Effect of particle size on early heat evolution of interground natural pozzolan blended cements

Construction and Building Materials 206 (2019) 210–218

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Effect of particle size on early heat evolution of interground natural pozzolan blended cements M. Kemal Ardog˘a ⇑, Sinan T. Erdog˘an, Mustafa Tokyay Department of Civil Engineering, Middle East Technical University, Ankara, Turkey

h i g h l i g h t s
A natural pozzolan-blended cement is sieved to different size subgroups.
The nucleation effect dominates in cements containing finer pozzolans.
Hydration kinetics are controlled by the dilution effect in coarser cements.
Adjusting cement size subgroups, the unsieved cement hydration can be predicted.

a r t i c l e

i n f o

Article history: Received 15 August 2017 Received in revised form 22 January 2019 Accepted 9 February 2019

Keywords: Hydration Heat Particle size Cement Natural pozzolan Intergrinding

a b s t r a c t Natural pozzolans are increasingly being used in blended cements due to their compositional uniformity and local abundance in some countries. They influence hydration, both at early and late ages. A control portland cement and three interground cements containing natural pozzolan were produced using a laboratory ball mill and sieved through 10-mm, 35-mm and 50-mm sieves to yield different cement size subgroups. The heat evolution of each subgroup was investigated up to 2 days using isothermal calorimetry. The compositions of the size subgroups and their early hydration behaviors were found to differ from those of the unsieved cement subgroups. Subgroups containing finer pozzolan particles had higher heat of hydration due to the nucleation effect. In the coarse cement subgroups with relatively lower amounts of clinker, the dilution effect was dominant. It was shown that early heat evolution characteristics of cements can be controlled, among other ways, by adjusting the particle size distribution. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction The hydration kinetics of blended cements are different than ordinary portland cements and the difference can be observed in their heat evolution curves [1]. Pozzolanic materials contribute to heat of hydration through pozzolanic reactions [1,2]. Once the concentration of calcium hydroxide release from hydrating cement reaches a critical value in the pore solution, the reactive fractions of the pozzolan react with it to form C-S-H, C-A-H gels, as well as AFm and AFt phases [1]. Until this time, the contribution of pozzolanic reactions to the hydration process is not significant [3]. However, like all finely divided mineral matter, pozzolans can accelerate (or decelerate) early hydration through a variety of physical effects even prior to reacting chemically [1].

⇑ Corresponding author. E-mail addresses: [email protected] (M.K. Ardog˘a), [email protected] (S.T. Erdog˘an), [email protected] (M. Tokyay). https://doi.org/10.1016/j.conbuildmat.2019.02.055 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

The physical effects of a mineral additive can basically be divided into the dilution and nucleation effect. In the dilution effect, due to the presence of the pozzolan, the amounts of clinker and gypsum are reduced. Thus, especially in the short term, the heat evolved from the hydration reactions of clinker and gypsum in the cement will be lower [1,4]. The nucleation effect on the other hand, dominates when a small amount of clinker is replaced by a fine pozzolan. At the beginning of hydration, more products collect on the surfaces of the mineral additives which, being smaller than the clinker particles, serve as nucleation sites. For this effect, the average particle size of the additive must be sufficiently small [1,5,6]. The physical effect of mineral additives on early hydration is complex and other explanations have also been suggested [5,7– 11]. Mineral additives not only modify the rate of heat evolution during hydration but they also modify different parts of the rate of heat evolution curve [1,9,12]. The adsorption of some ions of the pore solution on the surface of the mineral admixture leads to the increase of the main peak of rate of heat evolution curve

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and the change in the concentration of the ions causes the prolonged dormant period [1,9]. Particle size distribution (PSD) plays a significant role in the heat of hydration evolution of cement [2,13–15]. Increased specific surface area of cement leads to higher strength and decreased porosity of the interfacial transition zone (ITZ) especially at early ages [13–18]. The hydration and property development of cement pastes and mortars can also be ‘‘tailored” by carefully controlling the PSD of the cement and the relative sizes of the cement and mineral additive [10,12,14,19]. In the production of cements, grinding is an important factor, too. In natural pozzolanincorporated cements, compared to the separate grinding method, intergrinding provides a more homogeneous cement and leads to some interactions between the particles of different ingredients in the cements [20]. This study is a part of an extensive project on the effects on early hydration of different mineral additives interground with clinker and gypsum. Initially, the effects of different amounts of natural pozzolan were investigated in two cement size subgroups where the dividing line was 45 mm [21]. Then, blended cements containing ground granulated blast furnace slag (GGBFS) were sieved through 10, 35 and 50 mm-opening sieves and the effect of GGBFS incorporation on the hydration of blended cements was examined by comparing heat evolution characteristics [22]. The present study investigates the contributions of different cement size subgroups of natural pozzolan-blended cements to heat evolution during early hydration.

2. Materials and methods All materials used in the study were obtained from Bursa Cement Factory in Turkey. Their chemical compositions, found according to TS EN 196-2 [23], are given in Table 1. Three blended cement groups (named 3P, 11P and 22P) were prepared, in which 3, 11 and 22% (by mass) of the clinker portion was replaced by a natural pozzolan, respectively. The natural pozzolan used was a volcanic tuff. All cement groups including the control (0P) were ground using a laboratory ball mill to 3950 ± 200 cm2/g Blaine specific surface area. The reactive silica content of the natural pozzolan was found as 33.6% according to TS EN 196-2 and TS EN 197-1 [23,24]. The reactive silica content of the pozzolan two days after mixing with water was determined to be 34.4%. The similar reactive silica contents imply that the pozzolan is not remarkably self-cementing, up to two days. The 28-day strength activity index of the pozzolan was determined as 75.6% according to ASTM C 618 [25]. The X-ray diffractrogram of the natural pozzolan is given in Fig. 1. The hump of the diffractogram between 18° and 35° 2 theta indicates that the natural pozzolan also has a non-diffractable portion. 5% gypsum (by mass of clinker + gypsum) was added to the 0P and 3P cement groups to control setting. For the 11P and 22P cement groups, the amount of added gypsum was reduced to 4% due to the decrease in the clinker content. The cement groups were divided into <10, 10–35, 35–50, and >50 mm cement size subgroups by sieving. The sieving was performed using a mechanical shaker that also used ultrasonic pulses to rotate particles on the sieve surfaces and to unclog sieve openings. Thus, for each cement group, 5 cement subgroups (4 sieved and 1 unsieved [original] subgroup) were obtained. In naming the subgroups, the letter ‘‘U” indicates the unsieved, ‘‘all-in” cements. The numbers following the letter ‘‘P” represent the sizes of sieves which the cement is retained on and passes through, respectively. So, 11P10-35 has been sieved from a cement containing 11% natural pozzolan and 4%

Table 1 Chemical compositions of the materials used.

SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O LOI

Clinker (%)

Gypsum (%)

Natural pozzolan (%)

20.49 4.49 4.29 66.41 0.97 0.77 0.21 0.80 1.10

1.85 0.05 0.19 32.40 0.21 44.47 <0.01 0.07 20.89

59.26 17.21 4.65 5.94 1.72 0.25 3.47 1.84 4.58

211

Fig. 1. The X-ray diffractrogram of the natural pozzolan.

gypsum (cement group 11P), to pass the 35 mm-opening sieve and is retained on the 10 mm-opening sieve. In contrast, 0PU is the unsieved cement subgroup which contains no natural pozzolan (only 5% gypsum). The particle size distributions of the cement subgroups were obtained by the low-angle light scattering (laser diffraction). The real parts of the refractive indices were chosen as 1.73 and 1.50, respectively, for the portland cement and natural pozzolan. The heats of hydration of the cement subgroups were measured at 23 °C using an isothermal conduction calorimeter and using the in-situ procedure in ASTM C 1702 [26]. For each cement subgroup, 3.5 g of cement was mixed with 1.4 g of distilled water (water – conglomerate ratio of 0.4) and their cumulative heat of hydration and rate of heat evolution curves were obtained. Because the in-situ procedure was used, the heat evolution in the first minutes of hydration could be measured. The measurement was continued for 48 h. The heat released was normalized according to the total mass of powder (clinker + gypsum + natural pozzolan).

3. Results and discussion 3.1. Chemical composition and sieving Oxide analyses of the cement subgroups are given in Table 2. The sampling and analysis were performed according to TS EN 196-7 [27] and TS EN 196-2 [23]. Prior to the analysis, the materials were dried at 105 ± 5 °C to remove any moisture. The oxide contents of the cement subgroups change with changing pozzolan content and average particle size. With increasing pozzolan, the lime content decreases and the silica content increases. It can be clearly seen that the oxide contents of the cement size subgroups differ from those of the unsieved cement subgroups. Generally, the lime content is lower and silica content is higher for the 0–10 mm cement size subgroups compared to other cement subgroups. Similarly, SO3 is higher in this size subgroup due to the higher grindability (easier to grind) of gypsum. Loss on ignition (LOI) increases with increasing pozzolan content and decreasing average particle size. This is attributed to the porous structure of the natural pozzolan [1] which results in greater moisture absorption from the atmosphere. The smaller average particle size of the natural pozzolan (due to its higher grindability compared with that of the clinker) is another reason for the moisture absorption. Furthermore, the fine particles are more susceptible to carbonation and prehydration. In addition, the amount of the gypsum is higher in the finer cement size subgroups as well (due to its higher grindability) and the water present in the gypsum increases the LOI in smaller cement size subgroups. Relative proportions of the clinker, natural pozzolan and gypsum in the different cement size subgroups (Table 3) greatly

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212 Table 2 Chemical analysis of cement subgroups.

SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) SO3 (%) Na2O (%) K2O (%) LOI (%)

SiO2 (%) Al2O3 (%) Fe2O3 (%) CaO (%) MgO (%) SO3 (%) Na2O (%) K2O (%) LOI (%)

0PU

0P0-10

0P10-35

0P35-50

0P50-2000

3PU

3P0-10

3P10-35

3P35-50

3P50-2000

19.82 4.76 4.65 64.78 1.09 1.53 0.24 0.65 2.07

18.45 4.73 4.58 63.24 1.06 2.54 0.22 0.67 4.05

20.52 4.81 4.58 65.20 1.09 1.19 0.22 0.57 1.69

20.45 5.00 4.75 65.07 1.11 1.13 0.20 0.57 1.65

20.29 5.08 4.80 64.23 1.09 1.89 0.20 0.62 1.69

20.33 4.95 4.51 62.15 1.09 2.52 0.33 0.67 2.53

19.32 4.99 4.51 59.51 1.06 4.14 0.28 0.67 4.80

20.74 4.94 4.45 62.64 1.08 2.22 0.24 0.57 2.63

20.84 5.18 4.61 63.07 1.09 2.19 0.22 0.57 2.18

20.74 5.19 4.63 61.62 1.08 2.73 0.26 0.63 2.39

11PU

11P0-10

11P10-35

11P35-50

11P50-2000

22PU

22P0-10

22P10-35

22P35-50

22P50-2000

23.88 6.09 4.65 58.08 1.16 1.56 0.58 0.70 2.79

24.03 6.50 4.75 54.21 1.16 2.92 0.57 0.74 4.98

23.81 6.02 4.57 58.79 1.14 1.58 0.47 0.64 2.70

23.74 6.31 4.69 60.07 1.16 1.27 0.41 0.62 1.71

24.93 6.56 4.67 56.47 1.17 1.98 0.55 0.71 2.94

28.04 7.55 4.59 51.74 1.21 1.67 0.96 0.92 3.27

29.19 7.99 4.77 46.91 1.21 2.69 0.92 0.81 5.47

28.04 7.41 4.50 52.43 1.21 1.63 0.79 0.69 3.25

27.38 7.22 4.59 53.77 1.23 1.55 0.75 0.71 2.76

30.28 8.05 4.58 49.25 1.24 1.89 0.98 0.81 2.90

Table 3 Relative mass fractions of the materials for each cement subgroup.

Clinker (%) Gypsum (%) Natural pozzolan (%)

Clinker (%) Gypsum (%) Natural pozzolan (%)

0PU

0P0-10

0P10-35

0P35-50

0P50-2000

11PU

11P0-10

11P10-35

11P35-50

11P50-2000

95.2 4.8 –

90.7 9.3 –

96.4 3.6 –

96.1 3.9 –

93.6 6.4 –

84.1 4.8 11.1

75.2 10.6 14.2

85.7 3.9 10.4

88.6 2.0 9.4

81.5 4.8 13.7

3PU

3P0-10

3P10-35

3P35-50

3P50-2000

22PU

22P0-10

22P10-35

22P35-50

22P50-2000

89.8 7.2 3.0

82.6 13.8 3.6

91.4 5.4 3.2

92.6 4.4 3.0

89.1 6.7 4.1

73.7 4.6 21.7

63.6 9.4 27.0

75.4 3.5 21.2

77.8 3.0 19.2

70.3 3.0 26.7

influence their oxide compositions. These were calculated using the CaO and SiO2 (as the two main oxides) contents of the clinker, pozzolan, gypsum, shown in Table 1, and the cement itself, shown in Table 2. The lowest clinker contents were obtained in 0–10 mm cement size subgroups for all cement groups due to the lower grindability of the clinker compared to those of natural pozzolan and gypsum [28]. It should be noted that in each of the different cement size subgroups (obtained by sieving the cement group), there was a considerable amount of particles of sizes outside of the limits of the specific subgroup, as shown in Table 4. Two possible reasons for this are: (i) the laser particle sizer used operates on the assumption that the particles are spherical which obviously is not the case for cements [29] hence sieving results do not match laser diffraction results and (ii) the secondary attractive forces between the cement particles lead to a decrease in sieving efficiency [30]. The relative proportion of particles of the different size fractions in a given

cement subgroup can be calculated considering its particle size distribution curve. 3.2. Cumulative heat evolution of cement samples The cumulative heat of hydration evolved up to 48 h for each cement subgroup is given in Fig. 2. For all cement subgroups, the 0–10 mm cement size subgroups have the highest heat of hydration. Those of the unsieved cement subgroups and 10–35 mm cement size subgroups are nearly equal to each other due to their similar relative proportions (Table 4) and they are higher than the heats of hydration for 35–50 mm and 50–2000 mm cement size subgroups. The heat of hydration naturally decreases with the increasing average particle size of the cement size subgroups [14,15]. Among all the unsieved cement subgroups and 10–35 mm cement size subgroups, the 3P cements have the highest cumulative heat of hydration. However, this is not the case for the coarser

Table 4 Relative proportion of cement size subgroups. Particle Size Range

0PU (%)

0P0-10 (%)

0P10-35 (%)

0P35-50 (%)

0P50-2000 (%)

11PU (%)

11P0-10 (%)

11P10-35 (%)

11P35-50 (%)

11P50-2000 (%)

0–10 mm 10–35 mm 35–50 mm 50–2000 mm

36.4 36.1 12.6 14.9

86.6 13.4 0 0

20.4 64.8 12.4 2.5

17.8 22.7 27.9 31.6

15.5 9.0 16.3 59.3

35.5 36.6 12.5 15.4

90.4 9.6 0 0

35.6 54.6 8.4 1.4

9.1 20.1 39.0 31.1

14.5 10.8 14.7 60.0

Particle Size Range

3PU (%)

3P0-10 (%)

3P10-35 (%)

3P35-50 (%)

3P50-2000 (%)

22PU (%)

22P0-10 (%)

22P10-35 (%)

22P35-50 (%)

22P50-2000 (%)

0–10 mm 10–35 mm 35–50 mm 50–2000 mm

35.3 35.7 12.3 16.7

81.9 15.7 1.7 0.7

27.5 60.9 10.0 1.6

24.0 29.4 23.1 23.1

19.6 12.5 15.5 52.4

36.0 34.7 12.2 17.1

85.9 14.0 0.2 0

32.7 56.9 8.9 1.5

23.8 28.3 23.1 24.9

14.2 10.6 13.6 61.6

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213

Fig. 2. Cumulative heat of hydration of: a) 0P; b) 3P; c) 11P; d) 22P cements up to 48 h.

cement size subgroups. During intergrinding, the average particle size of the natural pozzolan decreases more than that of the clinker particles. Therefore, these particles act as nucleation sites for hydration products and increase the heat of hydration. However, for all 0–10 mm cement size subgroups, 0P0-10 has the highest heat of hydration due to its higher amount of clinker than 3P0-10. The clinker contents of 0P0-10 and 3P0-10 are 90.7% and 82.6%, respectively (Table 3). The greater amount of clinker in 0P0-10 results in higher cumulative heat of hydration than 3P0-10 (Fig. 2a and b). The cumulative heats of hydration of coarser cements decrease with increasing mineral additive incorporation into the cement, due to the dilution effect. However, a few exceptions exist. In spite of its higher mineral additive content, the heat of hydration of 22P35-50 is higher than 11P35-50 at the end of 2 days. This can be attributed to problems with sieving. According to Table 4, the amount of the fine particles (particles between 0 and 10 lm) serving as nucleation sites for the hydration products in 11P35-50 is lower than in 22P35-50. The fine particle amount is approximately 9% in 11P35-50, but about 24% in 22P35-50. 3.3. Rate of heat evolution of cement samples The changes in the rate of heat evolution of the cement samples up to 48 h are shown in Fig. 3. 3.3.1. Pre-induction and induction (dormant) period The time to reach the maximum rate of heat evolution during the pre-induction period (the first peak corresponding to the first

few minutes of hydration in Fig. 3), the minimum rate during the dormant period and the time corresponding to this minimum are given for each cement subgroup in Table 5. The duration of the dormant period is also given. The start and the end of the dormant period were chosen as the points corresponding to 1.2 times the minimum heat flow value during the dormant period. In the dormant period, the minimum rates of hydration (column 5 in Table 5) increase with decreasing average particle size of the cements. The pre-induction peaks (column 2 in Table 5) occur sooner and the time to reach the minimum rate within the dormant period (column 4 in Table 5) is less in the unsieved cement subgroups than in their cement size subgroups. Due to the lower clinker content of the 0–10 mm cement size subgroups than that of the unsieved cement subgroups (Table 3), the first peaks of the unsieved cement subgroups occur earlier. For other cement size subgroups (10–35, 35–50 and 50–2000 mm), although the clinker contents are higher, the average particle sizes are higher. So, the first peak occurs at earlier ages than in the unsieved cement subgroups. Naturally, the rate of heat evolution of 0P cements are the highest among all. As the amount of natural pozzolan in a cement increases, its rate of heat evolution can be expected to decrease. However, the rate of heat evolution of 11P cements are the lowest when identical cement size subgroups are considered. The clinker particles of the 11P cements are surrounded by a sufficient amount of the natural pozzolan particles that block water access. The hydration products in the very beginning of the reaction cover the natural pozzolan and clinker particles. Namely, the nucleation effect occurs locally in the pre-induction period. The hydration

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214

Fig. 3. Rate of heat evolution of: a) 0P; b) 3P; c) 11P; d) 22P cements up to 48 h.

Table 5 Time of pre-induction peak, minimum rate of heat evolution and its corresponding time during the dormant period, and the length of the dormant period. Cements

Time Corresponding to Peak in Pre-induction Period (min)

Duration of Dormant Period (h)

Time Corresponding to the Minimum RHE (min)

Minimum RHE during Dormant Period (J/g.h)

0PU 0P0-10 0P10-35 0P35-50 0P50-2000 3PU 3P0-10 3P10-35 3P35-50 3P50-2000 11PU 11P0-10 11P10-35 11P35-50 11P50-2000 22PU 22P0-10 22P10-35 22P35-50 22P50-2000

4.2 5.4 5.8 5.5 6.2 3.8 6.3 5.7 6.0 4.5 3.6 4.4 4.6 3.9 5.1 4.7 5.8 5.2 5.5 5.1

1.23 1.17 1.40 1.48 1.53 1.21 1.31 1.15 1.76 1.21 1.40 1.36 1.51 1.86 2.18 2.02 2.00 2.10 1.82 2.45

77.7 85.9 87.9 90.1 93.6 76.0 90.7 87.9 122.2 105.6 98.1 117.9 109.4 114.9 144.4 108.8 123.7 109.3 111.0 124.0

2.70 4.83 2.03 1.69 1.35 1.73 3.76 1.30 0.89 0.89 1.44 2.27 1.26 0.70 0.65 1.67 2.66 1.37 1.20 1.07

products collecting on the surfaces form barriers against water passing and decrease the minimum overall rate of heat evolution in induction period. Also supporting this, the maximum rates of heat evolution of the 11P cements shown in Table 5 during the

pre-induction period occur usually sooner than the others (column 2 in Table 5). The higher rates of heat evolution of 0P are also due to the amount of hydration products. The amount of hydration products in the beginning of the reactions is lower (since there is

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no nucleation effect) and the hydrated paste layers around the clinker particles are not thick enough to inhibit the reactions. In other words, some amount of clinker particles continue reacting during the dormant period. In the 22P cements, the dilution effect dominates the nucleation effect and the rate of heat evolution curves are not the lowest as would be expected. The length of the dormant period generally increases with the addition of pozzolan (from 3P to 22P, column 3 in Table 5). However, compared to 0P cements, the addition of only 3% pozzolan shortens the dormant period. Also, finer cement size subgroups show shorter dormant periods because of their higher physicochemical reactivity. 3.3.2. Acceleration period The maximum rate of heat evolution during the acceleration period (including immediately before the second peak) and the time to reach it are given in Table 6, for each cement subgroup. Time to reach the second peak increases with increasing pozzolan content. This delay is attributed to the prolonged dormant periods (column 3 in Table 5) in the pozzolanic cements due to the increase of mineral additive dosage (Table 5) which delays the acceleration period. The maximum rates of heat evolution were the lowest for the 50–2000 mm and highest for the 0–10 mm cement size subgroups. Generally, the maximum rates decrease with the increasing amount of pozzolan. However, 22P35-50 and 22P50-2000 had higher rates than the 11P35-50 and 11P50-2000, respectively. This is explained by the presence of a considerable amount of fine material (0–35 mm) in the former (Table 4). The shoulder seen in the rate of heat evolution curve of 3P0-10 (Fig. 3b) is attributed to the excess gypsum reacting with C3A and C4AF to produce ettringite or iron analogs of it [15]. 3.4. Estimating the contribution of natural pozzolan incorporation to the early heat evolution of cements The heat flows of pozzolan-blended cements were normalized in order to compare them with their 0P counterparts. Normalization of the clinker contents was performed to eliminate the dilution effect of the pozzolan and the normalized heat of hydration was calculated as:

HHnorm ¼ HHexp  ðK 0Px =K iPx Þ

ð1Þ

where, HHnorm and HHexp are the normalized and experimentallydetermined heat of hydration values, and K0Px and KiPx are the clinker fractions of the 0P (control) and iP (containing i % pozzolan) cements for size group x. Thus, for example, at 2 days, HHexp of 3P0-10 is 272.84 J/g (Fig. 2b). K3P0-10 is 82.6% and K0P0-10 = 90.7% (Table 3). So, HHnorm for 3P0-10 is calculated as 272.84* (90.7/82.6) = 299.60 J/g. With this normalization, it would be expected that the 3P0-10 would evolve 27 J/g more heat than it does if it contained the same amount of clinker as in the 0P0-10

215

(its ‘‘no pozzolan” size equivalent). Hence, HHnorm can be calculated similarly, at any age using HHexp. Fig. 4 shows the normalized heat flows for cement size subgroups comparing these with the 0P cements of the same size. It can be seen that due possibly to the nucleation effect, the cement size subgroups containing higher amounts of finer particles (such as the unsieved cement subgroups or 0–10 mm cement size subgroups) have higher normalized cumulative heat of hydration than their 0P equivalents. In such subgroups, the content of natural pozzolan is higher and it contributes to hydration physically. But, as the average particle size of cements becomes higher (e.g. 50– 2000 mm size subgroups), the nucleation effect disappears and the dilution effect dominates. 3.5. Possibility of adjusting the early heat evolution characteristics of cements It is possible to control the heat evolution characteristics of pozzolan-incorporated cements at early ages by considering the heat evolution and the amount of constituents of its cement size subgroups. Firstly, considering the heat evolution, chemical analysis and particle size distribution results of the cement size subgroups, the behavior of the ‘‘theoretical” cement size subgroup, assumed to contain only the particles with sizes within the size limits, is estimated. Then, with a similar approach, using the behavior of the ‘‘theoretical” cement size subgroups, the heat evolution characteristics of unsieved cements are predicted. Fineness and total amount of clinker and gypsum are the two main factors affecting the physicochemical reactivity of a cement. Since the pozzolan does not undergo significant reaction at early ages, the early heat evolution characteristics of blended cements are predicted using its particle size distribution and the cumulative heat evolved or the rate of heat evolution of the clinker + gypsum in its cement size subgroups. However, almost all the cement size subgroups contain particles with sizes beyond the limits of the specific subgroup (Table 4). Thus, for further calculations, a theoretical cement size subgroup needs to be developed. It is assumed that this ‘‘theoretical” cement size group consists only of the particles with sizes between the boundaries of the related size group. In addition, a normalization needs to be performed using the clinker and gypsum contents of the cement size subgroups. The calculations are performed using:

HHXPi;t ¼ ðK þ GÞXPi 

502000 X j¼010

HH XPj;t  PSDj in ðK þ GÞXPj

XPi

ð2Þ

where X refers to the pozzolan content of the cement group, i and j refer to the cement size subgroup (0–10, 10–35, 35–50, 50–2000) and ‘‘theoretical” cement size subgroup (0–10, 10–35, 35–50, 50– 2000) (the sizes of these particles are within the limits of the group), t refers to the age of the cement paste, HHXPi,t and HH*XPj,t refer to the measured heat evolution of the related cement size

Table 6 The time and rate of heat evolution for cement samples around the second peak. Cements

Time (h)

RHE (J/g.h)

Cements

Time (h)

RHE (J/g.h)

0PU 0P0–10 0P10–35 0P35–50 0P50–2000 3PU 3P0–10 3P10–35 3P35–50 3P50–2000

6.6 5.6 7.0 7.4 8.1 8.6 7.2 9.9 10.8 8.7

9.46 18.14 7.94 7.12 4.97 8.34 16.55 7.68 4.41 4.46

11PU 11P0–10 11P10–35 11P35–50 11P50–2000 22PU 22P0–10 22P10–35 22P35–50 22P50–2000

10.5 8.3 10.0 12.5 11.5 12.0 9.2 12.8 13.1 9.5

8.20 16.59 8.83 3.39 2.48 6.58 12.44 6.07 5.02 3.11

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Fig. 4. Comparison of normalized heat of hydration values of pozzolan-incorporated cement groups with the actual heat of hydration of the control cement for: a) unsieved; b) 0–10 mm size; c) 10–35 mm size; d) 35–50 mm size; e) 50–2000 mm size subgroups.

subgroup ‘‘XPi” up to age ‘‘t” (HH3P0-10,2 h for the measured heat evolution at 2 h for the cement size group, 3P0-10) and the heat evolution of the ‘‘theoretical” cement size subgroup ‘‘XPj” up to age ‘‘t” (HH*3P0-10,2 h for the estimated heat evolution at 2 h for the theoretical cement size subgroup, 3P0-10), respectively. PSDj in XPi indicates the particle size fraction of cement size subgroup ‘‘j” in cement ‘‘XPi”. (K + G)XPi and (K + G)XPj shows the total content of clinker and gypsum (in %) of the related cement size subgroup and the ‘‘theoretical” cement size subgroup, respectively. Determining the heat characteristics of the ‘‘theoretical” size subgroup from the

cement size subgroups, it is possible to estimate the heat characteristics of the unsieved cement subgroup using:

HHXPU;test ¼ ðK þ GÞXPU 

502000 X j¼010

HH XPj;t  PSDj in ðK þ GÞXPj

XPU

ð3Þ

As an example, for 11P0-10, the measured cumulative heat evolved (HH11P0-10,2h) up to 2 h is (from Fig. 2c) 34.69 J/g. The clinker + gypsum content for this cement size subgroup ((K + G)11P0-10) is 85.8% (from Table 3). The clinker + gypsum contents of

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Fig. 5. a) Measured vs. estimated heat of hydration of cement groups and; b) comparison of estimated and measured rate of heat evolution of unsieved cement subgroups.

11P10-35, 11P35-50 and 11P50-2000 ((K + G)11P10-35, (K + G)11P35-50, (K + G)11P50-2000) are 89.6%, 90.6% and 86.3%, respectively (Table 3). According to Table 4, in 11P0-10, the proportions of the particles with sizes of 0–10 mm, 10–35 mm, 35–50 mm and 50–2000 mm (PSD0-10 in 11P0-10, PSD10-35 in 11P0-10, PSD35-50 in 11P0-10, PSD50-2000 in 11P0-10) are 90%, 10%, 0% and 0%, respectively. Putting the data into Eq. (2):

HH11P010;2

so,

h

  HH 11P010;2 h  PSD010 in ¼ ðK þ GÞ11P010  ðK þ GÞ11P010 HH 11P1035;2 h  PSD1035 in 11P010 þ ðK þ GÞ11P1035 HH 11P3550;2 h  PSD3550 in 11P010 þ ðK þ GÞ11P3550  HH 11P502000;2 h  PSD502000 in 11P010 þ ðK þ GÞ11P502000

11P010

  HH 11P010;2 h  90% HH 11P1035;2 h  10% þ 85:8% 89:6%  HH 11P3550;2 h  0% HH 11P502000;2 h  0% þ þ 90:6% 86:3%

34:69 ¼ 85:8% 

Similarly, Eq. (2) can also be written for the 11P10-35, 11P35-50 and 11P50-2000 cement size subgroups. From these four equations, the unknowns, the heats evolved of 0–10 mm, 10–35 mm, 35–50 mm and 50–2000 mm theoretical cement size subgroups (HH*11P0-10,2 h, HH*11P10-35,2 h, HH*11P35-50,2 h, HH*11P50-2000,2 h) are calculated as 38.14, 3.81, 7.57 and 1.97 J/g, respectively. According to Table 4, for 11PU, the relative proportions of the particles with sizes of 0–10 mm, 10–35 mm, 35–50 mm and 50–2000 mm (PSD0-10 in 11PU, PSD10-35 in 11PU, PSD35-50 in 11PU, PSD50-2000 in 11PU) are 35%, 37%, 13% and 15%, respectively. The behavior of the unsieved cement can then be estimated using the ‘‘theoretical” results in Eq. (3):

HH11PU;2

hest

  HH 11P010;2 h  PSD010 in ðK þ GÞ11P010 HH 11P1035;2 h  PSD1035 in 11PU þ ðK þ GÞ11P1035 HH 11P3550;2 h  PSD3550 in 11PU þ ðK þ GÞ11P3550  HH 11P502000;2 h  PSD502000 in 11PU þ ðK þ GÞ11P502000

¼ ðK þ GÞ11PU 

11PU

so,

HH11PU;2

hest

 38:14  35% 3:81  37% þ 85:8% 89:6%  7:57  13% 1:97  15% þ ¼ 16:50 J=g þ 90:6% 86:3%

¼ 88:9% 

In comparison, the measured value of 11PU at 2 h is 15.55 J/g. Fig. 5a compares the estimated and measured cumulative heats of hydration for the unsieved cement groups. Each (estimated, measured) data point on the curves is for a distinct time/age, up to 48 h. The slopes of the best fit lines are close to 1.00 and the intercepts close to 0, which shows that the estimation is satisfactory. A similar estimation of the rate of heat evolution of the cements using Eqs. (2) and (3), is again quite successful (Fig. 5b). Fig. 5 hence shows that it is possible to obtain a cement with the desired heat evolution characteristics, in agreement with the findings of similar studies [14,21,22,31–33]. 4. Conclusions This study investigated the effects of different size subgroups of natural pozzolan-containing cements on their heat evolution characteristics and the following conclusions were reached:
The cumulative heat of hydration and the rate of heat evolution are greater for finer cement size subgroups. The length of the dormant period changes with pozzolan content and particle size. An increase in pozzolan content generally lengthens the dormant period except for 3P cements with their low amount of pozzolan, in which the nucleation effect dominates and the dormant period is shorter. An increase in average particle size of reduces physicochemical reactivity, leading to a longer dormant period.
Changes in average particle size can affect both the specific surface area and the relative contents of constituents in blended cements. Amounts of gypsum and natural pozzolan in finer cement size subgroups are higher than in coarse cement size subgroups, owing to their higher grindabilities. Size subgroups containing finer pozzolan particles evolve greater heat of hydration due to the nucleation effect where as in coarse size subgroups with relatively lower amounts of clinker, the dilution effect dominates.
Addition of the selected natural pozzolan dilutes the cements leading to decreases in rate and amount of heat evolution. However, when only the clinker contents of the cement size groups

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are considered, the nucleation effect dominates in cements containing finer particles, thus the normalized heat values are higher than in cements without any pozzolan.
The early heat evolution characteristics of cements can be controlled, by adjusting the particle size distribution. Using the heat evolution curves of different cement size subgroups, the particle size distributions, and the total amounts of clinker and gypsum normalized with the clinker + gypsum contents of the unsieved cement subgroups, the heat characteristics for unsieved cement subgroups could be predicted successfully. Conflict of interest The authors declare that there is no conflict of interest. Acknowledgements The authors appreciate the support of Turkish Cement Manufacturers’ Association (TCMA) for oxide, XRD analyses and for providing the materials used in this study. References [1] F. Massazza, Pozzolana and pozzolanic cements, in: Lea’s Chem. Cem. Concr., Elsevier, 2003, pp. 471–635. doi:10.1016/B978-075066256-7/50022-9. [2] N.Y. Mostafa, P.W. Brown, Heat of hydration of high reactive pozzolans in blended cements: Isothermal conduction calorimetry, Thermochim. Acta 435 (2005) 162–167, https://doi.org/10.1016/j.tca.2005.05.014. [3] W.A. Gutteridge, J.A. Dalziel, Filler cement: The effect of the secondary component on the hydration of Portland cement, Cem. Concr. Res. 20 (1990) 778–782, https://doi.org/10.1016/0008-8846(90)90011-L. [4] P. Siler, J. Kratky, N. De Belie, Isothermal and solution calorimetry to assess the effect of superplasticizers and mineral admixtures on cement hydration, J. Therm. Anal. Calorim. 107 (2012) 313–320, https://doi.org/10.1007/s10973011-1479-8. [5] P. Lawrence, M. Cyr, E. Ringot, Mineral admixtures in mortars, Cem. Concr. Res. 33 (2003) 1939–1947, https://doi.org/10.1016/S0008-8846(03)00183-2. [6] V. Rahhal, R. Talero, Early hydration of portland cement with crystalline mineral additions, Cem. Concr. Res. 35 (2005) 1285–1291, https://doi.org/ 10.1016/j.cemconres.2004.12.001. [7] V. Rahhal, V. Bonavetti, L. Trusilewicz, C. Pedrajas, R. Talero, Role of the filler on Portland cement hydration at early ages, Constr. Build. Mater. 27 (2012) 82– 90, https://doi.org/10.1016/j.conbuildmat.2011.07.021. [8] B. Lothenbach, K. Scrivener, R.D. Hooton, Supplementary cementitious materials, Cem. Concr. Res. 41 (2011) 1244–1256, https://doi.org/10.1016/j. cemconres.2010.12.001. [9] B.W. Langan, K. Weng, M.A. Ward, Effect of silica fume and fly ash on heat of hydration of Portland cement, Cem. Concr. Res. 32 (2002) 1045–1051, https:// doi.org/10.1016/S0008-8846(02)00742-1. [10] T. Zhang, Q. Yu, J. Wei, P. Zhang, P. Chen, A gap-graded particle size distribution for blended cements: Analytical approach and experimental validation, Powder Technol. 214 (2011) 259–268, https://doi.org/10.1016/j. powtec.2011.08.018. [11] R.K. Dhir, Pulverized fly ash, Cem. Replace. Mater. (1986) 197–250. _ [12] M. Tokyay, T. Delibasß, I.Ö. Yaman, Heat of hydration of GGBFS and natural pozzolan incorporated cements, Cem. Concr. World. 17 (2012) 70–87.

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