Insights into the “accelerating effect” of sucrose on cement pastes

Sugars have been widely used as setting retarders due to their efficiency and low cost. At low levels, they delay the hardening of the cement paste, reducing the hydration rate of the cement paste, especially alite. Some studies, however, have observed an accelerating effect through rheological tests, depending on the additive content and the water to cement (w/c) ratio, among other factors. So far, there is no explanation of the reason for this phenomenon supported by solid evidence. In this context, this work investigates the effect of sucrose on the structure (through in situ XRD and isothermal induction calorimetry) and on the rheological behavior (stationary rotational rheometry) of cement pastes. The results show that sucrose acts as an efficient setting retarder at low concentrations, delaying the setting time of the pastes with the reduction of the alite hydration rate, and without significant changes in the yield stress evolution. At high concentrations, the hydration delay of alite is even longer, followed by an intense formation of ettringite, whose main precursor is the ferrite phase. The formation of needle-shaped crystals of ettringite is the reason behind the acceleration phenomenon.