The chemistry of hydration of high alumina cement in the presence of accelerating and retarding admixtures

The effect of accelerators, and in particular lithium salts and citric acid solutions, on the setting time of high alumina cement has been studied using calorimetry, solution analysis and X-ray diffraction techniques. Results are discussed with respect to the ternary CaOAl2O3H2O solubility diagram. It appears that there is a nucleation barrier to the precipitation of the main products of hydration, CAH10 and C2AH8 and that lithium salts function as accelerators by precipitation of a lithium aluminate hydrate which acts as a heterogenous nucleation substrate. It is suggested that retardation by citric acid is due to the precipitation of protective gel coatings around the cement grains which impede hydrolysis or inhibit growth of the hydration products.