Hydration Kinetics of Tri-calcium Silicate in the Presence of Superplasticizers

A recently developed method based on Differential Scanning Calorimetry (DSC) has been used to study the hydration kinetics of tri-calcium silicate (C3S) in the presence of superplasticizers. We studied some of the most common additives employed in the cement industry, i.e., a sulfonated naphthalene-formaldehyde polycondensate, a polycarboxylate and a polyacrylate. The overall hydration kinetics of the superplasticized pastes have been obtained and compared to that of C3S cured in pure water. Superplasticizers do not change the kinetic laws involved in the hydration processes, the first according to an Avrami-Erofeev nucleation and growth law, and the second a three-dimensional diffusion equation. However, the induction times of the tri-calcium silicate paste are affected by the presence of additives. The polycarboxylate and the polyacrylate additives reduce the temperature dependence of the so-called "dormant period". The activation energy of the nucleation and growth stage is about doubled in the presence of superplasticizers, and is the largest for the polyacrylate additive, that is the most effective additive. DSC analysis shows that a larger amount of water reacts with C3S during the acceleration period in the presence of polycarboxylic and the polyacrylic additives, meaning that the water is more available, i.e., the paste is more fluid. SEM images show that the acrylic additive produces a change in the morphology of the formed hydrated calcium silicate gel, from a fiber-like structure to a sheet-like structure. The quantitative determination on how additives affect the curing process of cement is reported for the first time, allowing a determination of a scale of additives efficacy based on activation energies of the nucleation process.