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Fundamental studies of the early age desiccation of cement-based materials with and without a shrinkage-reducing admixture 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, and autogenous deformation. Interestingly, although the shrinkage-reducing admixture accelerates the drying of bulk solutions, in cement paste with a water-to-cement 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 shrinkage-reducing admixture maintains a greater internal relative humidity and reduces the induced autogenous deformation. Thus, these admixtures should be beneficial to low water-to-cement ratio concretes undergoing self-desiccation, in addition to their normal usage to reduce drying shrinkage.
Keywords: Cement paste (D); drying (A); humidity (A); shrinkage (C); surface tension.