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Concrete durability and CaOSiO2 mole ratio of CSH

1995; Elsevier BV; Volume: 25; Issue: 5 Linguagem: Inglês

10.1016/0008-8846(95)00085-q

1873-3948

S. Chatterji,

Concrete Corrosion and Durability

CaOSiO2 mole ratio of calcium silicate hydrate (CSH) in Portland cement pastes has been studied by various authors using diverse techniques. Some of these authors claimed a mole ratio of 1.5 and others a higher ratio of about 1.7 to 1.8. However no attempt seems to have been made to correlate the claimed mole ratios to the chemical and mechanical properties of CSH especially the durability of hardened mortar or concrete. This paper addresses this correlation. It has been shown that the durability of Portland cement concrete requires a CaOSiO2 mole ratio of 1.5 and any excess CaO is detrimental to concrete durability under some circumstances. The reported CaOSiO2 mole ratios above 1.5 may be due to a contamination of the analyzed volumes or the excess CaO is so loosely bound that it may take part in some concrete break-down processes. Neither does a higher CaOSiO2 ratio in CSH adds to its strength giving property. Hydration and strength development of Portland cement involves, among other reactions, the formation of calcium silicate hydrate (CSH) and Ca(OH)2. It is a general assumption that CSH is the main strength giving component in a set Portland cement paste; this is particularly so in a hardened C3S paste. Another general assumption is that CSH particles that form in Portland cement and C3S pastes are identical in structure except for some substitution of other ions in the structure of CSH in the case of Portland cement paste. In a hydrating Portland cement, especially in hydrating C3S, the formation of CSH and Ca(OH)2 are related processes in the sense that the proportion of each increases with the degree of cement hydration. The processes are complementary to each other in the sense that for a given degree of cement hydration an increase in CaOSiO2 mole ratio of CSH lowers the proportion of Ca(OH)2. CaOSiO2 mole ratio of CSH has been studied by a number of workers using diverse techniques. The results so far obtained may be divided into two main groups. The workers using the phase equilibrium, the theoretically valid electron probe micro-analytical and the extraction techniques obtained a ratio of about 1.5 (fx. 1, 2, 3). Other workers, using mainly the analytical electron microscopic, dta and tga techniques have reported a variable CaOSiO2 mole ratio centred on 1.7 (fx. 4, 5). The literature cited above is not exhaustive but only indicative of the existence of claims of two values.