Liquid self-diffusion in pores of hardened gypsum: pulsed field gradient NMR study

Abstract

The self-diffusion of liquid confined in pores of gypsum was studied using NMR method with pulsed magnetic field gradient at 30 °C. The starting weight ratios of water to gypsum were equal to 0.4, 0.5, 0.6, 0.8 and 1. The hardening process occurred at 23 °C. The pores of gypsum form a permeable system. This fact allows us to use two liquids with the diffusion coefficients differing by a factor of 100 such as water and polypropyleneoxide with M=1025. Thus, the length scale of the study was broadened to 2.5×10⁻⁷–4.2×10⁻⁵ m. Dependencies of echo-attenuation shape and of diffusion coefficient versus diffusion time were investigated. The slopes of echo-attenuations and values of diffusion coefficients determined for the same length scale are different for the systems with starting ratios of water to gypsum 0.4–0.6 and 0.8–1. The experimental results for the samples with lower water to gypsum ratios were interpreted in terms of a model with two levels of organization of the porous structure. The first level comprises the interconnected layers with all possible orientations. The second level comprises the barriers distributed inside the layers. Diffusion of the liquid molecules inside the layer is restricted due to collisions with the barriers. In the samples with greater water-to-gypsum starting ratios, pores larger than 50 μm were discovered. The width of layer, the barrier separation and the minimum of the layer stretching were estimated.