Characteristics of the hydration heat evolution of composite binder at different hydrating temperature

Thermochimica Acta 586 (2014) 52–57

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Characteristics of the hydration heat evolution of composite binder at different hydrating temperature Fanghui Han a,b , Rengguang Liu b , Dongmin Wang a , Peiyu Yan b,∗ a b

School of Chemical and Environmental Engineering, China University of Mining and Technology (Beijing), Beijing, China Key Laboratory of Civil Engineering Safety and Durability of China Education Ministry, Department of Civil Engineering, Tsinghua University, Beijing, China

a r t i c l e

i n f o

Article history: Received 25 January 2014 Received in revised form 24 March 2014 Accepted 8 April 2014 Available online 19 April 2014 Keywords: Composite binder Cement Slag Fly ash Hydration heat Temperature

a b s t r a c t Hydration heat evolution rate and quantity of cement–slag binder and cement–fly ash binder were measured at 25 ◦ C, 45 ◦ C and 60 ◦ C by isothermal calorimetry. Composite binder hydrates slower than pure cement at 25 ◦ C. The hydration of composite binder is accelerated by the elevated temperature. The hydration heat of composite binder containing no more than 50% of slag and that of pure cement have little difference in the later period at 45 ◦ C. It becomes the same or even higher at 60 ◦ C. The hydration heat between composite binder containing fly ash and pure cement still has certain gap at 45 ◦ C, but the gap is narrowed obviously at 60 ◦ C. Raising temperature promotes the hydration of composite binder greater than that of pure cement. The hydration of composite binder containing slag is very sensitive to temperature. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Portland cement is one of the most important binders used in modern concrete [1,2]. More than 2.2 billion tons of cement was produced in China during 2012 [3]. Due to the exothermic hydrating process of cement and the low thermal conductivity of concrete, the hydration of cement often leads to a raise of internal temperature of massive concrete, which may make concrete cracking during the cooling of concrete. In order to reduce the hydration heat released from the cementitious materials, mineral admixtures are used extensively in modern concrete [4]. Addition of mineral admixture affects the hydration degree, hydration heat evolution rate and total hydration heat of binder. As a result, the formation of microstructure as well as the development of physical and mechanical properties of concrete are varied [5–9]. Isothermal calorimetry is often used to characterize the hydration of composite binder. Kumar et al. [10] studied the isothermal heat evolution curve of composite binders containing 20% of fly ash at 35 ◦ C and 45 ◦ C, and reported that the addition of fly ash retarded their hydration while raising temperature accelerated it. Narmluk et al. [11] found that fly ash retarded the early hydration of cement but accelerated the hydration of cement in

∗ Corresponding author at : Department of Civil Engineering, Tsinghua University, Beijing, China. Tel.: +86 13501215836; fax: +86 01062785836. E-mail address: [email protected] (P. Yan). http://dx.doi.org/10.1016/j.tca.2014.04.010 0040-6031/© 2014 Elsevier B.V. All rights reserved.

the later period at 20 ◦ C and 35 ◦ C, but at 50 ◦ C 50% dosage of fly ash retarded the hydration of cement at later age. Mostafa et al. [12] studied the isothermal exotherm of composite binder containing both 30% of silica fume and kaolin. They thought that the pozzolanic material reduced the heat emission at the C3 S hydration stage; the heat released from pozzolanic reaction also has an impact on the main heat peak. Binici et al. [13] reported that with the content of slag increased, the hydration heat emission reduced, but the increased fineness of cement or slag will lead to the increase of hydration heat emission of composite binder. Xu et al. [14] proposed that the degree of cement hydration can be characterized by hydration parameters including activation energy and hydration curve parameters, ultimate hydration degree decreased and hydration time increased with the increase of fly ash replacement. The most studies done now are determination of hydration heat curve of composite binders with a single blend or at room temperature. But the type and dosage of mineral admixture, hydrating temperature, water to binder ratio, chemical admixtures, particle size distribution of binder et al. are important factors, too [15–18]. They affect the hydration rate and the reaction mechanism of composite binder. The interaction between them determines the hydration rate and the ultimate hydration degree of systems. In order to understand the hydration process in the presence of mineral admixtures at different hydrating temperature, study of detailed heat evolution behavior is needed. In this paper the hydration heat curves of composite binder containing different content of slag and fly ash hydrating at 25 ◦ C, 45 ◦ C and 60 ◦ C were

F. Han et al. / Thermochimica Acta 586 (2014) 52–57

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Table 1 Chemical compositions of binder (w/%). Composition

SiO2

Al2 O3

Fe2 O3

CaO

MgO

SO3

Na2 Oeq

f-CaO

Cl−

LOI

Cement Slag Fly ash

20.55 34.55 57.60

4.59 14.36 21.90

3.27 0.45 2.70

62.50 33.94 3.87

2.61 11.16 1.68

2.93 1.95 0.41

0.53 0.63 1.05

0.83 – –

0.010 – –

2.08 0.70 7.65

Na2 Oeq = Na2 O + 0.658K2 O; w – mass fraction.

measured, hereby to explore the characteristics of hydration heat of composite binder.

Table 2 Mix proportion of binder. Sample

W/B

Mass fraction (%) Cement

2. Experimental 2.1. Materials P.I 42.5 Portland cement, Class I fly ash, and S95 ground granulated blast furnace slag complying with related Chinese national standards were used. The specific surface areas of cement and slag are 350 m2 /kg and 442 m2 /kg, respectively. The water requirement rate of fly ash is 95%. The water-to-binder ratio (W/B) of prepared binder paste is 0.4. The chemical compositions of cement and mineral admixtures are given in Table 1.

CM Slag30 Slag50 Slag70 Slag90 FA20 FA35 FA50 FA65

0.4

Slag

100 70 50 30 10 80 65 50 35

Fly ash

0 30 50 70 90 0 0 0 0

0 0 0 0 0 20 35 50 65

W/B is mass ratio of water-to-binder.

3. Results and discussion 2.2. Test methods The hydration heat evolution rate and total hydration heat of composite binder were measured with an isothermal calorimeter (TAM Air) at 25 ◦ C, 45 ◦ C and 60 ◦ C within 168 h. TAM Air has eight parallel twin-chamber measuring channels maintained at a constant temperature: one chamber containing the sample, another containing the reference. The mix proportions of binder are shown in Table 2. After mixing homogeneously, the samples were immediately placed into the chamber. The hydration heat evolution rate and total hydration heat of composite binder can be continuously monitored as a function of time.

The ending time of induction period of composite binder hydration, the time and rate of the second heat emission peak, and the total heat emission at different hydration ages determined from heat evolution curves of binders are shown in Table 3. 3.1. Characteristics of hydration heat evolution of composite binder at 25 ◦ C The hydration heat evolution curves of cement–slag binders and cement–fly ash binders at 25 ◦ C are shown in Figs. 1 and 2, respectively. In a few minutes after mixing binder with water, a sharp

Table 3 Characteristic values of hydration heat evolution curves of binder. Temperature

Sample

Ending time of the induction period (h)

Time of the second heat emission peak (h)

Rate of the second heat emission peak qmax (J/g h)

25 ◦ C 25 ◦ C 25 ◦ C 25 ◦ C 25 ◦ C 25 ◦ C 25 ◦ C 25 ◦ C 25 ◦ C 45 ◦ C 45 ◦ C 45 ◦ C 45 ◦ C 45 ◦ C 45 ◦ C 45 ◦ C 45 ◦ C 45 ◦ C 60 ◦ C 60 ◦ C 60 ◦ C 60 ◦ C 60 ◦ C 60 ◦ C 60 ◦ C 60 ◦ C 60 ◦ C

CM Slag30 Slag50 Slag70 Slag90 FA20 FA35 FA50 FA65 CM Slag30 Slag50 Slag70 Slag90 FA20 FA35 FA50 FA65 CM Slag30 Slag50 Slag70 Slag90 FA20 FA35 FA50 FA65

1.90114 2.23326 1.93316 2.18825 2.79176 2.59696 2.83910 3.53854 4.40140 1.52560 1.50706 1.49582 1.50706 1.73693 1.69538 1.83464 2.08764 2.26424 1.23078 1.27438 1.29467 1.09613 1.34161 1.68280 1.97011 2.09281 2.08167

9.44533 8.74172 8.61546 7.14428 – 10.24639 11.12550 12.49524 15.54820 5.03961 4.95121 5.54022 6.04620 5.07332 5.39744 5.71546 6.28741 6.76956 3.74886 3.70480 4.11641 3.40920 2.79918 5.13976 5.79166 5.91223 6.07632

9.73245 7.82229 5.25151 3.90366 – 8.54588 7.13526 5.59975 3.85093 26.63802 19.56862 15.03892 13.89487 10.42259 20.43694 16.71809 12.82451 8.25642 47.77907 36.85133 34.99483 29.38576 24.60173 35.72359 27.27060 21.51052 14.85322

Total heat emission Q (J/g)

12 h

48 h

96 h

168 h

78.2 66.9 45.5 49.5 18.6 63.2 48.6 33.5 20.4 174.0 147.2 129.4 105.6 77.7 130.4 111.8 89.4 59.9 229.6 208.0 192.0 167.8 84.4 178.2 146.1 120.0 88.9

209.7 187.0 146.2 141.7 91.9 191.5 162.5 133.2 96.3 286.3 258.5 238.0 199.2 117.2 224.5 196.5 164.8 120.5 316.8 313.6 304.9 236.6 98.5 270.4 230.2 190.2 139.5

257.0 234.6 193.9 184.5 117.4 230.4 193.4 157.1 115.0 315.4 299.5 282.9 229.7 127.1 253.1 224.8 190.5 136.3 324.4 325.7 325.0 251.5 104.7 288.9 242.6 197.7 144.5

281.1 252.8 227.3 198.6 128.5 242.8 205.4 167.2 122.5 331.0 317.5 303.4 244.5 133.0 264.2 233.7 197.2 140.5 337.1 340.2 338.8 261.2 115.8 300.2 250.0 202.2 148.1

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F. Han et al. / Thermochimica Acta 586 (2014) 52–57

Fig. 1. Hydration heat evolution rate and quantity of cement–slag binder at 25 ◦ C. (a) Hydration heat evolution rate and (b) hydration heat.

Fig. 2. Hydration heat evolution rate and quantity of cement–fly ash binder at 25 ◦ C. (a) Hydration heat evolution rate and (b) hydration heat.

and transient exothermic peak is formed by a quick formation of AFt and syngenite due to the rapid dissolution of sulfate and aluminate from cement clinker. Then the heat emission rate decreases rapidly. The system is in a C-S-H gel nucleation and Ca(OH)2 supersaturation process [19]. The induction period of hydration for pure cement ends after 1.90 h (Table 3). The induction period of hydration for composite binder is prolonged gradually with the increased dosage of mineral admixture. Addition of fly ash can more distinctly retard the hydration of cement than slag. The induction period of composite binder containing both 50% of slag and fly ash is 1.93 h and 3.54 h, respectively (Table 3). The retarding effect of fly ash on the pure cement hydration has been reported previously [20–23]. This is related to the condition of the fly ash surface. A large amount of Ca2+ produced during the hydration of cement clinker is absorbed on the surface of fly ash particles, which reduces Ca2+ concentration in the solution, delays Ca(OH)2 nucleation and generates unstable C-S-H gel with low Ca/Si ratio, which will slowly change into a stable C-S-H gel. Thus the ending time of the induction period is prolonged [11]. The activity of slag is higher than that of fly ash. Therefore, the extension effect of slag on the induction period is not as obvious as fly ash. Subsequently the acceleration period comes with rapid hydration of C3 S, fast formation of C-S-H gel and Ca(OH)2 . The more content of mineral admixture is, the smaller peak value of the second exothermic peak is (Figs. 1(a) and 2(a)). The time of the second exothermic peak for pure cement occurs after 9.45 h but that for

binders containing slag appears ahead of time (Table 3). With the increased amount of slag, the time of maximum heat evolution rate of binder is shortened (Fig. 1(a), Table 3). The dilution effect of slag at the early stage of hydration increases the effective waterto-cement ratio of binder but shortens the duration of hydration process of binder. Slag provides nucleation points for the hydration products of cement and reaction of slag increases the hydration rate of binder. However, with the increased dosage of fly ash, the time of maximum exothermic peak of binders is prolonged. The dosage of fly ash can increase effective water-to-cement ratio of mix and shows nucleation effect due to its low activity. It acts as an inert filler in the mix in the early period of hydration. Fly ash may reduce the hydration rate of binder because large amount of Ca2+ and water is absorbed on its spherical particles. An obvious exothermic effect in the later period appears on the hydration heat evolution rate curve of cement–slag binder because of the pozzolanic reaction of slag. This effect comes early and is much obvious with the increased content of slag. This effect is higher than the second one when the content of slag in binder is beyond 70%. Even only this effect is observed when the slag content reaches 90% (Fig. 1(a)). The third exothermic peak cannot be observed obviously on the heat evolution rate curve of cement–fly ash composite binder since the activity of fly ash is lower than that of slag. A weak exothermic effect caused by the pozzolanic reaction of fly ash appears after about 30 h of hydration when the dosage of fly ash is greater than 50% (Fig. 2(a)).

F. Han et al. / Thermochimica Acta 586 (2014) 52–57

Fig. 3. Hydration heat evolution rate and quantity of cement–slag binder at 45 ◦ C. (a) Hydration heat evolution rate and (b) hydration heat.

Figs. 1(b) and 2(b) show that the total hydration heat of composite binder decreases disproportionally with the increase of mineral admixture. As slag is gradually activated in the later period of hydration, the reaction of slag is accelerated and it consumes Ca(OH)2 generated by the cement hydration, which in turn promotes the cement hydration. Thus, the hydration heat of composite binder increases, the gap of total hydration heat between composite binder containing no more than 70% of slag and pure cement becomes small. The weak alkalinity of paste is insufficient to activate slag for the binder containing 90% of slag due to the very low percentage of cement. Therefore, it has the lowest hydration heat. The heat emission of cement–fly ash composite binder is less than that of composite binder containing slag at same replacement level. The increasing trend of cement–fly ash composite binder is not as obvious as that of binders containing slag. 3.2. Characteristics of hydration heat evolution of composite binder at 45 ◦ C Figs. 3 and 4 show hydration heat evolution curves for the cement–slag binders and cement–fly ash binders at 45 ◦ C, respectively. From Figs. 3(a) and 4(a) it can be seen that the early hydration of composite binder is accelerated with the rise of temperature. The ending time of induction period and the reaching time of the second peak are shortened significantly. The peak value increases greatly. Table 3 shows that, when the hydration

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Fig. 4. Hydration heat evolution rate and quantity of cement–fly ash binder at 45 ◦ C. (a) Hydration heat evolution rate and (b) hydration heat.

temperature increases from 25 ◦ C to 45 ◦ C, the ending time of induction period is shortened from 1.90 h to 1.53 h for pure cement, from 1.93 h to 1.50 h for the composite binder containing 50% of slag and from 3.54 h to 2.09 h for composite binder containing 50% of fly ash. The time reaching the second exothermic peak of binders is shortened from 9.45 h to 5.04 h, from 8.62 h to 5.54 h and from 12.50 h to 6.29 h, respectively. The maximum value of the second exothermic effect of binders increases by 173.8% from 9.73 J/g h to 26.64 J/g h, by 186.5% from 5.25 J/g h to 15.04 J/g h and, by 128.9% from 5.60 J/g h to 12.82 J/g h, respectively. The hydration of slag is activated by the alkali released by clinker hydration in composite binder and temperature rise provides energy to activate alkali-hydroxide attack on the slag particles [24]. The elevation of temperature accelerates both the hydration of pure cement and fly ash [25], too. Therefore, quick consumption of humidity in composite binder pastes and large amount of hydration products generated by the rapid hydration of binder hinder the diffusion and migration of water and ions. As a result, the hydration reaction rate decreases rapidly. It forms the second exothermic peak with a high but narrow shape. The reaction goes into the diffusion control phase after about 20 h (Figs. 3(a) and 4(a)). The reaction of slag is accelerated when the active phase of slag is corroded by a strong alkaline solution because much Ca(OH)2 is generated at high temperature [26]. Thus, the third exothermic peak is overlapped by the second one. The hydration of composite binder blended with slag is sensitive to the temperature. The change of hydration rate curve for composite binder blended with fly ash is not so obvious.

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Fig. 5. Hydration heat evolution rate and quantity of cement–slag binder at 60 ◦ C. (a) Hydration heat evolution rate and (b) hydration heat.

Figs. 3(b) and 4(b) and Table 3 show that the total heat emission of binder in the early period has a substantial increase when the hydration temperature rises from 25 ◦ C to 45 ◦ C. It can be seen from Table 3 that the 12 h heat emission increases by 122.5% from 78.2 J/g to 174 J/g for pure cement, by 184.4% from 45.5 J/g to 129.4 J/g for the binder containing 50% of slag and by 166.9% from 33.5 J/g to 89.4 J/g for the binder containing 50% of fly ash. Raising temperature promotes the early hydration of composite binder more than that of pure cement. As Table 3 shows, after 168 h of hydration, the heat emission increases by 17.8% from 281.1 J/g to 331.0 J/g for pure cement, by 33.5% from 227.3 J/g to 303.4 J/g for the binder containing 50% of slag and by 17.9% from 167.2 J/g to 197.2 J/g for the binder containing 50% of fly ash. The hydration of the composite binder blended with slag is sustainably developed in the later period. It leads to a great increase of the total heat emission. There is little difference between the hydration heat emission of composite binders containing no more than 50% of slag and that of pure cement in the later period (Fig. 3(b)). The composite binders containing 70% and 90% of slag have much low total heat emission due to the small mass fraction of cement in binders. Therefore, higher curing temperature increases the reactivity of the slag, and as the increased dosage of slag, its reactivity is reduced. These results are in agreement with the mechanism explained by literature [24,27] and previously described. The hydration heat emission of cement–fly ash composite binders under high temperature in the later period does not increase much resulting in a certain difference of heat emission with pure cement.

Fig. 6. Hydration heat evolution rate and quantity of cement–fly ash binder at 60 ◦ C.

3.3. Characteristics of hydration heat evolution of composite binder at 60 ◦ C Figs. 5 and 6 show hydration heat evolution curves for the cement–slag binders and cement–fly ash binders at 60 ◦ C, respectively. Fig. 5(a) shows that the reaction is fast when the temperature rises to 60 ◦ C, the induction period of composite binders blended with slag ends after about 1.20 h of hydration (Table 3). The composite binder containing no more than 50% of slag reaches the second exothermic peak after about 4 h of hydration (Table 3). The second exothermic peak for composite binders containing 70% and 90% of slag occur much early. Ca(OH)2 is generated quickly by the hydration of cement and slag is activated significantly by the high temperature. From Fig. 6(a) and Table 3 it can be seen that when the hydration temperature reaches 60 ◦ C, fly ash is greatly activated, the ending time of induction period and the reaching time of the second exothermic peak come much early, forming a high but very narrow second exothermic peak. F. Deschner et al. [28] showed that due to the enhanced pozzolanic reaction of fly ash at elevated temperature, the change of the composition of C-S-H and the pore solution toward lower Ca and higher Al and Si concentration is shifted toward earlier hydration times. An obvious exothermic effect appears on the heat evolution rate curve of composite binder containing 65% of fly ash after about 13 h of hydration because the depolymerizing ability of vitreous fly ash increases with the rise of temperature. After about 10 h of hydration, the hydration of composite binder containing mineral admixture comes into a diffusion-controlled process (Figs. 5(a) and 6(a)).

F. Han et al. / Thermochimica Acta 586 (2014) 52–57

Table 3 shows that when the hydration temperature rises from 25 ◦ C to 60 ◦ C, for pure cement, the maximum heat evolution rate increases by 391.1% from 9.73 J/g h to 47.78 J/g h; the 12 h heat emission increases by 193.6% from 78.2 J/g to 229.6 J/g; the 168 h heat emission increases by 19.9% from 281.1 J/g to 337.1 J/g. For the binder containing 50% of slag, it increases by 566.5% from 5.25 J/g h to 34.99 J/g h, by 322.0% from 45.5 J/g to 192.0 J/g, by 49.1% from 227.3 J/g to 338.8 J/g, respectively. And for the binder containing 50% of fly ash, it increases by 284.1% from 5.60 J/g h to 21.51 J/g h, by 258.2% from 33.5 J/g to 120.0 J/g, by 20.9% from 167.2 J/g to 202.2 J/g, respectively. When the temperature rises to 60 ◦ C, the hydration of binder is promoted greatly, the early hydration rate increases significantly and the total heat emission of binder increases in the later period of hydration, too. The hydration heat emission of composite binder containing no more than 50% of slag and that of pure cement is about the same or even higher in the later period (Table 3, Fig. 5(b)). Its total heat evolution curve still tends to increase over time. The latent heat of crystallization of amorphous phase in slag is 200 J/g [29]. The total heat emission of composite binder mixed with small amount of slag will exceed that of pure cement if this latent heat releases under the stimulation of high temperature. The total heat evolution curve of cement–fly ash composite binder is leveling off after 40 h of hydration (Fig. 6(b)). The total heat emission of composite binder blended with fly ash still cannot exceed that of pure cement because of the low activity of fly ash although raising temperature substantially increases the total heat emission of binder and diminishes the gap between cement–fly ash composite binder and pure cement in the later period of hydration. 4. Conclusion 1. The hydration heat evolution rate of composite binder decreases rapidly with the increase of dosage of mineral admixtures at room temperature. The reduced hydration heat is not proportional to the dosage of mineral admixtures. 2. The hydration of composite binder is accelerated at 45 ◦ C. The ending time of induction period and the reaching time of the second peak are shortened significantly and the peak value is increased greatly. The hydration heat emission of composite binder containing no more than 50% of slag and that of pure cement have little difference in the later period, but there is still a certain gap between composite binder blended with fly ash and pure cement. 3. A high but very narrow second exothermic peak is formed at 60 ◦ C. The hydration heat emission of composite binder containing no more than 50% of slag is the same or higher than that of pure cement in the later period. The difference of total hydration heat emission between the composite binder blended with fly ash and pure cement becomes small. 4. The promotion effect of the elevated temperature on the hydration of composite binder containing mineral admixture is greater than that of pure cement. 5. The hydration of composite binder blended with slag is very sensitive to temperature. Acknowledgment Authors would like to acknowledge National Natural Science Foundation of China (No. U1134008 and No. 51278277).

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