The solution chemistry and early hydration of ordinary portland cement pastes with and without admixtures

Solution extraction and analysis in the first 3 h of hydration have been us ed to study the effects of calcium chloride, calcium nitrate, calcium thioc yanate, and sucrose on the hydration of Portland cement. Studies were made at a water : cement ratio of 0.5, which is similar to that to be expected i n normal use. The addition of calcium salts has a pronounced effect on the calcium, hydroxyl and sulphate ion concentrations. This is in agreement wit h the results from X-ray diffraction and differential scanning calorimetry, which show rapid precipitation of more gypsum and calcium hydroxide within the first 5 min on adding these salts than in neat OPC pastes. The data fo r sucrose show opposite trends in line with the chelation of the calcium io n. No significant changes in sodium and potassium levels were observed in t he first 3 h with any of the additions. Differences in degrees of hydration at early stages from the addition of calcium salts are attributed to diffe rent relative degrees of solvation of the ionic reactants and transition st ates, in addition to the nature and extent of sulphate and hydroxide format ion from solution. The combined effect of cations and anions on the acceler ation produced are discussed. (C) 1999 Elsevier Science B.V. All rights res erved.