CONCRETE DURABILITY AND CAO SIO2 MOLE RATIO OF CSH/

CaO/SiO2 mole ratio of calcium silicate hydrate (CSH) in Portland ceme nt pastes has been studied by various authors using diverse techniques . Some of these authors claimed a mole ratio of 1.5 and others a highe r ratio of about 1.7 to 1.8. However no attempt seems to have been mad e to correlate the claimed mole ratios to the chemical and mechanical properties of CSH especially the durability of hardened mortar or conc rete. This paper addresses this correlation. It has been shown that th e durability of Portland cement concrete requires a CaO/SiO2 mole rati o of 1.5 and any excess CaO is detrimental to concrete durability unde r some circumstances. The reported CaO/SiO2 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 pr ocesses. Neither does a higher CaO/SiO2 ratio in CSH adds to its stren gth giving property.