The Accelerating Action of Calcium Thiosulfate on the Hydration of Portland Cement and Comparison with Other Inorganic Salts

Abstract

Calcium chloride is now regarded as an excellent accelerator to the hydration of portland cement, however, its corrosive character to the steel in reinforced concrete is hardly ignored. Therefore, the accelerators without corrosive character are especially indispensable for concrete practice.In this paper, the accelerating action of calcium thiosulfate discovered or by the authors is investigated, in comparison with other inorganic soluble salts, by means of various methods.The accelerating ability of calcium thiosulfate on the setting of portland cement is very identical to that of calcium chloride. The early development of strength of portland cement increases in proportion to the amount of calcium thiosulfate added to portland cement. The early strength of mortar of portland blastfurnace slag cement (granulated blastfurnace slag 65%, portland cement 35%) is developed near that of ordinary portland cement, when calcium thiosulfate was used at the concentration of 3%. However, the accelerating action of calcium thiosulfate on the development of early strength of portland cement is slightly poorer than that of calcium chloride.The most characteristic of calcium thiosulfate as an accelerator will be that calcium thiosulfate does not give any corrosive action to the steel in concrete. The results of corrosion test showed that the test pieces of mild steel buried in mortar containing respectively 2% and 5% of calcium thiosulfate were scarcely corroded after the mortars were exposed under the atmosphere of outdoors for 6 months. Under a same condition, the test pieces of steel buried in the mortar containing 2% of calcium chloride was markedly corroded after only one month. Calcium nitrate did not show corrosive action. However, calcium nitrate can not be regarded as an accelerator.The heat liberation curves of the hydration of portland cement obtained by the use of a devised adiabatic calorimeter indicated that the accelerating action of inorganic salts on the hydration of portland cement is focused on the hydration of alite phase. The orders of effectiveness of anions and cations as accelerators of the hydration of portland cement are Cl->S2O3-->SO4-->NO3-, Cl->Br->I- and Ca++>Mg++>Na+.The hydration products of tricalcium silicate is CSH (II) when calcium chloride was present, is CSH (I) when gypsum was present and is not evidently identified when calcium thiosulfate was present. DTA curve of the 24 hours hydration product of C3S paste (H2O/C3S:50%) with 3% of calcium chloride showed a sharp exothermic peak at 640°C. The peak showed that the produced CSH (II) in hydration products was converted into β-C2S by heating. In the case of calcium thiosulfate, the continuous two exothermic peaks were obseved in DTA curve. The first peak could not be exactly assigned, however, the second was assigned as the oxidation of calcium sulfite which produced by the decomposition of calcium thiosulfate.