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Effect of calcium lignosulfonate on the hydration of the tricalcium aluminate–anhydrite system
Cement and Concrete Research 42 (2012) 1549–1554
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Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp
Effect of calcium lignosulfonate on the hydration of the tricalcium aluminate–anhydrite system Xiaoping Wang, Yuxia Pang, Hongming Lou, Yonghong Deng, Xueqing Qiu ⁎ School of Chemistry and Chemical Engineering, State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou, Guangdong, China
a r t i c l e
i n f o
Article history: Received 14 December 2011 Accepted 9 August 2012 Keywords: Calcium lignosulfonate Ca3Al2O6 (D) Hydration (A) Ettringite (D)
a b s t r a c t As a plasticizer and retarder, calcium lignosulfonate (CL) frequently leads to quick set of Portland cement containing anhydrite. For exploring the cause of this problem, the hydration characteristics of the tricalcium aluminate–anhydrite system in the saturated calcium hydroxide solution with and without CL were researched from two aspects: the compositions of liquid phases and the formations of hydration products. Results show that the CL can promote the formation of ettringite, which induces a significant decrease in sulfate ion concentration at the initial time of hydration. The size of ettringite crystals becomes large in the presence of CL, which seems to be related to a decrease in calcium sulfate saturation ratio. It can be deduced that the quick set in the presence of CL is mainly caused by the accelerated reaction between the tricalcium aluminate and the anhydrite at the initial time of hydration. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction In some parts of the world such as China, natural anhydrite is more abundant form of calcium sulfate than gypsum, and the price is relatively cheap. Nowadays, more and more cement factories use anhydrite to replace gypsum as a retarder of Portland cement for economic reason; however, this replacement has encountered a barrier when the concrete uses lignosulfonate as a plasticizer. Lignosulfonate, especially calcium lignosulfonate (CL), is a commonly-used water reducer of concrete. Since 1935, great progress has been made by chemical modification to improve CL's performance attributes, such as increasing water-reducing capability, reducing air-entraining tendency and reducing hydration-delaying capacity [1–6]. But some problems remain unsolved. When the CL is admixed with a Portland cement containing gypsum, the two are compatible and concrete performance is usually normal. However, the compatibility between CL and Portland cement containing anhydrite is poor, which frequently leads to quick set of cement and rapid stiffening of the concrete [7]. In this case, the initial set of cement in some extreme cases can be as short as 10 min, with much evolution of heat; plasticity cannot be regained on continued mixing, and the subsequent development of strength is poor [8]. Once the quick or flash set happens, workability and mechanical properties of concrete decline sharply, and construction becomes very difficult. This phenomenon seriously affects
⁎ Corresponding author. E-mail address: [email protected] (X. Qiu). 0008-8846/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cemconres.2012.08.008
the application of CL in concrete and building industries, especially in China where anhydrite is widely used. In order to address this problem, efforts must be done to clarify the quick set mechanism between CL and cement containing anhydrite, and the related research should emphasize the hydration reactions of various mineral components. Tricalcium aluminate, C3A, is known to be the most reactive of the four main minerals in cement. The common understanding of researchers in this field is that the acceleration of the setting is primarily related to the acceleration mechanism of C3A reactions at the initial time of hydration, and the corresponding reaction rate is mainly controlled by the actual amount of available calcium sulfate [8]. Different mole ratios of actual CaSO4/C3A can result in different forms of C3A hydrates, which in turn, can contribute to very different setting characteristics. It has been reported that in the presence of CL, the solubility of anhydrite is reduced in a short time, in the meanwhile, the hydration of C3A is accelerated and then the hardening process of cement is speeded up, consequently leading to quick set [7,9–11]. However, there are few reports about the relationship between the solubility of anhydrite and the hydration of C3A, and the corresponding action mechanism has not been fully understood. In the present paper, experiments were performed to investigate the hydration process of the C3A–anhydrite system with and without CL from two aspects: the compositions of liquid phase and the formations of hydration products. The aim of this investigation was to elucidate the acceleration mechanism of C3A hydration in the presence of CL, and explore the cause of the quick set by studying the effect of CL on the hydration characteristics of the C3A–anhydrite system.
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2. Materials and methods 2.1. Materials Commercial CL, solid powder recovered from spent sulfite pulping liquor, was provided by Shixian Papermaking Co. Ltd. Jilin, China. It consisted of about 74 wt.% of calcium lignosulfonate, about 11 wt.% of reducing substance and some other impurities. Natural anhydrite with a CaSO4 content of 85.7 wt.% was provided by New Building Materials Co. Ltd. Hunan, China. C3A of high purity was prepared by calcining the mixture of CaCO3 and Al2O3 at 1350 °C for three times and each time for 3 h. The molar ratio of CaCO3/Al2O3 in the mixture was fixed to 3. Before the second and third calcinations, the material was crushed and ground in a laboratory mill. After XRD analysis, it was found that free lime was scarcely detectable in the synthetic C3A, so it had little influence on the following experiments. Analytical reagent of Ca(OH)2 (purity ≥ 95%) was used to prepare saturated Ca(OH)2 solution. 2.2. Methods 2.2.1. Determination of the solubility of anhydrite To test the solubility of anhydrite with time in the presence and absence of CL, the following experiments were carried out: 0.6 g anhydrite powder was dissolved in 200 ml saturated Ca(OH)2 solution, and placed in tightly covered polythene containers and rotated continuously with constant temperature vibrator at 20 °C. After dissolution for 1–60 min, the liquid phase of the anhydrite suspension was separated by suction filter. Similar procedure was undertaken to obtain samples of liquid phase with the addition of 1.0 wt.% dosage of solid CL (based on the mass of the anhydrite powder). Appropriate dilution was prepared for the determination of sulfate by ion chromatography (IC) (ICS-1000, Dionex INC, U.S.). 2.2.2. Measurement of sulfate and calcium concentrations The C3A and the anhydrite powders were mixed by blending mechanically at a C3A/anhydrite mass ratio of 2 (the C3A/CaSO4 molar ratio is 1.175), which was close to their actual proportion in common Portland cement. The C3A–anhydrite mixture was hydrated in the saturated Ca(OH)2 solution with a liquid to solid ratio of 10 (L/S= 10), and placed in tightly covered polythene containers and rotated continuously with constant temperature vibrator at 20 °C. After hydration for 1–180 min, the liquid phase of the C3A–anhydrite suspension was separated by suction filter. All filtering process was finished within 20 s. The results of setting times of cement containing anhydrite showed that the initial set was as short as 10 min when the dosage of solid CL was up to 1.0 wt.% by weight of cement. A similar procedure to the one mentioned above, with the addition of 1.0 wt.% dosage of solid CL (based on the mass of the C3A–anhydrite mixture powder), was undertaken to obtain samples of liquid phase. Two methods were adopted for the addition of CL: one was a simultaneous addition, and the other was a delayed addition (of about 1 min). The solid CL was fully dissolved in proper amount of saturated Ca(OH)2 solution before participating in hydration. Appropriate dilutions were prepared for the determination of sulfate by IC and calcium by atomic adsorption spectrophotometer (AAS) (Z-2300, Hitachi Ltd., Japan) using standard methods. 2.2.3. Analysis of hydration products The C3A and the anhydrite powders were also mixed by mass ratio of 2 and hydrated in the saturated Ca(OH)2 solution, while the liquid to solid ratio was 0.8. The pastes were stirred for 2 min with a mechanical stirrer and placed for specified intervals, which varied from 10 min to 28 d, under a standard-curing condition. Then the samples
were soaked and washed with excess pure ethanol in order to interrupt the hydration. After vacuum drying at 40 °C, phase characterization was done by X-ray diffractometer (XRD) (D8 Advance, Bruker Co., Ltd, Germany) using Cu Kα radiation (40 kV, 40 mA) and complex thermal analysis (TG-DSC) (STA449C, NETZSCH Co., Ltd, Germany) in a nitrogen atmosphere at heating rate of 10 °C/min. The morphology of hydrates was observed by scanning electron microscope (SEM) (Evo18, Carl Zeiss Far East Co., Ltd, Germany). A similar method was followed to study hydration products with the addition of 1.0 wt.% dosage of solid CL (based on the mass of the C3A–anhydrite mixture powder). 3. Results and discussion 3.1. Effect of CL on the solubility of anhydrite When the anhydrite dissolves in the saturated Ca(OH)2 solution, the curves of concentrations of sulfate ion (SO42 −) with time in the presence and absence of CL are plotted in Fig. 1. As shown, during the dissolution of anhydrite, SO42 − concentration in the liquid phase containing CL is lower than that without CL. This result indicates that the solubility of anhydrite is reduced in the presence of CL, which is consistent with the conclusion drawn by Dodson and Hayden [9]. 3.2. Effect of CL on the compositions of liquid phase Effect of CL on SO42− concentration during the hydration of the C3A–anhydrite system is shown in Fig. 2. SO42− concentration in the liquid phase without CL increases obviously with the increase of hydration time from 1 min to 30 min, and then gradually decreases. This result suggests that the amount of SO42 − generated from the dissolution of anhydrite is more than the amount that was consumed in the corresponding reactions during the first 30 min. For both the simultaneous addition and the delayed addition (of about 1 min), SO42− concentration in the liquid phase containing CL is lower in comparison to that without CL, especially during the first 2 min. There are two possible explanations for this case. The first one is that the CL is interfering with the rate of available sulfate supplied by the anhydrite. The second one is that the CL can promote the hydration of C3A and lead to higher consumption of SO42 − during the formations of hydrates containing sulfate. The effect of CL on Ca2+ concentration during the hydration of the C3A–anhydrite system is shown in Fig. 3. Ca 2+ concentration in the liquid phase without CL increases during the first 10 min and then changes slightly. The trend in Ca2+ concentration in the liquid phase
Fig. 1. SO42− concentration vs. time during the dissolution of anhydrite in the saturated Ca(OH)2 solution.
X. Wang et al. / Cement and Concrete Research 42 (2012) 1549–1554
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Fig. 2. SO42− concentration vs. time during the C3A–anhydrite hydration in the saturated Ca(OH)2 solution (L/S=10).
Fig. 4. CaSO4 saturation ratio vs. time during the C3A–anhydrite hydration in the saturated Ca(OH)2 solution (L/S=10).
with CL is similar to that without CL. One distinction is that Ca 2+ concentration in the liquid phase containing CL is higher than that without CL. The level of Ca 2+ concentration is related to the saturation of Ca(OH)2 solution, the dissolution of anhydrite and C3A. The amount of Ca 2+ supplied by the saturated Ca(OH)2 solution is throughout constant. Owing to the solubility of anhydrite that is reduced in the presence of CL, the amount of Ca 2+ supplied by the anhydrite decreases. It can be deduced that the amount of Ca 2+ supplied by the C3A increases in the presence of CL, which results in the increase of Ca 2+ concentration. When the C3A dissolves, it leads to ions in solution according to Eq. (1) and the solution becomes quickly supersaturated [12].
ratio is close to zero. It means that the CL can reduce CaSO4 saturation ratio, and the extent of reduction is relatively large at the initial time of hydration.
2þ
Ca3 Al2 O6 þ H2 O→3Ca
3þ
þ 2Al
þ 12OH :
ð1Þ
After the addition of CL, the C3A can dissolve faster and generate more Ca 2+ and Al 3+, which indicates that the CL can accelerate the hydration of C3A and speed up the formations of hydration products containing aluminate. CaSO4 saturation ratio is calculated from the concentrations of SO42− and Ca 2+ according to Ref. [13]. The curves of CaSO4 saturation ratios with time in the presence and absence of CL are plotted in Fig. 4. As shown, during the hydration of the C3A–anhydrite system, CaSO4 saturation ratios in both liquid phases rapidly increase during the first 10 min, and then decrease somewhat. However, CaSO4 saturation ratio in the liquid phase containing CL is lower in comparison to that without CL. Especially, during the first 1 min, CaSO4 saturation
Fig. 3. Ca2+ concentration vs. time during the C3A–anhydrite hydration in the saturated Ca(OH)2 solution (L/S=10).
3.3. Influence of CL on hydration products 3.3.1. XRD analysis XRD results of phase compositions during the hydration of the C3A–anhydrite system in the presence and absence of CL are shown in Figs. 5 and 6. At 10 min, the peak intensities of both C3A and anhydrite in the sample containing CL are evidently lower in comparison to those without CL (see Fig. 5), which implies that the CL can immediately enhance the reaction rate between the C3A and the anhydrite after the C3A–anhydrite mixture and the saturated Ca(OH)2 solution are mixed. As the hydration progresses, the intensity of ettringite peaks initially increases with time and then decreases. Within the first 7 day of hydration, the main hydration product is ettringite. It can be seen from Figs. 5 and 6 that the intensity of characteristic peaks of ettringite in the sample containing CL is relatively stronger than that without CL. Especially at 10 min, key characteristic peak of ettringite (crystal distance is about 0.971 nm) in the sample containing CL has stronger intensity by several times compared to that without CL (see Fig. 6), which is attributed to the formation of a larger amount of ettringite crystals in the presence of CL. At 10 min, in the presence of CL, the lower intensity peaks of anhydrite couple with the relatively stronger peaks associated with ettringite, which suggests that at the initial time of hydration, the primary reason for the significant decrease of SO42− concentration in the liquid phase containing CL is not due to a decrease in the solubility of anhydrite, but rather the high consumption of sulfate caused by the sharp increase in the formation of ettringite. 3.3.2. TG-DSC analysis From the DSC curves of the samples in the absence and presence of CL (see Fig. 7), the peaks at 99–123 °C are due to the decomposition of ettringite, and the peaks at 161–178 °C can be attributed to the dehydration of monosulfate aluminate. From the TG analysis (as shown in Table 1), it is possible to calculate the approximate content of ettringite. For calculation purposes, it is assumed that theoretical value of weight loss on TG caused by all ettringite decomposition at 99–123 °C is 28.4%, corresponding to the loss of about 20 molecules of water [14]. On the basis of this assumption, according to the weight loss caused by ettringite decomposition, the ettringite contents of the hydrated samples are calculated (as shown in Fig. 8). The calculated results confirm the conclusions drawn from XRD analysis that the
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Fig. 7. DSC analyses of the C3A–anhydrite system. (a) In the absence of CL, (b) in the presence of CL.
content of ettringite in sample with CL is more than that without CL, especially at 10 min. It is further demonstrated that the reaction rate of the components in the sample containing CL is much faster compared to that without CL at the initial time of hydration. Fig. 5. XRD analyses of hydration products. (a) In the C3A–anhydrite system without CL, (b) in the C3 A–anhydrite system containing CL. A = anhydrite, E = ettringite, T = tricalcium aluminate, M = monosulfate aluminate, H = hydrated aluminate.
3.4. Effect of CL on the ettringite morphology The morphology of ettringite formed by the hydration of the C3A–anhydrite system with and without CL is observed by SEM. In the absence of CL, at 30 min, a small amount of fine ettringite crystals (about 1 μm in length) can be found, and the hydrates assume a gel-like shape at 3 h (as shown in Fig. 9, and big blocks observed in Fig. 9(a) are large anhydrite particles by EDS analysis). On the other hand, in the presence of CL, large amount of ettringite crystals is mostly present in the form of large clusters and irregular rods with the size of 2–4 μm (as shown in Fig. 10). Associating with Fig. 4, it can be found that the large ettringite crystals are corresponding to Table 1 Weight loss corresponding to ettringite decomposition at 99–123 °C by TG analysis. The C3A–anhydrite system
Fig. 6. Integral intensity of key characteristic peak of ettringite (crystal distance is about 0.971 nm) obtained from XRD.
Blank With CL
Weight loss of ettringite (wt.%) 10 min
30 min
1d
7d
28 d
1.49 9.86
6.23 14.55
13.50 19.29
17.06 19.00
8.94 16.96
X. Wang et al. / Cement and Concrete Research 42 (2012) 1549–1554
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do not easily form complete coating layers around C3A grains to hinder the diffusion of liquid phase, especially at the initial time of hydration. Consequently, the liquid phase continues to react with C3A rapidly, which accelerates the reaction rate between the C3A and the anhydrite.
4. Conclusions
low CaSO4 saturation ratio in liquid phase, and this result is consistent with earlier research reported by H. Uchikawa [13]. One of the retardation mechanisms in C3A–gypsum–Ca(OH)2 system is that the effect of gypsum can be attributed to the formation of an impermeable layer of ettringite, which is long needles, stubby and prismatic crystals (up to 1 μm in length), and this layer can cover the C3A surfaces and prevent further hydration between C3A and water [8]. Present research indicates that ettringite crystals grow faster and larger in the presence of CL, and that the large ettringite crystals
With the addition of CL, SO42− concentration and CaSO4 saturation ratio in the liquid phase of the C3A–anhydrite system decrease especially at the initial time of hydration, whereas Ca 2 + concentration increases. XRD and TG analyses have shown that the formation of ettringite in the C3A–anhydrite system is accelerated in the presence of CL, which is consistent with the concentration changes of SO42− and Ca 2 +. The presence of CL increases the size of ettringite crystals, which is related to the decrease of CaSO4 saturation ratio in the liquid phase containing CL. This effect is more pronounced at the initial time of hydration. Considering the opinions of other researchers, it can be deduced that the large ettringite crystals do not easily form complete coating layers around C3A grains. As a result, the liquid phase continues to diffuse through the crystalline ettringite coating and react with C3A rapidly. In general, it seems that when CL is used, the principal reason for the quick set is the accelerated reaction between the C3A and the anhydrite at the initial time of hydration, which generates plenty of large ettringite crystals that have little hindrance effect to the subsequent hydration of cement.
Fig. 9. SEM of the C3A–anhydrite system without CL. (a) At 30 min of hydration, (b) at 3 h of hydration.
Fig. 10. SEM of the C3A–anhydrite system containing CL. (a) At 30 min of hydration, (b) at 3 h of hydration.
Fig. 8. Ettringite content calculated by weight loss corresponding to decomposition peak of ettringite on TG.
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Acknowledgments The authors would like to acknowledge the financial supports of the China Excellent Young Scientist Fund (20925622), the National Science Foundation of Guangdong Province of China (8351064101000002) and the Fundamental Research Funds for the Central Universities of China (2011ZP0012). References [1] T. Zhang, S. Shang, F. Yin, A. Aishah, A. Salmiah, T.L. Ooi, Adsorptive behavior of surfactants on surface of Portland cement, Cem. Concr. Res. 31 (2001) 1009–1015. [2] X.P. Ouyang, X.Q. Qiu, P. Chen, Physicochemical characterization of calcium lignosulfonate—a potentially useful water reducer, Colloids Surf., A 282–283 (2006) 489–497. [3] C. Li, Q.W. Pan, J. Zhang, X.J. Qin, Z.F. Wang, L.J. Liu, M.S. Pei, The modification of calcium lignosulfonate and its applications in cementitious materials, J. Dispersion Sci. Technol. 28 (2007) 1205–1208. [4] Andrew C. Jupe, Angus P. Wilkinson, Karen Luke, Gary P. Funkhouser, Class H oil well cement hydration at elevated temperatures in the presence of retarding agents: an in situ high-energy X-ray diffraction study, Ind. Eng. Chem. Res. 44 (2005) 5579–5584.
[5] H.M. Lou, X.Q. Qiu, D.J. Yang, X.P. Ouyang, H.Q. Chen, Study on the action mechanism of modified calcium lignosulfonate GCL1 water reducer, J. S. China Univ. Technol. 30 (2002) 48–52. [6] X.W. Wu, H. Zhang, Preparation of cement water reducer of modified lignosulfonate, Adv. Mater. Res. 168–170 (2011) 541–544. [7] Pierre-Claude Aitcin, Carmel Jolicoeur, James G. MacGregor, Superplasticizers: how they work and why they occasionally don't, Concr. Int. 16 (1994) 45–52. [8] H.F.W. Taylor, Cement Chemistry, 2nd Ed. Thomas Telford, London, 1997. [9] V.H. Dodson, T.D. Hayden, Another look at the Portland cement/chemical admixture incompatibility problem, Cem. Concr. Aggr. 11 (1989) 52–56. [10] Y.C. Hu, Effect of water reducer on the setting of Portland cement containing anhydrite, Chin. Concr. Cem. Prod. 3 (1983) 9–13. [11] X.C. Hu, C.Y. Liu, B.P. Yao, Compatibility of concrete water reducer with cements, Ind. Constr. 8 (1987) 33–38. [12] S. Pourchet, L. Regnaud, J.P. Perez, A. Nonat, Early C3A hydration in the presence of different kinds of calcium sulfate, Cem. Concr. Res. 39 (2009) 989–996. [13] H. Uchikawa, S. Uchida, K. Ogawa, S. Hanehara, Influence of CaSO4·2H2O, CaSO4·1/2H2O and CaSO4 on the initial hydration of clinker having different burning degree, Cem. Concr. Res. 14 (1984) 645–656. [14] E.Z. He, Formation condition and character of ettringite, Silic. Constr. Prod. 2 (1983) 38–43.
Contents lists available at SciVerse ScienceDirect
Cement and Concrete Research journal homepage: http://ees.elsevier.com/CEMCON/default.asp
Effect of calcium lignosulfonate on the hydration of the tricalcium aluminate–anhydrite system Xiaoping Wang, Yuxia Pang, Hongming Lou, Yonghong Deng, Xueqing Qiu ⁎ School of Chemistry and Chemical Engineering, State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou, Guangdong, China
a r t i c l e
i n f o
Article history: Received 14 December 2011 Accepted 9 August 2012 Keywords: Calcium lignosulfonate Ca3Al2O6 (D) Hydration (A) Ettringite (D)
a b s t r a c t As a plasticizer and retarder, calcium lignosulfonate (CL) frequently leads to quick set of Portland cement containing anhydrite. For exploring the cause of this problem, the hydration characteristics of the tricalcium aluminate–anhydrite system in the saturated calcium hydroxide solution with and without CL were researched from two aspects: the compositions of liquid phases and the formations of hydration products. Results show that the CL can promote the formation of ettringite, which induces a significant decrease in sulfate ion concentration at the initial time of hydration. The size of ettringite crystals becomes large in the presence of CL, which seems to be related to a decrease in calcium sulfate saturation ratio. It can be deduced that the quick set in the presence of CL is mainly caused by the accelerated reaction between the tricalcium aluminate and the anhydrite at the initial time of hydration. © 2012 Elsevier Ltd. All rights reserved.
1. Introduction In some parts of the world such as China, natural anhydrite is more abundant form of calcium sulfate than gypsum, and the price is relatively cheap. Nowadays, more and more cement factories use anhydrite to replace gypsum as a retarder of Portland cement for economic reason; however, this replacement has encountered a barrier when the concrete uses lignosulfonate as a plasticizer. Lignosulfonate, especially calcium lignosulfonate (CL), is a commonly-used water reducer of concrete. Since 1935, great progress has been made by chemical modification to improve CL's performance attributes, such as increasing water-reducing capability, reducing air-entraining tendency and reducing hydration-delaying capacity [1–6]. But some problems remain unsolved. When the CL is admixed with a Portland cement containing gypsum, the two are compatible and concrete performance is usually normal. However, the compatibility between CL and Portland cement containing anhydrite is poor, which frequently leads to quick set of cement and rapid stiffening of the concrete [7]. In this case, the initial set of cement in some extreme cases can be as short as 10 min, with much evolution of heat; plasticity cannot be regained on continued mixing, and the subsequent development of strength is poor [8]. Once the quick or flash set happens, workability and mechanical properties of concrete decline sharply, and construction becomes very difficult. This phenomenon seriously affects
⁎ Corresponding author. E-mail address: [email protected] (X. Qiu). 0008-8846/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cemconres.2012.08.008
the application of CL in concrete and building industries, especially in China where anhydrite is widely used. In order to address this problem, efforts must be done to clarify the quick set mechanism between CL and cement containing anhydrite, and the related research should emphasize the hydration reactions of various mineral components. Tricalcium aluminate, C3A, is known to be the most reactive of the four main minerals in cement. The common understanding of researchers in this field is that the acceleration of the setting is primarily related to the acceleration mechanism of C3A reactions at the initial time of hydration, and the corresponding reaction rate is mainly controlled by the actual amount of available calcium sulfate [8]. Different mole ratios of actual CaSO4/C3A can result in different forms of C3A hydrates, which in turn, can contribute to very different setting characteristics. It has been reported that in the presence of CL, the solubility of anhydrite is reduced in a short time, in the meanwhile, the hydration of C3A is accelerated and then the hardening process of cement is speeded up, consequently leading to quick set [7,9–11]. However, there are few reports about the relationship between the solubility of anhydrite and the hydration of C3A, and the corresponding action mechanism has not been fully understood. In the present paper, experiments were performed to investigate the hydration process of the C3A–anhydrite system with and without CL from two aspects: the compositions of liquid phase and the formations of hydration products. The aim of this investigation was to elucidate the acceleration mechanism of C3A hydration in the presence of CL, and explore the cause of the quick set by studying the effect of CL on the hydration characteristics of the C3A–anhydrite system.
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2. Materials and methods 2.1. Materials Commercial CL, solid powder recovered from spent sulfite pulping liquor, was provided by Shixian Papermaking Co. Ltd. Jilin, China. It consisted of about 74 wt.% of calcium lignosulfonate, about 11 wt.% of reducing substance and some other impurities. Natural anhydrite with a CaSO4 content of 85.7 wt.% was provided by New Building Materials Co. Ltd. Hunan, China. C3A of high purity was prepared by calcining the mixture of CaCO3 and Al2O3 at 1350 °C for three times and each time for 3 h. The molar ratio of CaCO3/Al2O3 in the mixture was fixed to 3. Before the second and third calcinations, the material was crushed and ground in a laboratory mill. After XRD analysis, it was found that free lime was scarcely detectable in the synthetic C3A, so it had little influence on the following experiments. Analytical reagent of Ca(OH)2 (purity ≥ 95%) was used to prepare saturated Ca(OH)2 solution. 2.2. Methods 2.2.1. Determination of the solubility of anhydrite To test the solubility of anhydrite with time in the presence and absence of CL, the following experiments were carried out: 0.6 g anhydrite powder was dissolved in 200 ml saturated Ca(OH)2 solution, and placed in tightly covered polythene containers and rotated continuously with constant temperature vibrator at 20 °C. After dissolution for 1–60 min, the liquid phase of the anhydrite suspension was separated by suction filter. Similar procedure was undertaken to obtain samples of liquid phase with the addition of 1.0 wt.% dosage of solid CL (based on the mass of the anhydrite powder). Appropriate dilution was prepared for the determination of sulfate by ion chromatography (IC) (ICS-1000, Dionex INC, U.S.). 2.2.2. Measurement of sulfate and calcium concentrations The C3A and the anhydrite powders were mixed by blending mechanically at a C3A/anhydrite mass ratio of 2 (the C3A/CaSO4 molar ratio is 1.175), which was close to their actual proportion in common Portland cement. The C3A–anhydrite mixture was hydrated in the saturated Ca(OH)2 solution with a liquid to solid ratio of 10 (L/S= 10), and placed in tightly covered polythene containers and rotated continuously with constant temperature vibrator at 20 °C. After hydration for 1–180 min, the liquid phase of the C3A–anhydrite suspension was separated by suction filter. All filtering process was finished within 20 s. The results of setting times of cement containing anhydrite showed that the initial set was as short as 10 min when the dosage of solid CL was up to 1.0 wt.% by weight of cement. A similar procedure to the one mentioned above, with the addition of 1.0 wt.% dosage of solid CL (based on the mass of the C3A–anhydrite mixture powder), was undertaken to obtain samples of liquid phase. Two methods were adopted for the addition of CL: one was a simultaneous addition, and the other was a delayed addition (of about 1 min). The solid CL was fully dissolved in proper amount of saturated Ca(OH)2 solution before participating in hydration. Appropriate dilutions were prepared for the determination of sulfate by IC and calcium by atomic adsorption spectrophotometer (AAS) (Z-2300, Hitachi Ltd., Japan) using standard methods. 2.2.3. Analysis of hydration products The C3A and the anhydrite powders were also mixed by mass ratio of 2 and hydrated in the saturated Ca(OH)2 solution, while the liquid to solid ratio was 0.8. The pastes were stirred for 2 min with a mechanical stirrer and placed for specified intervals, which varied from 10 min to 28 d, under a standard-curing condition. Then the samples
were soaked and washed with excess pure ethanol in order to interrupt the hydration. After vacuum drying at 40 °C, phase characterization was done by X-ray diffractometer (XRD) (D8 Advance, Bruker Co., Ltd, Germany) using Cu Kα radiation (40 kV, 40 mA) and complex thermal analysis (TG-DSC) (STA449C, NETZSCH Co., Ltd, Germany) in a nitrogen atmosphere at heating rate of 10 °C/min. The morphology of hydrates was observed by scanning electron microscope (SEM) (Evo18, Carl Zeiss Far East Co., Ltd, Germany). A similar method was followed to study hydration products with the addition of 1.0 wt.% dosage of solid CL (based on the mass of the C3A–anhydrite mixture powder). 3. Results and discussion 3.1. Effect of CL on the solubility of anhydrite When the anhydrite dissolves in the saturated Ca(OH)2 solution, the curves of concentrations of sulfate ion (SO42 −) with time in the presence and absence of CL are plotted in Fig. 1. As shown, during the dissolution of anhydrite, SO42 − concentration in the liquid phase containing CL is lower than that without CL. This result indicates that the solubility of anhydrite is reduced in the presence of CL, which is consistent with the conclusion drawn by Dodson and Hayden [9]. 3.2. Effect of CL on the compositions of liquid phase Effect of CL on SO42− concentration during the hydration of the C3A–anhydrite system is shown in Fig. 2. SO42− concentration in the liquid phase without CL increases obviously with the increase of hydration time from 1 min to 30 min, and then gradually decreases. This result suggests that the amount of SO42 − generated from the dissolution of anhydrite is more than the amount that was consumed in the corresponding reactions during the first 30 min. For both the simultaneous addition and the delayed addition (of about 1 min), SO42− concentration in the liquid phase containing CL is lower in comparison to that without CL, especially during the first 2 min. There are two possible explanations for this case. The first one is that the CL is interfering with the rate of available sulfate supplied by the anhydrite. The second one is that the CL can promote the hydration of C3A and lead to higher consumption of SO42 − during the formations of hydrates containing sulfate. The effect of CL on Ca2+ concentration during the hydration of the C3A–anhydrite system is shown in Fig. 3. Ca 2+ concentration in the liquid phase without CL increases during the first 10 min and then changes slightly. The trend in Ca2+ concentration in the liquid phase
Fig. 1. SO42− concentration vs. time during the dissolution of anhydrite in the saturated Ca(OH)2 solution.
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Fig. 2. SO42− concentration vs. time during the C3A–anhydrite hydration in the saturated Ca(OH)2 solution (L/S=10).
Fig. 4. CaSO4 saturation ratio vs. time during the C3A–anhydrite hydration in the saturated Ca(OH)2 solution (L/S=10).
with CL is similar to that without CL. One distinction is that Ca 2+ concentration in the liquid phase containing CL is higher than that without CL. The level of Ca 2+ concentration is related to the saturation of Ca(OH)2 solution, the dissolution of anhydrite and C3A. The amount of Ca 2+ supplied by the saturated Ca(OH)2 solution is throughout constant. Owing to the solubility of anhydrite that is reduced in the presence of CL, the amount of Ca 2+ supplied by the anhydrite decreases. It can be deduced that the amount of Ca 2+ supplied by the C3A increases in the presence of CL, which results in the increase of Ca 2+ concentration. When the C3A dissolves, it leads to ions in solution according to Eq. (1) and the solution becomes quickly supersaturated [12].
ratio is close to zero. It means that the CL can reduce CaSO4 saturation ratio, and the extent of reduction is relatively large at the initial time of hydration.
2þ
Ca3 Al2 O6 þ H2 O→3Ca
3þ
þ 2Al
þ 12OH :
ð1Þ
After the addition of CL, the C3A can dissolve faster and generate more Ca 2+ and Al 3+, which indicates that the CL can accelerate the hydration of C3A and speed up the formations of hydration products containing aluminate. CaSO4 saturation ratio is calculated from the concentrations of SO42− and Ca 2+ according to Ref. [13]. The curves of CaSO4 saturation ratios with time in the presence and absence of CL are plotted in Fig. 4. As shown, during the hydration of the C3A–anhydrite system, CaSO4 saturation ratios in both liquid phases rapidly increase during the first 10 min, and then decrease somewhat. However, CaSO4 saturation ratio in the liquid phase containing CL is lower in comparison to that without CL. Especially, during the first 1 min, CaSO4 saturation
Fig. 3. Ca2+ concentration vs. time during the C3A–anhydrite hydration in the saturated Ca(OH)2 solution (L/S=10).
3.3. Influence of CL on hydration products 3.3.1. XRD analysis XRD results of phase compositions during the hydration of the C3A–anhydrite system in the presence and absence of CL are shown in Figs. 5 and 6. At 10 min, the peak intensities of both C3A and anhydrite in the sample containing CL are evidently lower in comparison to those without CL (see Fig. 5), which implies that the CL can immediately enhance the reaction rate between the C3A and the anhydrite after the C3A–anhydrite mixture and the saturated Ca(OH)2 solution are mixed. As the hydration progresses, the intensity of ettringite peaks initially increases with time and then decreases. Within the first 7 day of hydration, the main hydration product is ettringite. It can be seen from Figs. 5 and 6 that the intensity of characteristic peaks of ettringite in the sample containing CL is relatively stronger than that without CL. Especially at 10 min, key characteristic peak of ettringite (crystal distance is about 0.971 nm) in the sample containing CL has stronger intensity by several times compared to that without CL (see Fig. 6), which is attributed to the formation of a larger amount of ettringite crystals in the presence of CL. At 10 min, in the presence of CL, the lower intensity peaks of anhydrite couple with the relatively stronger peaks associated with ettringite, which suggests that at the initial time of hydration, the primary reason for the significant decrease of SO42− concentration in the liquid phase containing CL is not due to a decrease in the solubility of anhydrite, but rather the high consumption of sulfate caused by the sharp increase in the formation of ettringite. 3.3.2. TG-DSC analysis From the DSC curves of the samples in the absence and presence of CL (see Fig. 7), the peaks at 99–123 °C are due to the decomposition of ettringite, and the peaks at 161–178 °C can be attributed to the dehydration of monosulfate aluminate. From the TG analysis (as shown in Table 1), it is possible to calculate the approximate content of ettringite. For calculation purposes, it is assumed that theoretical value of weight loss on TG caused by all ettringite decomposition at 99–123 °C is 28.4%, corresponding to the loss of about 20 molecules of water [14]. On the basis of this assumption, according to the weight loss caused by ettringite decomposition, the ettringite contents of the hydrated samples are calculated (as shown in Fig. 8). The calculated results confirm the conclusions drawn from XRD analysis that the
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Fig. 7. DSC analyses of the C3A–anhydrite system. (a) In the absence of CL, (b) in the presence of CL.
content of ettringite in sample with CL is more than that without CL, especially at 10 min. It is further demonstrated that the reaction rate of the components in the sample containing CL is much faster compared to that without CL at the initial time of hydration. Fig. 5. XRD analyses of hydration products. (a) In the C3A–anhydrite system without CL, (b) in the C3 A–anhydrite system containing CL. A = anhydrite, E = ettringite, T = tricalcium aluminate, M = monosulfate aluminate, H = hydrated aluminate.
3.4. Effect of CL on the ettringite morphology The morphology of ettringite formed by the hydration of the C3A–anhydrite system with and without CL is observed by SEM. In the absence of CL, at 30 min, a small amount of fine ettringite crystals (about 1 μm in length) can be found, and the hydrates assume a gel-like shape at 3 h (as shown in Fig. 9, and big blocks observed in Fig. 9(a) are large anhydrite particles by EDS analysis). On the other hand, in the presence of CL, large amount of ettringite crystals is mostly present in the form of large clusters and irregular rods with the size of 2–4 μm (as shown in Fig. 10). Associating with Fig. 4, it can be found that the large ettringite crystals are corresponding to Table 1 Weight loss corresponding to ettringite decomposition at 99–123 °C by TG analysis. The C3A–anhydrite system
Fig. 6. Integral intensity of key characteristic peak of ettringite (crystal distance is about 0.971 nm) obtained from XRD.
Blank With CL
Weight loss of ettringite (wt.%) 10 min
30 min
1d
7d
28 d
1.49 9.86
6.23 14.55
13.50 19.29
17.06 19.00
8.94 16.96
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do not easily form complete coating layers around C3A grains to hinder the diffusion of liquid phase, especially at the initial time of hydration. Consequently, the liquid phase continues to react with C3A rapidly, which accelerates the reaction rate between the C3A and the anhydrite.
4. Conclusions
low CaSO4 saturation ratio in liquid phase, and this result is consistent with earlier research reported by H. Uchikawa [13]. One of the retardation mechanisms in C3A–gypsum–Ca(OH)2 system is that the effect of gypsum can be attributed to the formation of an impermeable layer of ettringite, which is long needles, stubby and prismatic crystals (up to 1 μm in length), and this layer can cover the C3A surfaces and prevent further hydration between C3A and water [8]. Present research indicates that ettringite crystals grow faster and larger in the presence of CL, and that the large ettringite crystals
With the addition of CL, SO42− concentration and CaSO4 saturation ratio in the liquid phase of the C3A–anhydrite system decrease especially at the initial time of hydration, whereas Ca 2 + concentration increases. XRD and TG analyses have shown that the formation of ettringite in the C3A–anhydrite system is accelerated in the presence of CL, which is consistent with the concentration changes of SO42− and Ca 2 +. The presence of CL increases the size of ettringite crystals, which is related to the decrease of CaSO4 saturation ratio in the liquid phase containing CL. This effect is more pronounced at the initial time of hydration. Considering the opinions of other researchers, it can be deduced that the large ettringite crystals do not easily form complete coating layers around C3A grains. As a result, the liquid phase continues to diffuse through the crystalline ettringite coating and react with C3A rapidly. In general, it seems that when CL is used, the principal reason for the quick set is the accelerated reaction between the C3A and the anhydrite at the initial time of hydration, which generates plenty of large ettringite crystals that have little hindrance effect to the subsequent hydration of cement.
Fig. 9. SEM of the C3A–anhydrite system without CL. (a) At 30 min of hydration, (b) at 3 h of hydration.
Fig. 10. SEM of the C3A–anhydrite system containing CL. (a) At 30 min of hydration, (b) at 3 h of hydration.
Fig. 8. Ettringite content calculated by weight loss corresponding to decomposition peak of ettringite on TG.
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Acknowledgments The authors would like to acknowledge the financial supports of the China Excellent Young Scientist Fund (20925622), the National Science Foundation of Guangdong Province of China (8351064101000002) and the Fundamental Research Funds for the Central Universities of China (2011ZP0012). References [1] T. Zhang, S. Shang, F. Yin, A. Aishah, A. Salmiah, T.L. Ooi, Adsorptive behavior of surfactants on surface of Portland cement, Cem. Concr. Res. 31 (2001) 1009–1015. [2] X.P. Ouyang, X.Q. Qiu, P. Chen, Physicochemical characterization of calcium lignosulfonate—a potentially useful water reducer, Colloids Surf., A 282–283 (2006) 489–497. [3] C. Li, Q.W. Pan, J. Zhang, X.J. Qin, Z.F. Wang, L.J. Liu, M.S. Pei, The modification of calcium lignosulfonate and its applications in cementitious materials, J. Dispersion Sci. Technol. 28 (2007) 1205–1208. [4] Andrew C. Jupe, Angus P. Wilkinson, Karen Luke, Gary P. Funkhouser, Class H oil well cement hydration at elevated temperatures in the presence of retarding agents: an in situ high-energy X-ray diffraction study, Ind. Eng. Chem. Res. 44 (2005) 5579–5584.
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