Laboratory Experiments on the Alteration of Highly-Compacted Bentonite by Alkaline Solution and the Effects on Physical Properties

The laboratory experiment was done that 1.0mol/L sodium hydroxide solution was injected to the compacted bentonite whose density is the same as the prospected value in the concept of the intermediate-level disposal in Japan in the circumstance of 70°C temperature. After the injection of the alkali solution for approximately 600 days, the bentonite was taken out of the apparatus and some sorts of analysis were done. The accompanying minerals in the bentonite, calcedony and quartz, were dissolved and disappeared in XRD charts. Then analcime was precipitated as a secondary mineral. Although montmorillonite was dissolved, the mass fraction of it was kept approximately. The hydraulic conductivity of the bentonite calculated using the flow rate at the end of the injection of alkali solution was smaller than the prospected value based on a widely-used empirical model of the hydraulic conductivity of compacted bentonite as a function of the equivalent concentration of pore solution, montmorillonite partial void ratio, and the ratio of sodium ion equivalent to the exchangeable cation equivalent. The reasons for the difference were supposed to be the decrease of pore size brought by mineral dissolution and the large viscosity of pore solution involving high concentration aqueous silicon.

Effect of Various Dispersants on the Gel-Casting of Silicon Nitride

A high solid loading of dispersed silicon nitride slurry was prepared with various dispersants, which allowed a stable silicon nitride suspension with sintering additives (cordierite) in alkaline regions to a solid content of 45 vol%. Gum arabic, PMAA-NH4, TMAH, PAA-NH4 and their composites were selected as dispersants. The results suggested that the composite dispersant which was obtained using PAA-NH4 and TMAH with the ratio in 4:1 was more effective in aqueous silicon nitride powder suspensions system than simplex dispersant. The green bodies derived exhibited a mechanical strength as high as 8.7MPa and SEM photos revealed that the green body also had a high homogeneity.