LIQUID SELF-DIFFUSION IN PORES OF HARDENED GYPSUM - PULSED-FIELD GRADIENT NMR-STUDY

The self-diffusion of liquid confined in pores of gypsum was studied u sing NMR method with pulsed magnetic field gradient at 30 degrees C. T he 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 degrees C. The por es of gypsum form a permeable system. This fact allows us to use two l iquids with the diffusion coefficients differing by a factor of 100 su ch as water and polypropyleneoxide with M = 1025. Thus, the length sca le of the study was broadened to 2.5 x 10(-7)-4.2 x 10(-5) m. Dependen cies of echo-attenuation shape and of diffusion coefficient versus dif fusion time were investigated. The slopes of echo-attenuations and val ues 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 wa ter to gypsum ratios were interpreted in terms of a model with two lev els 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 collisio ns with the barriers. In the samples with greater water-to-gypsum star ting ratios, pores larger than 50 mu m were discovered. The width of l ayer, the barrier separation and the minimum of the layer stretching w ere estimated.