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1、189 189 5 Accelerating Admixtures 1.0INTRODUCTION Admixtures can influence the properties of concrete from the moment water comes into contact with the concrete ingredients. In the fresh state, the water requirement, workability, bleeding, segregation, rate of hydration, setting time, air content, p
2、umpability, etc., and in the hardened state, compressive, flexural and tensile strengths, creep, shrinkage, perme- ability, and durability are affected to different extents, depending on the type and amount of admixture added. The general types of admixtures used in concrete and their applications h
3、ave already been described in Ch. 4 of this book. Thermal analysis techniques have been applied widely for the investigation of the role of admixtures, especially that related to the hydration of cement and cement components. Application of thermal analysis permits determination of the heat of react
4、ion, mechanism of reaction, kinetics of reactions, compatibility of admixtures with cements, prediction of some properties, durability problems, material characteriza- tion and selection, development of new admixtures, quick assessment of some physical properties, etc. In some instances, they yield
5、results that are not possible to obtain with the use of other techniques. 190Chapter 5 - Accelerating Admixtures In this chapter, typical examples of the application of various thermal techniques to the study of the effect of admixtures on the hydration of cement and cement compounds is emphasized.
6、Where relevant, the results obtained with these techniques are compared with those derived from other tools. 2.0CALCIUM CHLORIDE Calcium chloride is a unique accelerator in the sense that, of the various cation-anion combinations, the Ca+2 and Cl- combination ranks as the best accelerator for cement
7、s. The accelerating influence of CaCl2 on the hydration of calcium silicates was observed more than sixty years ago by Sloane, et al.,1 and Haegerman,2 and has been confirmed by subsequent work.3 The accelerating influence of calcium chloride on the hydration of C3S is conveniently followed by adopt
8、ing any of the following methods, viz., estimating at different times the amount of residual unhydrated C3S, the amount of Ca(OH)2, loss on ignition, electrical conductivity, heat liberation, etc. The techniques of DTA, TG, and Conduction Calorimetry have proven to be valuable to follow the hydratio
9、n of C3S in the presence of calcium chloride.3 Typical results of the application of DTA are illustrated in Figs. 1, 2, and 3.4 Thermograms of C3S hydrated to different times in the presence of 0, 1, or 4% CaCl2 enable a study of the progress of hydration, identification, and estimation of some of t
10、he products. Unhydrated C3S exhibits endothermal effects at about 680, 930, and 970980C represent- ing crystalline transitions (Fig. 1). Onset of hydration is indicated by the endothermal effects below 300C. They are caused by expulsion of loosely, as well as firmly, bound water from the C-S-H gel.
11、The increase in the intensity of this effect with time is indicative of the formation of larger amounts of C-S-H product. A very small endothermal effect at about 480C, which appears within a few minutes becoming more evident at one hour and after, may be attributed to the dehydration effect of Ca(O
12、H)2. In the first eight hours, the amount of Ca(OH)2 produced is about 25% of all the calcium hydroxide that is formed in thirty days. 191 Figure 1. Rate of hydration of 3CaOSiO2 by DTA. Section 2.0 - Calcium Chloride 192Chapter 5 - Accelerating Admixtures Figure 2. Rate of hydration of 3CaOSiO2 in
13、the presence of 1% CaCl2 by DTA. 193 Figure 3. Rate of hydration of 3CaOSiO2 in the presence of 4% CaCl2. Section 2.0 - Calcium Chloride 194Chapter 5 - Accelerating Admixtures In the presence of 1% CaCl2, the thermograms of hydrating C3S show significant differences from those hydrated without calci
14、um chloride addition (Fig. 2). The endothermal effects below 300C in the presence of CaCl2 are much larger than those obtained in samples without the addition of calcium chloride (Figs. 1 and 2). An endotherm at 550C appearing up to two hours in the presence of 1% CaCl2 is absent in C3S hydrated wit
15、hout CaCl2. There is also evidence that the endothermal effect due to Ca(OH)2 is more intense in samples containing 1% CaCl2 than without it. Of the total amount of Ca(OH)2 formed at 30 days, 33% is formed within 8 hours of hydration. A remarkable feature of these thermograms is the onset of an inte
16、nse exothermic peak at four hours at a temperature of 690C. This peak is always followed by a large endothermal dip at about 800840C. There is some evidence5 that it may be caused by the chemisorbed chloride on the C-S-H surface and possibly also by chloride ions in the interlayer positions. In the
17、presence of 4% CaCl2, some thermal effects become more intense at earlier times than the corresponding ones hydrated in the presence of 1% CaCl2 (Fig. 3).4 Exothermal peaks are also evident at temperatures above 600C at three hours and beyond. The possibility of a surface complex of chloride on the
18、hydrating silicate phase is suggested by an endothermal effect in the range 570590C. A larger effect at 810850C following an exothermal effect is present from three hours to thirty days. If the rate of hydration of C3S is determined in terms of the amount of Ca(OH)2 formed at different times, at six
19、 hours the sample containing 4% chloride will have the largest amounts of calcium hydroxide. At 24 hours and 30 days, the sample containing 1% will have higher amounts of calcium hydroxide. If the hydration is determined by the disappearance of C3S, then at 30 days C3S with 4% CaCl2 is hydrated to t
20、he maximum extent followed by that containing 1% CaCl2. The apparent discrepancy is due the differ- ences in the CaO/SiO2 ratios of the C-S-H products formed during the hydration. Calcium chloride accelerates the hydration of C3S even at higher temperatures. The effect is particularly greater at ear
21、lier periods of hydra- tion. Heat evolution curves show that at temperatures of 25, 35, and 45C, the addition of 2% CaCl2 not only influences the total heat developed at early periods but also the time at which the maximum heat evolution peak occurs.6 Increasing the concentration of CaCl2 up to 20%
22、with respect to C3S has been found to influence the conduction calorimetric curves.7 In Fig. 4, conduction calorimetric curves of C3S containing 020% calcium chloride are given. The sample containing no chloride shows a hump with 195 a peak effect at about 1314 hours. This peak occurs at lower tempe
23、ratures and is also sharper as the amount of added chloride is increased. At 20% CaCl2, a sharp peak occurs at about two hours. By applying thermal analysis, XRD, and chemical methods, it has been concluded that calcium chloride may exist in different states in the system tricalcium silicate-calcium
24、 chloride-water.5 The chloride may be in the free state, as a complex on the surface of the silicate during the dormant period, as a chemisorbed layer on the hydrate surface, in the interlayer spaces, and in the lattice of the hydrate. Figure 5 gives the estimate of the states of chloride in the sil
25、icate hydrated for different periods.5 The results show that the amount of free chloride drops to about 12% within 4 hours, becoming almost nil in about 7 days. At 4 hours, the amount of chloride existing in the chemisorbed and/or interlayer positions rises sharply and reaches about 75%. Very strong
26、ly held chloride that cannot be leached, even with water, occurs to an extent of about 20% of the initially added chloride. Since this will not be in a soluble state in water, it would not be available for corrosion processes. The formation of com- plexes may explain effects such as the acceleration
27、 of hydration, the increase in surface area, morphological changes, and the inhibition of formation of afwillite (a crystalline form of calcium silicate hydrate) in the presence of calcium chloride. Figure 4. Influence of CaCl2 on the heat evolution characteristics of hydrating C3S. Section 2.0 - Ca
28、lcium Chloride 196Chapter 5 - Accelerating Admixtures Collepardi, et al., studied the hydrates formed during the hydration of C3S in the presence of calcium chloride.89 In an autoclaved sample containing CaCl2, they detected an exothermic peak in the thermogram that was attributed to the chemisorpti
29、on of chloride on the C-S-H surface and its penetration into the molecular layers of C-S-H. Compared to the extensive thermal investigations of C3S hydration in the presence of CaCl2, only a limited amount of work has been reported on the effect of calcium chloride on the hydration of C2S. Calcium c
30、hloride accelerates the hydration of dicalcium silicate. In the thermograms of C2S paste hydrated for 13 months, lower amounts of calcium hydroxide are formed in the presence of calcium chloride, compared to that hydrated Figure 5. Possible states of chloride in tricalcium silicate hydrated for diff
31、erent periods. 197 without the chloride.10 This would imply that a higher C/S ratio C-S-H product results in the presence of calcium chloride. There is also evidence that in the hydration of C2S some chloride is bound rigidly. The reaction of C3A with calcium chloride results in the formation of hig
32、h and low forms of tricalcium chloroaluminates. Under normal conditions of hydration, the low form, viz., C3ACaCl2XH2O is obtained. The DTA technique may be used to differentiate between the two forms. Endothermal effects at about 190 and 350C are caused by the low form and the endotherm at about 16
33、0C is exhibited by the high form. In the system C3A-CaO-CaCl2-H2O, at higher concentrations of calcium chloride, cal- cium hydroxychloride is formed that is identified by peaks at 130, 145, and 485C.11 Calcium chloride influences the rate of hydration of C3A + gypsum mixtures. In Fig. 6, the conduct
34、ion calorimetric curves of the mixtures C3A + 20% gypsum + 12.5% CaCl2 are given along with the identified com- pounds at different times.37 A comparison of this curve with that obtained with C3A + gypsum (G) or C3A + CaCl2 would lead to the following conclusions. The reaction between C3A and gypsum
35、 is accelerated by calcium chloride. Monochloroaluminate (MCA) is formed after gypsum is consumed in the reaction with C3A. Conversion of ettringite (TSA) to monosulfoaluminate occurs only after all CaCl2 has reacted.12 Figure 6. The rate of consumption of various components in the C3A-gypsum-CaCl2
36、-H2O system. Section 2.0 - Calcium Chloride 198Chapter 5 - Accelerating Admixtures The effect of calcium chloride on the hydration of portland cement has been studied by various techniques such as XRD, TG, conduction calorimetry, chemical analysis, etc. Thermograms of portland cement hydrated with o
37、r without 2% CaCl2 are shown in Fig. 7.3 The endothermal effect at about 450475C is indicative of the presence of Ca(OH)2. Compared to the DTA of plain cement paste, that hydrated in the presence of calcium chloride indicates substantial differences in the form and size of the endotherm appearing in
38、 the region 150200C. The broad endothermal valley is caused mainly by the presence of a high form of sulfoaluminate and the C-S-H phase. The relatively larger intensity of these effects in the presence of the chloride demonstrates the increased formation of these products. Particularly significant i
39、s the increased intensity of the Ca(OH)2 peak between 2 and 4 hours, in the presence of the chloride. The small endothermal effect around 350C observed only in the presence of calcium chloride is probably due to calcium chloroaluminate hydrate or to its solid solution with the hexagonal calcium alum
40、inate hydrate. Figure 7. Differential thermograms of portland cement hydrated with 2% CaCl2. 199 The thermogravimetric method may also be used to estimate the amount of Ca(OH)2 formed by the hydration of cement. Determination of the weight loss between 450 and 550C caused by the decomposition of Ca(
41、OH)2, and that between 550 and 900C caused by the decomposition of CaCO3 permits the estimation of the amount of lime formed during the hydration of cement with 0, 1, and 2% CaCl2. Table 1 gives the results for cement samples hydrated by adding the chloride to the mixing water or by adding it a few
42、minutes after the cement is mixed with water.13 The lime formed is reported in terms of CaO. The amount of CaO at 1 day in the presence of calcium chloride is almost twice that formed in the absence of the admixture. The degree of hydration is generally higher in samples hydrated with 2% CaCl2. It i
43、s also observed that even at 90 days the samples with calcium chloride are hydrated to a greater extent than those without it. Hydration is promoted by delayed addition of the chloride. This is ex- plained as followsin the absence of chloride, the reaction that occurs initially involves the aluminat
44、e and ferrite phases, and gypsum. In the presence of CaCl2, where the aluminate and ferrite phases preferentially react with the chloride, less chloride is available for accelerating the hydration of C3S. If, however, chloride is added a few minutes after the cement is mixed with water, the sulfate
45、would have already reacted with the aluminate and ferrite phases, and there is more chloride available to accelerate the silicate reaction. Table 1. Effect of CaCl2 on the Amount of CaO Formed in Portland Cement Pastes Section 2.0 - Calcium Chloride Calcium ChlorideMethod of AdditionPercentage CaO f
46、ormed at (days) Additionof CaCl21392890 0% 3.56.88.07.98.1 1%with water6.77.08.08.38.4 1%2 minutes later6.97.78.18.48.5 1%4 minutes later7.07.78.38.48.5 1%8 minutes later6.57.28.18.08.4 2%with water7.17.78.28.58.6 2%2 minutes later7.88.08.38.68.8 2%4 minutes later7.98.08.38.68.8 2%8 minutes later7.3
47、7.87.98.08.3 200Chapter 5 - Accelerating Admixtures The setting and hardening of concrete are accelerated in the presence of calcium chloride. These are related to the effect of CaCl2 on the rate of hydration of cement in concrete. The hydration of cement is exothermic, resulting in the production o
48、f heat, and if the heat is produced at a faster rate, larger amounts of the hydrates are formed at earlier times in the presence of accelerators. This is particularly significant in the first 1012 hours. The influence of different amounts of calcium chlo- ride on the rate of heat development is depi
49、cted by conduction calorimet- ric curves (Fig. 8).14 The position of the peak corresponding to the maximum liberation of heat moves towards shorter times as the amount of CaCl2 is increased. It occurs at about eight hours for the reference cement and at about three hours with 2% CaCl2. Calcium chloride also increases the amount of heat liberated during the first few hours. Their reaction can also be accelerated by hydration at higher temperatures. The addition of 2% CaCl2 was found to have the same accelerating ef