Sulfate Optimization in the Cement-Slag Blended System Based on Calorimetry and Strength Studies

Optimization of sulfur trioxide (SO3) content is typically conducted for plain cements (OPC-based) paste or mortar systems using compressive strength testing and isothermal calorimetry for quality control. The objective of these procedures is to ensure adequate strength and set properties by controlling the time of occurrence of the aluminate peak by adjusting the SO3 content by incremental addition of sulfate in the form of hemihydrate at three or more levels, at the same time attempting to maximize either strength or heat of hydration at 1 and 3 days. The inclusion of supplementary cementitious materials such as blast furnace slag at high replacement levels in concrete mixtures is common. The incorporation of slag, typically with higher alumina contents than OPC, can additionally impact the sulfate consumption as they are expected to influence the reaction of the aluminate phase at an early age. Additionally, slags have variable alumina content ranging between 7% and 18%, which can further influence this tendency. This study investigates the influence of fineness, C3A, C4AF content of cement, and Al2O3 content of slag based on compressive strength and isothermal calorimetry testing at one and three days. A matrix consisting of four cements with variable fineness, C3A, C4AF content of cement, and two slags with low and high alumina contents was investigated for optimum SO3 content. A single replacement level of 50% by mass of slag was used. The results suggest the strong influence of C3A, C4AF content of cement, and Al2O3 content of slag and mean particle size and fineness of both cement and slag on optimum SO3 content based on the heat of hydration and compressive strength testing. An expression incorporating these parameters is suggested for the optimization of SO3 content for use in slag-blended systems