Effects of water-to-cement ratio and temperature on diffusion of water in hardened cement pastes

Apparent diffusion coefficients (D_a) of water and activation energies (E_a) of diffusion in hardened cement pastes (HCPs) were determined as a function of water-to-cement (w/c) ratio (0.36–0.60) and temperature (293–323 K) using HTO and H₂¹⁸O as tracers. The values of D_a and E_a ranged from 1.1×10⁻¹¹ to 1.7×10⁻¹⁰ m² s⁻¹ and from 21.5 to 31.3 kJ mol⁻¹, respectively. No significant difference between the D_a values of HTO and H₂¹⁸O suggests that water predominantly diffuses as H₂O molecule and dissociation of water is not significant even at high pH range in HCP. The values of E_a at low w/c ratio were higher than in bulk liquid water, suggesting a contribution of a different water regime, such as supercooled bulk water. Two simple models consisting of capillary and gel pores were considered to estimate the volume ratio of gel pores to total pores by optimizing the model to fit with the experimental data. The result suggests that HCP has a pore network mostly consisting of capillary pores with some very narrow pores plugged with hydrates, where HTO must diffuse through gel pores. This view of the HCP pore network was made available through analysis of E_a values. (author)