Effect of Curing Conditions on Hydration Reaction and Compressive Strength Development of Fly Ash-Cement Pastes

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1 Effet of Curing Conditions on Hydration Reation and Development of Fly Ash-Cement Pastes Warangkana Saengsoy Candidate for the degree of Dotor of Philosophy Supervisor: Prof. Dr. Toyoharu Nawa Division of Solid Waste, Resoures and Geoenvironmental Engineering Introdution Fly ash is one of the pozzolani materials whih has been widely used to effetively improve the various properties of onrete. The reation between pozzolan and alium hydroxide is referred to as the pozzolani reation. The produt of the pozzolani reation, alium siliate hydrates (C-S-H), is highly effiient in filling up large apillary spaes due to its lower density whih is lower than that of C-S-H produed from the reation of Portland ement and water. Thus, it improves the strength and impermeability of ementbased materials. The term for uring of onrete stands for proedures devoted to hydration reation of ement and pozzolani reation of the fly ash, onsisting of ontrol of time, temperature, and humidity onditions. Several studies have undergone to understand the influene of uring on the ompressive strength of the fly ash onrete. In addition, it has been reported that the fly ash onrete is more sensitive to uring onditions and requires a longer uring period than that required by OPC onrete. However, the mehanism on the effet of uring onditions on the strength development is yet to be asertained. Due to their hydration harateristi, both pozzolani reation and the reation of Portland ement require water to omplete their reation. The prevention of rapid and exessive loss of water ourring during the evaporation proess in the early age is neessary for the adequay of water for the slower pozzolani proess. Objetive and Sope of Study The objetive of this study is to ontrol and predit mehanial properties and durability of fly ash onrete from the viewpoint of the progress on the hydration of ement, and pozzolani reation of fly ash at any uring onditions. The sope of this researh is threefold: Firstly, the existing predition model of strength development is applied to hardened pastes ured at different temperatures and relative humidity uring onditions. Next, the effet of uring onditions on individual hydration degree of ement and fly ash in the fly ashement paste is quantified by the X-ray diffration- Rietveld analysis and seletive dissolution analysis, respetively. The predition model that haraterizes the temperature and relative humidity influenes on hydration of both Portland ement and fly ash is developed based on the Arrhenius equation. Finally, the modified gel/spae ratio theory is proposed to enhane the predition model of strength development and verified by taking into aount the differene in the density of C-S-H with fly ash addition, uring temperature, and uring relative humidity. Methodologies Materials and Mix Proportion Ordinary Portland Cement (OPC) and fly ash type II were used aording to JIS R521 and JIS A621, respetively. The paste speimens were prepared with a water-to-binder ratio (W/B) of 1. by volume. The replaement ratios of ement by fly ash were %, 25%, and 5% by volume. A polyarboxylate-based superplastiizer was used to ontrol the fluidity of the paste. Curing Conditions In order to investigate the effet of uring temperature, the paste speimens were ured in water with three different uring temperatures at 2±2 C, 35±2 C, and 5±2 C. As for the effet of uring relative humidity, the speimens were plaed in a ontrolled temperature hamber at 2±2 C. At three days after mixing, the speimens were removed from their moulds and ured in five different environmental onditions until the required age was reahed. The uring onditions were water uring, sealed uring, and at relative humidity levels of 6%, 8% and 95%. Relative humidity was ontrolled by saturated salt solutions. Hydration Reation Measurement The degree of hydration of the hydrated ement pastes was estimated with an XRD-Rietveld analysis. In the ase of the pastes that had been inorporated with fly ash, the degree of hydration was estimated with a ombined method using XRD-Rietveld analysis and seletive dissolution [1, 2]. At the required age, the speimens were broken into small piees (2.5-5.mm). Then, the hydration reation was stopped by soaking them in aetone and further drying at 15 C. Next, they were ground in a ball mill at a speed of 3rpm for eight minutes. The X-ray diffration equipment used in the XRD- Rietveld analysis was a Rigaku, CuK α X-ray type. The experiments were performed in the range of 5-7 2θ at

2 4kV, 2mA,.2 sampling width and 2 /min san speed. The divergene slit, sattering slit and reeiving slit were 1/2, 1/2 and.3 mm, respetively. Standard orundum, 1% by weight of sample, was used as an internal referene. The sample and orundum were mixed in the ball mill at a speed of 1rpm for three minutes. The software used in the Rietveld analysis for this study was SIROQUANT (version 3.). As for seletive dissolution, the unhydrated fly ash was determined using a solution of 2N HCl and 5% Na 2 CO 3. One the weight of unhydrated ement and fly ash was obtained, the degree of hydration of ement and fly ash an be alulated from their hydrated quantity divided by the unhydrated quantity at days [3]. Internal Relative Humidity Measurement The internal relative humidity in the pastes was measured with a erami sensor embedded in the entre of eah speimen during speimen preparation. The dimensions of the erami sensor were 1m in diameter and 1m in height. The sensor was onneted to the data logger to reord eletrial resistane every ten minutes throughout the experiment. In order to obtain the relationship between the eletrial resistane of the sensor and the relative humidity, the erami sensor was alibrated using salt solutions. In this study, relative humidity was alulated based on eletrial resistane as in Eq. (1)..42 RH =.5( ρ 327.5) + 1 (1) where RH is relative humidity (%) and ρ is eletrial resistane (Ω). Strength Development of Fly Ash Cement Pastes Effet of Curing Temperature There is no signifiant differene in the strength development of ement pastes ured in water at different uring temperatures as shown in Fig. 1. On the other hand, an inrease of the uring temperature signifiantly inreases the strength development of fly ash ement pastes as shown in Fig FA % 2 C 5 C Fig. 1 Compressive strength development of ement pastes ured in water at different temperatures C 5 C FA 5% Fig. 2 Compressive strength development of pastes with 5% fly ash ured in water at different temperatures Internal Relative Humidity (%) Sealed RH 95% RH 8% RH 6% Fig. 3 Change in internal relative humidity of paste with ambient relative humidity: W/B = 1. by volume Sealed RH 95% RH 8% RH 6% Fig. 4 Influene of uring methods and ambient relative humidity on ompressive strength of paste with W/B = 1. by volume Effet of Curing Relative Humidity Hydration of ordinary Portland ement onsumes the water, aordingly internal relative humidity of paste dereases with time even if the paste was ured under sealed uring ondition. It is also observed that when ured the paste at ambient relative humidity of 95%, the internal relative humidity of paste is almost the same as water immersing and sealed uring. In ontrast, at ambient relative humidity below 95%, the internal relative humidity of paste is dereased with a redution in the ambient relative humidity. A typial result of hange in the internal relative humidity of paste ured at different moisture onditions is shown in Fig. 3. The redution of internal relative humidity in paste is derived from two auses: one is the onsumption of water due to the hydration of ement and pozzolani reation of fly ash, and the other is the evaporation of

3 water by drying when pastes are exposed at low ambient relative humidity. The ompressive strength of the pastes ured at various methods is shown in Fig. 4. There is no signifiant differene in the strength development of speimens ured in water, in sealed uring and in moist uring at a relative humidity of 95%. However, the ompressive strength slowly develops when the pastes are exposed at a relative humidity below 8% regardless the replaement ratio of fly ash, in partiular the ompressive strength of pastes exposed at a relative humidity of 6% is hardly inreased after 7days. Predition Model of Strength Development of Pastes Bernhardt [4] proposed that the rate of strength gain at any age was a funtion of the urrent strength and the temperature and proposed the mathematial expression as in Eq. (2). ds dt ( S ) k(t ) = f (2) where S is the ompressive strength, f(s) is a funtion of strength and k(t) is a funtion of temperature. The strength funtion was proposed as a hyperboli equation as in Eq. (3) [5]. S f ( S ) = Su 1 (3) S u where S u is the ultimate ompressive strength. Beause the Arrhenius funtion estimates the effet of temperature more aurately than the previous temperature funtion. Kim et al. [5] proposed Eq. (4) and suggested that the funtion k(t,t) was based on the Arrhenius equation as shown in Eq. (5). The apparent ativation energy in Eq. (5) was a funtion of age as in Eq. (6). ds dt ( S ) k( T, t) 3 = f (4) E RT k( T, t) = Ae (5) αt E = E e (6) where A is a onstant, R is gas onstant and equal to J/K mol, and T is uring temperature (K). E is the apparent ativation energy (J/mol). E is the initial apparent ativation energy (J/mol). α is a onstant and equals to.615. t is age (days). The apparent ativation energy an be estimated and it is shown in Fig. 5. It an be seen that the apparent ativation energy inreases with inreasing fly ash replaement ratio. The apparent ativation energy is almost onstant and independent of age of pastes. Even though the Eq. (4) an haraterize the temperature influene on the strength development but it an not desribe the influene of the relative humidity. Hene, the model should be modified by taking into aount the influene of the relative humidity. It is shown in Fig. 6 that the ompressive strength is linearly dereased with a redution in the internal relative humidity regardless of replaement ratio of fly ash. As a result, it is assumed that the relative humidity effet is linearly related to the rate of strength development as in Eq. (7). RH k( T, t, RH ) = a Ae (7) 1 where RH is the relative humidity (%), and a is a onstant. The modified ompressive strength model with onsidering the influene of the relative humidity reasonably estimates the ompressive strength of pastes ured at any onditions as shown in Fig. 7. Apparent Ativation Energy (J/mol) E RT FA % FA 5% Fig. 5 Apparent ativation energy 12 FA % 1 R 2 =.969 FA 5% R 2 = R 2 = days Internal Relative Humidity (%) Fig. 6 Relationship between the ompressive strength and the internal relative humidity of pastes with W/B = 1. by volume Calulated Relative R 2 = Measured Relative Compressive Strength Fig. 7 A omparison between the alulated and measured relative ompressive strength of pastes ured at different relative humidity

4 Hydration Reation of Fly Ash Cement Pastes Effet of Curing Temperature An inrease of the uring temperature aelerates the degree of hydration of ement at early age but it has a small effet at later age of pastes as shown in Fig. 8. The degree of hydration of fly ash is strongly influened by the uring temperature. The inrease of the uring temperature aelerates the degree of hydration of fly ash both at early age and later age of pastes as shown in Fig. 9. This finding implies that the inreased strength development of fly ash ement paste is owing to the aelerated degree of hydration of fly ash due to the inrease of uring temperature. Effet of Curing Relative Humidity There is no signifiant differene in the degree of hydration of Portland ement in pastes ured at ambient relative humidity of 95%, sealed ondition, and water immersed ondition. However, the degree of hydration of ement is seen to derease with a redution in ambient relative humidity when the pastes were exposed to lower ambient relative humidity levels below 8% as shown in Fig. 1. The influene of uring relative humidity on the degree of hydration of fly ash also exhibits the similar tendeny as that of ement. It an be seen in Fig. 11 that the degree of hydration of fly ash is severely restrited at relative humidity below 8%. Predition Model of Hydration Reation It has been reported that the exponential funtion as shown in Eq. (7) an be used to effetively haraterize the development of the degree of hydration [6, 7]. β ( ) exp τ α t e = α u 1 (7) te where α(t e ) is degree of hydration at equivalent age t e (%), τ is hydration time parameter (h), β is hydration shape parameter, and α u is ultimate degree of hydration (%). The equivalent age an be expressed in the Arrhenius form as in Eq. (8). t E te Tr t R Tr T 1 1 ( ) = exp (8) where t e (Tr) is equivalent age at referene uring temperature (h). t is hronologial time interval (h). T is average onrete temperature during time interval t ( C); T r is referene temperature ( C). The ativation energy of the hydration reation of ement and the hydration reation of fly ash is shown in Fig. 12. It shows that the apparent ativation energy of ement hydration dereases with inreasing fly ash replaement ratio. It was reported that the aeleration Cement (%) C 5 C Fig. 8 Degree of hydration of OPC in fly ash-ement paste ured in water at different temperatures Fly Ash (%) C 5 C Fig. 9 Degree of hydration of fly ash in paste with W/B = 1. by volume Cement (%) FA 5% Sealed RH 95% RH 8% RH 6% Fig. 1 Influene of ambient relative humidity on degree of hydration of OPC in paste with W/B = 1. by volume Fly Ash (%) Sealed RH 8% RH 6% FA 5% Fig. 11 Influene of ambient relative humidity on degree of hydration of fly ash in paste with W/B = 1. by volume of ement hydration redued the apparent ativation energy [5, 8]. Therefore, the inrease in fly ash replaement ratio aelerates the degree of hydration of ement and redues the apparent ativation energy. In ase of the fly ash, the ativation energy of the

5 pozzolani reation is higher than that of ement hydration and it inreases with inreasing fly ash replaement ratio. The higher value of the ativation energy of the hydration reation of fly ash indiates that the reation is more sensitive to the temperature than that of ement. The redution of the internal relative humidity due to drying leads to prevent the hydration reation of ement and fly ash. In this study, the influene of the internal relative humidity on the hydration of ement and/or fly ash an be expressed by Eq. (9). The preventing influene of the hydration reation is modeled by a oeffiient C RH as in Eqs. (1) to (13). β ( ) exp τ α t e = αu 1 C RH (9) te C RHC =.51RH for RH < 95% (1) C = 1 for RH 95% (11) RHC C RHF =.47RH for RH < 95% (12) C = 1 for RH 95% (13) RHF where C RHC and C RHF are oeffiients of internal relative humidity for the degree of hydration of ement and fly ash, respetively. RH is internal relative humidity (%). It is shown in Fig. 13 that the proposed equations an be used to estimate the degree of hydration of ement and the degree of hydration of fly ash. Ativation Energy (kj/mol) Cement Fly Ash y = x R 2 =.952 y = 132.8x R 2 = Replaement Ratio of Fly Ash Fig. 12 Ativation energy of ement hydration and fly ash hydration Calulated Degree of Hydration (%) Cement R 2 =.9926 Fly Ash R 2 = Measured Degree of Hydration (%) Fig. 13 A omparison between the alulated and measured degree of hydration of ement and fly ash in paste with W/B = 1. by volume Predition Model of Strength Development of Pastes based on Hydration Reation The degree of hydration is one fator to ontribute to the ompressive strength development. However, the ompressive strength ould not be diretly estimated by only either the degree of hydration of ement or the degree of hydration of fly ash: The hydrates suh as C- S-H gel produed from both ement and fly ash hydration reations fill the spae and ontribute to strength development. For OPC pastes, assuming that 1 ml of ement will oupy 2.6 ml of spae when fully hydrated [9]. Pipat et al. [1] reported that the density of C-S-H produed in fly ash-ement paste is different from that of Portland ement paste as shown in Fig. 14. This implies the filling effet of hydrates is also different. Hene taking into aount of this effet, the hydration degree of fly ash-ement system is orrespond to about 1.5 times as large as that of Portland ement system. The volume of the hydrated gel an be obtained as a funtion of the degree of hydration of ement and fly ash and the volume of the hydrated produt of ement and fly ash as shown in Eq. (14). V Gel α α f = 2.6V + D fv f (14) 1 1 where, V Gel is volume of hydrated gel. V and V f are volume fration of ement and fly ash, respetively. α and α f are degrees of hydration of ement and fly ash (%), respetively. 2.6 is the volume of the hydration produts of ement at the ompletion of hydration of 1 ml of ement. D f is the volume of the produts of hydration of fly ash at the ompletion of reation of 1 ml of fly ash and equals to 3.. Model for Prediting Strength Development of Pastes at Different Curing Temperatures The density of the hydrated gel may hange aording to the uring temperature. Therefore, the volume of the hydrated gel is modified by taking into aount the effet of temperature as in Eq. (15). Density of C-S-H gel (g/m 3 ) V Gel α α f = 2.6V + DT D f V f (15) Cement Fly Ash Amount of hydrated gel (% by weight) Fig. 14 The density of hydrated gel

6 where D T D f is the volume of the produts of hydration of fly ash at the ompletion of reation of 1 ml of fly ash. D T equals to 1 for uring temperature at 2ºC and equals to.75 for higher uring temperature at 35ºC and 5ºC. These values represent the shrinkage of C-S-H gel in fly ash ement paste due to higher uring temperature. The ompressive strength of paste is losely related to the volume of the hydrated gel and the relationship between them an be estimated by Eq. (16). ' VGel f = (16) where f is the ompressive strength. Model for Prediting Strength Development of Pastes at Different Curing Relative Humidity The ompressive strength is governed by not only the volume of the hydrated gel but also other fators suh as the internal relative humidity. It is well known that the C-S-H gel shrinks by the drying, that is the redution of internal relative humidity [11]. Hene, Eq. (16) should be rewritten by taking into aount the C-S-H shrinkage. In this study, the oeffiient that shows the influene of shrinkage is assumed to be expressed as a funtion of the internal relative humidity. Therefore, the Eq. (16) an be rewritten into the following equation. ' 3.34 f =.11( C V ) (17) 947 RHS Gel 2.75 RH C RHS = (18) 1 where V Gel is the volume of hydrated gel from Eq. (15). C RHS is oeffiient of the shrinkage of the C-S-H gel. RH is the internal relative humidity (%). Fig. 15 shows a good orrelation between the ompressive strength and the multipliation of the oeffiient of the shrinkage of the C-S-H gel and the volume of the hydrated gel of the pastes. It is onfirmed that Eq. (17), onsidering both influene of the degree of hydration and the internal relative humidity, an be used to estimate the ompressive strength y = x 3.34 R 2 = C RHS *V Gel Fig. 15 Relationship between the ompressive strength and the multipliation of the oeffiient of the shrinkage of the C-S-H gel and the volume of the hydrated gel of paste with W/B = 1. by volume Conlusions The effet of uring onditions on the hydration reation and the strength development of pastes has been investigated and their predition models were proposed. The proposed models with onsidering the effet of uring temperature and uring relative humidity reasonably estimate the ompressive strength and degree of hydration of pastes ured at any onditions. The modified gel/spae ratio model with onsidering the effet of fly ash addition, uring temperature, and uring relative humidity was proposed and properly used to estimate the ompressive strength development. Referenes [1] Termkhajornkit, P., Nawa, T. and Kurumisawa, K.: A study of fly ash ement hydration by Rietveld analysis and seletive dissolution, JCI, Vol. 27, pp , 25. [2] Termkhajornkit, P., Nawa, T. and Kurumisawa, K.: Quantitative study on hydration of fly ash and Portland ement, Proeedings of ConMat 5, Vanouver, pp. 399, 25. [3] Termkhajornkit, P., Nawa, T., Yamashiro, Y. and Saito, T.: A Study of Self-Healing Ability of Fly Ash- Cement System by Hydration Reation and Porosity, Proeedings of CONSEC 7 Tours, Frane, pp , 27. [4] C.J. Bernhardt: Hardening of onrete at different temperatures, RILEM Symposium on Winter Conreting, Copenhagen, Danish, Institute for Building Researh, 1956, Session B-II. [5] J.K. Kim, S.H. Han: Estimation of ompressive strength by a new apparent ativation energy funtion, Cem. Conr. Res. 31 (2) (21) [6] Pane, I., and Hansen, W.: Conrete Hydration and Mehanial Properties Under Nonisothermal Conditions, ACI Materials Journal, V. 99, No. 6, Nov.-De., pp , 22. [7] Freiesleben Hansen, P., and Pedersen, E. J.: Curing of Conrete Strutures, Draft DEB-Guide to Durable Conrete Strutures, Appendix 1, Comité Euro- International du Béton, Switzerland, [8] K.O. Kjellsen, R.J. Detwiler: Later ages strength predition by a modified maturity model, ACI Mater. J. 9 (3) (1993) [9] T.C. Powers and T.L. Brownyard: Studies of the physial properties of hardened Portland ement paste (Nine parts), J. Amer. Conr. Inst., 43 (Ot to April 1947). [1] Termkhajornkit, P. and Nawa, T.: Composition of C-S-H in the Hydration Produts of Fly Ash-Cement System, Proeedings of the Ninth CANMET/ACI International Conferene on the use of Fly Ash, Silia Fume, Slag, and Natural Pozzolans in Conrete, Warsaw, Poland, pp , 27. [11] Aono, Y., Matsushita, F., Shibata, S. and Hama, Y.: Nano-struture Changes of C-S-H in Hardened Cement Paste during Drying at 5 C, Journal of Advaned Conrete Tehnology Vol. 5, No. 3, pp , 27.

Chemically bound water as measure of degree of hydration: method and potential errors

Chemically bound water as measure of degree of hydration: method and potential errors Chemially bound ater as measure of degree of hydration: method and potential errors Fagerlund, Göran Published: 29-1-1 Link to publiation Citation for published version (APA): Fagerlund, G. (29). Chemially