A diagnostic criterion for gravity-induced anomaly in water absorption

Capillary water absorption is a key transport mechanism influencing the durability of cementitious and other porous materials. Although absorption in unsaturated media typically follows a square root of time relationship (t^α=0.5), deviations, termed anomalous behaviour, are frequently observed. In materials with open and coarse pore structures, gravity can induce sub-diffusive behaviour (α < 0.5), yet the exponent alone does not quantify the competing influences of capillarity and gravity. The capillary-to-gravity pressure ratio (N_cg), derived from Darcy’s law and the sharp front theory, enables this comparison. Previous studies on highly porous rocks (>25% porosity) suggest that gravity effects become significant when N_cg < 3 at the end of the initial absorption stage. In this study, this criterion has been generalised using gravimetric absorption data for ten porous materials, including mortar, autoclaved aerated concrete and foamed concrete, from the literature, as well as two laboratory-cast lean concretes, collectively spanning porosities of 11.3–87.8%. The results show that gravity-induced anomaly occurs when N_cg < 3, with α = 0.30–0.40, clearly distinct from the anomaly induced by calcium silicate hydrate swelling, reported to have α ≈ 0.25. These findings demonstrate that N_cg provides a physically meaningful and practical criterion for identifying when gravitational effects must be incorporated in water absorption models.