Hydration of High-Alumina Calcium Aluminate Cements with Carbonate and Sulfate Additives

This study investigated the influence of limestone (LS) and calcium sulfate (C$) mineral additives on the hydration kinetics of high-α-Al2O3 calcium aluminate cement (CAC) utilizing experimental techniques and thermodynamic simulations. Increasing the replacement level of limestone or calcium sulfate increased the cumulative heat of the hydration reaction. The limestone exhibited limited acceleratory effects to the CAC hydration kinetics due to the coarseness of the powder. The coarse particle size distribution limited any heterogenous nucleation that would have occurred with a finer particle size as well as the intrinsic insolubility kinetically limits the formation of monocarboaluminate phases. Conversely, the cumulative heat release increased as the limestone content increased; however, this was not due to any enhanced reactivity provided by the limestone. Instead, this increase in the cumulative heat is due to a combination of the LS and the increase in the amount of water available to react with CAC via the dilution effect. In comparison, the increase in the C$ replacement level accelerated the heat flow rate of CAC with the C$ particles acting as a favorable surface for heterogenous nucleation of the hydrates during the initial stages of the hydration reaction. Increasing the C$ replacement level does not form more ettringite and does not translate in an increase in the compressive strength. After the 72-h hydration period, C$ remains in the microstructure, showing that the complete dissolution of C$ is not responsible for the monotonic increase in heat flow rate. It is expected that the amount of hydrates or residual unreacted particles cannot compensate for the decrease in strength caused by the reduction of α-Al2O3 present in the CAC