Hydration of tricalcium aluminate

These studies were of great interest to understand cement hydration and since then many in situ studies have been performed following the reactions using neutron and synchrotron X-ray powder diffraction. The development in time resolution is demonstrated by a synchrotron study of the hydration of tricalcium aluminate, C3A, which is an important component of Portland cement. Using a time resolution of down to 0.3 s a hitherto unknown shortlived intermediate phase was detected in the very early stage of hydration. 

It is also possible for several hydrate phases to be formed simrrltaneously, as in the hydration of tricalcium aluminate in water ... 

Ca2Al(0H)g.2H20].0H. In C4AHjc, an additional layer of H2O molecules is present between the principal layers. Tetracalcium aluminate hydrate is not among the hydrate phases formed in the hydration of calcium aluminates constituting calcium aluminate cement however, it may be formed in the hydration of tricalcium aluminate, a constituent of Portland cement. 

MuHite can be made below its fusion point from colloidal silica and basic aluminun chloride (756). Hydration of tricalcium aluminate is accelerated by low levels of" colloidal silica (757). 

Ramachandran, V. S., and Feldman, R. F., Significance of Low Water/ Solid Ratio and Temperatnre on the Physico-Chemical-Mechanical Characteristics of Hydrates of Tricalcium Aluminate, J. Appl. Chem. Biotech., 23 625-633 (1973)... 

Stukalova, N. P., and Andreeva, E. P., Chemical Interactions and Structure F ormation During Hydration of Tricalcium Aluminate in Calcium Chloride Solutions Satnrated with Calcium Hydroxide, Kolld. Zh., 31 446-450... 

Figure 10. Hydration of tricalcium aluminate with different amounts of triethanolamine.

Young, J. F., Hydration of Tricalcium Aluminate with Lignosulfonate Additives,A/ag. Concr. Res., 14 137-142 (1962)... 

Pauri, M., Ferrari, G., and Collepardi, M., Combined Effect of Lignosulfonate and Carbonate on Pure Portland Clinker Compound Hydration, IV Hydration of Tricalcium Aluminate-Sodium Hydroxide Solid Solution, Cement Concr. Res., 13 61-68 (1983)... 

Milestone, N. B., The Effect of Glucose and Some Glucose Oxidation Products on the Hydration of Tricalcium Aluminate, Cement Conor. Res., 7 45-52(1967)... 

Fig. 5. The early hydration reactions of tricalcium aluminate in the presence of gypsum and calcium hydroxide. Initial molar proportions l-C A ...

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. 

In Portland cements tricalcium aluminate usually exists in its cubic form however, in the presence of increased amounts of alkalis in the raw mix an orthorhombic or even a monoclinic modification may be formed instead. The stmcture of tricalcium aluminate is built from rings of six AIO4 tetrahedra and Ca ions. All three modifications of C3A hydrate in a similar way however, their reactivity may differ, depending on the quality and quantity of the foreign ions incorporated in their crystalline lattices. 

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 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... 

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. 

Ramachandran, V. S., Adsorption and Hydration Behavior of Tricalcium Aluminate-Water and Tricalcium Aluminate-Gypsum-Water Systems in the Presence of Superplasticizers, J. Am. Concr. Inst., 80 235-241 (1983)... 

Ramachandran, V. S., Differential Thermal Investigation of the System Tricalcium Silicate-Calcium Lignosulfonate-Water in the Presence of Tricalcium Aluminate and its Hydrates, 3rd Int. Conf. Therm. Analysis, Davos, Switzerland, pp. 255-267 (1971)... 

Sarsale, R., Sabetelli, V., and Valenti., G. L., Influence of Some Retarders on the Hydration at Early Ages of Tricalcium Aluminate, 7th Int. Symp. Chem. of Cements, Paris, 4 546-551 (1980)... 

Portland cement clinker. In addition to the major dicalcium silicate and tricalcium silicate phases, the structure shows silicate and tricalcium silicate phases, smaller amounts of tricalcium aluminate (3Ca0-Al203), and millerite (4Ca0-Al203-Fe203). On reaction with water, the clinker forms a complex hydrated product. This product is the cementitious material. It is a noncrystalline calcium silicate gel resulting from the tricalcium silicate and dicalcium silicate. The microstructure of set cement is shown in Figure 11.25. 

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