Investigation on colloid generation from cement hydrates and their characterization

The association of contaminants with mobile colloidal particles present in groundwater has been recognized as a potentially important mass transfer mechanism for contaminant migration, such as radionuclides, in the subsurface environment. In the case of a cementitious repository system, the cement hydrates may provide a significant source of colloids, especially for the low-ionic-strength ground waters and for the long-term degradation of cements by dissolution. Leaching experiments were performed using pulverized fuel ash-silica fume blended cement (HFSC) and blast furnace slag blended portland cement (OPC/BFS) mixed with low ionic strength simulated groundwater at liquid to solid ratio of 10:1, 50:1 and 100:1 either at room temperature or 60degC for 2-8 months. Colloids in the leachates were observed by TEM with the ultracentrifugal deposition method. The number concentration of colloid for the leachate of the HFSC was in the range of 1010-1012 N/l. It depended on the solid to liquid ratio in the leaching experiments. The colloid particles were composed dominantly of Si-Al-Ca. The mean particle size of colloids was 50 nm-200 nm. The colloid generated from HFSC was characterized by the zeta potential measurement of the particles and by the change in particle diameter distribution as a function of Ca concentration in the leachates. The results showed that the colloidal stability of the particles was affected by Ca concentration in the leachates. The application of DLVO theory to the obtained data could succeed in the estimation of the dependence of colloid population on solid to liquid ratio in the leaching experiments. (author)