REVISED VIEW ON CHEMICAL SHRINKAGE

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1 REVISED VIEW ON CHEMICAL SHRINKAGE S. Zhutovsky and K. Kovler National Building Research Institute - Faculty of Civil and Environmental Engineering Technion - Israel Institute of Technology, Haifa 32000, Israel Abstract Extensively studied, chemical shrinkage is considered to be an intrinsic property of Portland cement. It is generally assumed to be proportional to degree of hydration, since it has its origin in the hydration reaction stoichiometry. Furthermore, generally accepted point of view is that chemical shrinkage is independent of water/cement (w/c) ratio. The new representation and interpretation of the chemical shrinkage of hydrating cement paste is given in the paper. The chemical shrinkage of cement pastes with w/c ratios of 0.45, 0.33, 0.25 and 0.21 was tested by weighing-in-water method. The degree of hydration of the pastes was examined by thermogravimetric analyzer (TGA). Finally, the effect of water/cement ratio on chemical shrinkage of hydrating cement paste was demonstrated and compared with the literature data. 1. INTRODUCTION Interest to chemical shrinkage has grown with the wide spread of high-performance concrete (HPC). The major problem of HPC is early-age cracking sensitivity, which is caused by autogenous shrinkage and self-desiccation. Chemical shrinkage is considered the main driving force of self-desiccation and autogenous shrinkage [1]. Thus, correct calculation is indispensable in autogenous shrinkage modeling and internal curing applied mix design. Extensively studied, chemical shrinkage is considered an intrinsic property of Portland cement. For the most part, it is assumed proportional to degree of hydration, since it originates in the hydration reaction stoichiometry. Furthermore, generally accepted point of view is that chemical shrinkage is independent of water/cement ratio. However, the traditional way of data representation of chemical shrinkage experimental results does not take into account the actual degree of cement hydration. Although the chemical shrinkage is produced only by hydrated cement, the measured chemical shrinkage volume is conventionally related to the initial (total) cement content. In this paper, in order to identify effect of w/c ratio and degree of hydration, the chemical shrinkage of cement pastes with water/cement ratios of 0.45, 0.33, 0.25 and 0.21 was tested by weighing-in-water method. The degree of hydration of the pastes was examined by thermogravimetric analyzer (TGA). The experimental chemical shrinkage data were analyzed 775

2 considering the measured degree of hydration, and revised definition and interpretation of the chemical shrinkage of hydrating cement paste is given. The effect of water/cement ratio and degree of hydration on chemical shrinkage of hydrating cement paste was evaluated and compared with the literature data. 2. THEORETICAL BACKGROUND When cement reacts with water, the volume of reaction products is smaller than the sum of the volumes of reactants. This phenomenon is known as chemical shrinkage or Le Chatelier contraction after the name of the scientist that first discovered it [2]. In cement pastes with low w/c ratio, chemical shrinkage results in self-desiccation and autogenous shrinkage which can lead to cracking and even failure. Portland cement is poly-mineral compound and its hydration is extremely complex. However, it is conventional practice to represent the hydration of Portland cement by a set of simplified equations, as, for example, the following: C S+ 5.3H C SH +1.3CH C S+ 4.3H C SH + 0.3CH (1) (2) C3A + 3CSH2 + 26H C3A 3CS H (3) 32 2C A + C A 3CS H + 4H 3 C A CS H ( ) C AF+ 3 CSH + 27H C AF 3CS H + CH (5) ( ) ( ) 2 C4AF + C3 AF 3CS H32 + 6H 3 C3 AF CS H CH C3A + CH + 12H C4AH13 ( ) C AF + 2CH + 10H 2C AF H (4) (6) (7) (8) 1.1CH + S + 2.8H C1.1SH 3.9 (9) This set of equations represents the hydration reactions of main Portland cement minerals, and includes pozzolanic reaction - equation (9). Each reaction has its own stoichiometry resulting in different chemical shrinkage. If, for instance, equation (1) is considered, using the reaction stoichiometry and known molar volume, which is molar mass divided by density, the chemical shrinkage of alite can be calculated as follows: 776

3 C 3 S + 5.3H C 1.7 SH CH Molar volume (V m ), cm 3 /mole Density (ρ), g/cm Molar mass (M r ), g/mole cm cm cm cm 3 Chemical Shrinkage cm cm 3 cm 3 /mole of cm 3 /g of C 3 S = = C 3 S hydrated hydrated It can be seen that hydration of each gram of alite results in reduction of volume by 69 mm 3, i.e. to chemical shrinkage of alite is 69 mm 3 /g of C 3 S hydrated. The total chemical shrinkage of cement is the sum of chemical shrinkage of its constituting minerals. Let us define specific chemical shrinkage as the total volume reduction of solid and liquid substances per unit weight of cement hydrated, in cement paste. If the cement composition is known, the specific chemical shrinkage of cement can be quantified from the volume stoichiometry of the hydration reaction for any of general-type cement. If Powers model of cement paste [3] is considered and the molar mass and density of cement gel are assumed constant, then the total chemical shrinkage of any cement mineral ( V i cs) at any time (t) can be calculated as product of specific chemical shrinkage of the given mineral (CS i ), the relative content of the mineral in cement ([i]), the initial weight of cement (C) and degree of hydration of the mineral (α i ): i V () t = CS [] i C α () t cs i i Expressions for the calculation of the total chemical shrinkage similar to equation (10) were used by several researchers [4, 5]. It assumes that the specific chemical shrinkage is independent of both degree of hydration and w/c ratio. For this reason, chemical shrinkage measurement is often put forward as a method of the determination of the degree of cement hydration [6]. The chemical shrinkage of cement paste can be measured by two general approaches. The first approach implies the direct measurement of volume changes of the cement paste during hydration under water curing. This methodology was originally developed by Le Chatelier and is commonly referred as dilatometry method [7].The second approach uses Archimedes' principle in order to evaluate volume changes of hydrating cement paste. Chemical shrinkage of hydrating cement paste is gauged by weighting the hydrating cement paste in water. Accordingly, the measured weight change corresponds to the chemical shrinkage. This method is generally referred as buoyancy method, gravimetery method or simply weighing method [8]. The advantage of this method is very easy implementation of the measurement automation by connecting the balance to a data logging system. Another method, which is rather a modification of dilatometry method than independent approach, is pycnometery [9]. It implies keeping the total volume of paste and water constant, during continuous weighting that allows automatic data logging. The experimental set-up for these three methods is demonstrated in Figure 1. Since chemical shrinkage causes a number of problems in cementitious systems with low w/c ratio, scientific interest to chemical shrinkage aroused with the growth of the popularity (10) 777

4 of HPC. The effect of cement composition, temperature, mineral and chemical admixtures, w/c ratio, and degree of hydration on chemical shrinkage as well as correlation between chemical and autogenous shrinkage has been investigated [7-13]. Several researchers [10-13], reported the experimental data which support the contention that specific chemical shrinkage is independent of degree of hydration and w/c ratio. However, the method of data representation casts some doubts upon the conclusions. Traditionally chemical shrinkage is described as the volume reduction related to the initial cement content. However, since only hydrated cement causes chemical shrinkage, it cannot be concluded from such data representation in which manner the specific chemical shrinkage is affected by w/c ratio or degree of hydration. The applicability of equation (10) comes into question, considering that in cement pastes with lower w/c ratio the initial cement content is higher, while the final degree of hydration is lower. As was shown before, chemical shrinkage depends on the density of hydration products. Concept of constant chemical shrinkage suggests that the cement gel density is also constant and independent of degree of hydration and w/c. However, there is experimental evidence that cement gel has uneven microstructure, affected by degree of hydration and w/c and resulting in density changes in time. Recently Tennis and Jennings published a model of calcium silicate hydrate (CSH) that based on two types of gel: low-density (LD) CSH and high-density (HD) CSH [14, 15]. Relying on the experimental data of nitrogen sorption, they deduced that in cement paste exist HD CSH and LD CSH which can be associated with inner and outer hydration products. Since the distance between cement particles in lower w/c ratio pastes is shorter, there is less space for outer hydration products, i.e. for LD CSH. Hence, the LD/HD CSH ratio will decrease with w/c ratio. For the similar reasoning, the LD/HD CSH ratio will decline with increase in degree of hydration for cement pastes with w/c ratio below 0.4 [14]. Summarizing the above argumentation, further research is required in order to assess the effect hydration degree and w/c ratio on specific chemical shrinkage. In the case that experimental data will give a proof to LD and HD CSH model, equation (10) should be adopted for each of the two types of cement gel. a b c Figure 1: Experimental set-up for various methods of chemical shrinkage measurement: (a) dilatometry, (b) buoyancy, (c) picnometry 778

5 3. MATERIALS AND METHODS The chemical shrinkage of cement pastes with water/cement ratios of 0.45, 0.33, 0.25 and 0.21 was tested by weighing-in-water method. The degree of hydration of the pastes was examined by TGA. 3.1 Materials Commercially available ordinary Portland cement of CEM I 52.5 N type manufactured by Nesher - Israel Cement Enterprises Ltd. was used. The chemical composition of the cement according ASTM C is given in Table 1. The loss on ignition was 4.2 % by weight. Table 1: Chemical composition of Portland cement Oxide CaO SiO 2 Al 2 O 3 Fe 2 O 3 MgO TiO 2 K 2 O Na 2 O SO 3 % by weight Specific surface area of the Portland cement, tested according ASTM C204, was m 2 /kg. Setting times were determined in accordance with ASTM C191. The initial setting time comprised 160 minutes and final setting time 220 minutes. Cement paste samples were made at water to cement ratios of 0.21, 0.25, 0.33 and Commercially available high range water reducing agent "RHEOBUILD 2000b" of the naphthalene formaldehyde sulfonate type was used for mixes with w/b ratio of 0.21, at content of 3% and w/b ratio of 0.25 at content of 2% by weight of cement. 3.3 Chemical shrinkage Chemical shrinkage of hydrating cement pastes are tested via buoyancy method. This method implies weighting the hydrating cement paste in water. According to Archimedes' principle, the measured weight change corresponds to the chemical shrinkage. The experimental set up is depicted in Figure 1 (b). The test was performed in temperaturecontrolled room at 30 C. 3.4 Degree of hydration The degree of hydration was determined by means of TGA. The degree of hydration was calculated by the dividing of weight loss between 105 C and 1000 C by The samples were cast simultaneously and kept sealed in temperature-controlled room at 30 C until the target age and then dried at 105 C. After drying, the samples were ground to powder and tested using Perkin-Elmer Thermo-Gravimetric System (TGS-2). 4. RESULTS AND DISCUSSION The results of chemical shrinkage tests are shown in Figure 2 in a traditional form, as volume change in mm 3 per gram of cement initially present in the mix. It can be seen that in the conventional representation all the curves are virtually merged together. Neither effect of w/c ratio nor of degree of hydration is recognized. Degree of hydration is shown in Figure

6 80 Chemical shrinkage, mm 3 per g of cement h 5h 7h 8h9h 6h 12h 18h 1d 2d 3d 7d Age, hours Figure 2: Chemical shrinkage of cement pastes made with different w/c ratios vs. age Degree of hydration h 5h 9h 8h 7h 6h 12h 18h 1d 2d 3d 7d Age, hours Figure 3: Degree of hydration of cement pastes vs. age As can be seen in Figure 3, the degree of hydration is different for different w/c ratios at given time. This effect is most pronounced after the age of 24 hours. As stated above, only hydrated cement produces chemical shrinkage. Hence, the specific chemical shrinkage, i.e. chemical shrinkage per gram of hydrated cement, depends on w/c ratio. The specific chemical 780

7 shrinkage can be estimated by dividing the values of chemical shrinkage in Figure 2 by degree of hydration shown in Figure 3. The calculated specific chemical shrinkage is plotted in Figure 4, where the effect of w/c ratio can be clearly seen. In Figure 4, the specific chemical shrinkage is depicted versus the time scale, whereas, at given time point, the degree of hydration for different w/c ratios is still different. Furthermore, the majority of the modeling approaches predict the chemical shrinkage as a function of degree of hydration. Figure 5 demonstrates the specific chemical shrinkage curves versus degree of hydration. It can be clearly seen that specific chemical shrinkage depends on degree of hydration. Further analysis of Figure 5 discloses that there is an inflection point around 24 hour, after which the two strong tendencies are recognized. The first tendency is that the higher is degree of hydration, the higher is specific chemical shrinkage. The second is that the lower is w/c ratio, the higher is the specific chemical shrinkage. In literature, one can find chemical shrinkage curves as function of degree of hydration [16], as presented in Figure 6. However, it is often cut at 24 hours, while the trend is clearly distinguished only after the age of 1 day d 160 Specific chemical shrinkage, mm 3 per g of hydrated cement h 5h 7h 8h 9h 6h 12h 18h 1d 2d 3d Age, hours Figure 4: Specific chemical shrinkage of cement pastes made with different w/c ratios vs. age 781

8 d Specific chemical shrinkage, mm3 per g of hydrated cement h 5h 6h 7h 8h 9h 12h 18h 1d 3d 2d Predict Degree of hydration Figure 5: Specific chemical shrinkage, both tested (at w/c = 0.21, 0.25, 0.33 and 0.45) and predicted, vs. degree of hydration d Chemical shrinkage, mm 3 per g of cement h 8h 9h 7h 12h 18h 1d 3d 2d Predict. 4h 5h Degree of hydration Figure 6: Chemical shrinkage, both tested (at w/c = 0.21, 0.25, 0.33 and 0.45) and predicted, vs. degree of hydration 782

9 As mentioned before, the widely accepted concept suggests constant specific chemical shrinkage. This parameter is considered as a fundamental property of cement and intensively used in chemical shrinkage prediction [16], including autogenous shrinkage modeling [17] and internal curing design [1, 5]. The chemical shrinkage predicted by the traditional modeling concept is illustrated in Figure 5 and Figure 6 by dotted line. It can be seen that chemical shrinkage is considerably underestimated by the constant specific chemical shrinkage hypothesis. Moreover, the underestimation increases with the lower w/c ratio and the higher degree of hydration. Consequently, the autogenous shrinkage predicted with the assumption of constant specific chemical shrinkage is underestimated. In addition, the calculation of internal curing water content based on this concept leads to improper mix design. The conventional formula (10) should be modified for specific chemical shrinkage CS i as a function of both degree of hydration (α i ) and w/c ratio (ω): i Vcs () t = CSi ( αi, ω)[] i C αi () t (11) The concept of LD and HD CSH, referred earlier, provides an adequate explanation to the specific chemical shrinkage behavior demonstrated in Figure 5. Indeed, if LD CSH corresponds to the outer gel and HD CSH to the inner gel products, the LD/HD CSH ratio will decrease with w/c ratio and degree of hydration. This is because the space between cement particles in lower w/c ratio pastes is lower; there is less room for outer hydration products, i.e. for LD CSH. Furthermore, in cement pastes with w/c ratio below 0.4, where the pore space is insufficient to reach 100% cement hydration, an inflection point is expected when the interparticle space is filled with LD hydration products. Certainly, there is additional effect of aluminate phases, which contribute to higher chemical shrinkage at later age, since they have triple-stage reaction of ettringite formation and conversion to monosulphate, calcium aluminate hydrate and hydrogarnet formation see equations (3) - (8). 5. CONCLUSIONS Traditionally chemical shrinkage is described as the volume reduction related to the initial cement content. However, only hydrated cement produces chemical shrinkage. Therefore, chemical shrinkage (called in the paper specific chemical shrinkage ) should be represented as volume reduction related to unit weight of cement hydrated, and not to that of total cement content. It is found experimentally that specific chemical shrinkage depends on both w/c ratio and degree of hydration: the lower w/c ratio and the higher the degree of hydration, the higher is specific chemical shrinkage. Currently used model for degree of hydration, which assumes constant specific chemical shrinkage and, accordingly, the total chemical shrinkage proportional to degree of hydration, considerably underestimates chemical shrinkage after the age of 24 hours. The traditional concept of constant specific chemical shrinkage leads to underestimation of autogenous shrinkage and improper design of internal curing. 783

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