Improved model for capillary absorption in cementitious materials: Progress over the fourth root of time

Sorptivity is broadly used for characterising the pore connectivity of cementitious materials, with applications in design for durability. A water sorptivity coefficient (WSC) is typically obtained from the ratio between the amount of absorbed water and t0.5. This relationship is however not linear for cementitious materials, and conventions are needed for the computation. Variable criteria in the literature complicate the comparison of WSCs. This paper proposes a new approach for describing the entire absorption process. We substantiate the hygroscopicity of calcium silicate hydrates and the effect of swelling during the process as the main causes for the anomalous capillary absorption by cementitious materials. We present a theoretical model with a single descriptive coefficient of capillary absorption progressing linearly with t0.25. The model fits remarkably well to experimental data, and it solves the problem of lack of linearity with t0.5. A full description of the transport process is then offered.