org/10.2208/jscej.1994.496_89.[6] M. Suzuki, I. Maruyama, T. Kawabata. A study ondeformation of ultra high strength concrete containingcrushed roof tile aggregate. Japan Concrete Institute29:651-6, 2007.,.[7] Y. Ogawa, P. T. Bui, K. Kawai, et al. Effects ofporous ceramic roof tile waste aggregate on strengthdevelopment and carbonation resistance ofsteam-cured fly ash concrete. Construction andBuilding Materials 236, 2020. https://doi.org/10.1016/j.conbuildmat.2019.117462.[8] I. A. Smith. The Design of Fly-Ash Concretes.Proceedings of the Institution of Civil Engineers36(4):769-90, 1967.https://doi.org/10.1680/iicep.1967.8472.[9] S. Ohsawa, E. Sakai, M. Daimon. Reaction ratio of flyash in the hydration of fly ash-cement system. CementScience and Concrete Technology 53:96-101, 1999.237

Contribution of fly ash to mortar strength development under steam and internal curing

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Contribution of fly ash to mortar strength development under steam and internal curing

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