Hydration of tricalcium silicate

Omotoso, O. E., Ivey, D. G. Mikula, R. 1998b. Hexavalent chromium in tricalcium silicate Part II Effects of CrVI on the hydration of tricalcium silicate. Journal of Materials Science, 33, 515-522. 

The tobermorite obtained in the hydration of tricalcium silicate (Ca3Si02), / -dicalcium silicate (/ -Ca2Si04), portland cement, and concrete is a colloid, with a specific surface area of the order of 300 sq. meters per gram. To give an idea of how the elementary particles of tobermorite look, Figure 7 is an electron micrograph of a few particles (obtained by L. E. Copeland and Edith G. Schulz at the Portland Cement Association Research and Development Laboratories). These particles look like fibers, but if you watch them closely, you see that they are very thin sheets, rolled up as one would roll up a sheet of paper. At the lower end the sheets are partly unrolled. When one prepares tobermorite by the reaction of lime and silica, one usually obtains crumpled sheets, which are not rolled up. The electron microscopists tell us that the sheets are very thin, of the order of a single unit cell in thickness. 

The hydration of tricalcium silicate C3S and dicalcium silicate C2S (for abbreviations see below Table 5.3-6) are responsible for the further. solidification of Portland cement. This reaction only begins in earnest after ca. 4 hours. Initially long needles of calcium silicate hydrate are formed, which bond the cement particles together. Later, smaller needles of calcium silicate hydrate fill the gaps left. The more reactive tricalcium silicate hydrolyzes much faster than dicalcium silicate. 

The early strength of concrete is the result of hydration of tricalcium silicate C jS) and tricalcium aluminate (C.,A) phases of Portland cement. When mixed with water C.-,S hardens rapidly and both C.,S and C..,A release heat. 

The products of dicalcium silicate hydration are identical or almost identical to those formed in the hydration of tricalcium silicate however, the amount of calcium hydroxide formed is distinctly lower. The hydration rate of dicalcium silicate is sigrrificantly lower than that of tricalcium silicate, even though it may be influenced to a certain degree by the selection of the dopant ion and the cooling rate. 8ome highly reactive forms of dicalcium silicate have been synthesized in recent years, but are of theoretical importance only. 

Upon hydration, alinite yields a C-S-H phase similar to that formed in the hydration of tricalcium silicate, together with Ca(OH)2 (portlandite) and an AFm phase that is similar to, but not identical with, Friedel s salt (C3A.CaCl2.10H2O). In it, up to 15 mol% of CaCl+ is replaced by Ca(OH)2 and CaCOj (contamination from air) (Pollmann, 1986 Neubauer and POllmann, 1994). The composition of this phase may be expressed by the general formula (l-x-> )C3A.xCaCl2.TCaC03.Ca(0H)2. At the same time distinct amounts of chloride, aluminate and calcium ions also enter the liquid phase. The following concentrations were reported by Ji et al. (1997) after 15 min of hydration ... 

The amount of free calcium hydroxide in Portland cement-microsilica mixes increases initially, as its formation in the hydration of tricalcium silicate is faster than its consumption in the pozzolanic reaction with microsilica. Later on, however, the amount of free calcium hydroxide may start to decline, when the amount of it consumed in the pozzolanic reaction exceeds the rate by which it is formed in the hydration of tiicalcium and dicalcium silicate (Papadakis, 1999). This crossover point— that is, the time at which the rate of Ca(OH)2 consumption exceeds the rate of its formation— will depend on the amount and reactivity of the microsilica present, as well as on the reactivity of the clinker, and can occur after several hours or a few days of hydration, or not at all (especially at low microsilica additions). The amount of residual free calcium hydroxide in mature paste will generally decline with increasing amounts of nucrosilica in the original mix. It will also decline with decreasing watei/solid ratio, as under these eonditions the C/S ratio of the formed C-S-H phase tends to increase. 

Stadelmann C (1987) The influence of citric acid on the hydration of tricalcium silicate (CUjSiO ). Z Anorg Allg Chem 547 127-132... 

Sato, T., Diallo, F., 2010. Seeding effect of nano-CaCOs on the hydration of tricalcium silicate. 

Figure 2. Rate of heat development during the hydration of tricalcium silicate and portland cement. (With permission, Noyes Publications, Concrete Admixtures Handbook, V. S. Ramachandran, ed., 2 Ed. 1995)...

The alkali in clinker is combined as a solid solution with the C3A phase. The crystalline stmcture changes from cubic to orthorhombic or monoclininc structure, depending on the content of Na in the C3A phase. Shin and Han studied the effect of different forms of tricalcium aluminate on the hydration of tricalcium silicate by applying DTA, TG, and conduction calorimetry. It was concluded that the hydration of tricalcium silicate is accelerated when orthorhombic, monoclinic, or melt C3A was present in the mixture. The cubic form of tricalcium aluminate was least effective for accelerating the hydration of the silicate phase. 

It is important to acquire information on the hydration processes that occur when cement is subjected to low temperatures. Even at temperatures below zero cement hydrates slowly, as water does not freeze due to the presence of soluble materials in the pores. The products that form at low temperatures are calcium aluminate hydrates at earlier periods and after several months, the products of hydration of tricalcium silicate. A weak endothermal peak at about 330°C could be due to the presence of silica gel and this is preceded by the appearance of calcium hydroxide and C-S-H gel.t l... 

Ramachandran, V. S., Kinetics of Hydration of Tricalcium Silicate in the Presence of Calcium Chloride by Thermal Methods, Thermochimica Acta, 2 41-55 (1971)... 

The retarding effect on tricalcium silicate hydration is almost completely eliminated in the presence of C3A or NC3A + gypsum. Adsorption of TEA on ettringite may remove the admixture to facilitate the hydration of tricalcium silicate. Figure 4 shows that the amount of TEA in solution in contact with ettringite decreases with time, Another possibil-... 

The rate of hydration of tricalcium silicate in the presence of different amounts of calcium formate (0.5%-6.0%) has been reported by Singh and Abha.i Calcium formate accelerates the tricalcium silicate hydration at 2% addition. It is speculated that in the presence of formate the protective layer on the silicate surface is ruptured, resulting in an acceleratory effect. 

In cements, incorporation of calcium carbonate is permitted in some countries. In Canada, the maximum limit is set at 5%. Calcium carbonate is not an inert filler. It is known to react with calcium aluminate. In a study of the hydration of tricalcium silicate in the presence of finely divided calcium carbonate, Ramachandran observed that the carbonate acted as an accelerator. Ushiyama, et al.,t examined the effect of carbonates of Na, K, Li, Cs, and bicarbonates of Na, K, and Li on the hydration of alite. Although small amounts retarded the hydration, larger amounts acted as accelerators. 

Alkalis, such as NaOH, and Na salts of carbonate, aluminate, and silicate are known to accelerate the hydration of cement and cause early stiffening. In the hydration of tricalcium silicate with NaOH, there was an acceleratory effect even up to 28 days. After 7 days however, the strength of the reference was higher than that containing NaOH.[ l Evidence was also obtained for the incorporation of Na in the hydrated product. 

Volatilized silica, obtained as a by-product in the metallic silicon or ferrosilicon industry, is known as silica fume. It has been advocated for incorporation into cement to obtain high strength. It acts as a pozzolan and is also known to accelerate the hydration of tricalcium silicate. Therefore, it can be treated as an accelerator. 

There has been continued interest in developing an organic-based accelerator. Ramachandran and coworkers studied the effect of o, m, and p-nitrobenzoic acids on the hydration of tricalcium silicate.The m and p nitrobenzoic acids acted as accelerators. The acceleratory effect was attributed to the complex formation between the organic compound and the C-S-H phase on the surface of the tricalcium silicate phase. 

Ramachandran, V. S., Beaudoirt, J. J., and Patoli, R. M., The Effect of Nitrobenzoic and Aminibenzoic Acids on the Hydration of Tricalcium Silicate A Conduction Calorimetric Study, ThermochimicaActa, 190 325-333 (1991)... 

Figure 14. Calorimetric curves showing the effect of phosphonic acid on the rate of hydration of tricalcium silicate.

Monosi, S., Moriconi, G., and Collepardi, M., Combined Effect of Lignosulfonate and Carbonate on Pure Portland Clinker Compoimds Hydration, III Hydration of Tricalcium Silicate Alone and in the Presence of Tiicalcium Aluminate, Cement Concr. Res., 12 415-424(1982)... 

Superplasticizers affect the hydration of tricalcium silicate. It is generally known that superplasticizers retard the hydration of C3S [4][i2]-[i5] jjjg effect of different amounts of SMF on the hydration of C3S can be illustrated by conduction calorimetric curves (Fig. By the... 

Figure 16. Effect of SNF on the hydration of tricalcium silicate phase in a high alkali cement.

The silicates are the major components in Portland cement imparting strength to the matrix. No reactions have been detected between chloride ions and silicates. Calciiun chloride accelerates the hydration of the silicates when at least 1 percent by weight is added. Calcium chloride seems to act as an accelerator in the hydration of tricalcium silicate, as well as to promote the corrosion of steel. 

Dobson CM, Goberdhan DGC, Ramsey JDF, Rodger SA (1988) Si MAS NMR study of the hydration of tricalcium silicate in the presence of finely divided silica. J Mater Sci... 

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