Shrinkage-reducing admixtures and early-age desiccation in cement pastes and mortars

Fundamental studies of the early-age desiccation of cement-based materials with and without a shrinkage-reducing admixture (SRA) have been performed. Studies have been conducted under both sealed and drying conditions. Physical measurements include mass loss, surface tension, X-ray absorption to map the drying profile, internal relative humidity (RH), and autogenous deformation. Interestingly, although the SRA accelerates the drying of bulk solutions, in cement paste with a water-to-cement (w/c) ratio of 0.35, it actually reduces the measured drying rate. Based on the accompanying X-ray absorption measurements and a simple three-dimensional microstructure model, an explanation for this observation is proposed. In sealed systems, at equivalent hydration times, the SRA maintains a greater internal RH and reduces the induced autogenous deformation. Thus, these admixtures should be beneficial to low w/c ratio concretes undergoing self-desiccation, in addition to their normal usage to reduce drying shrinkage.