Effects of volume fractions of activated carbon and silicon dioxide in bentonite on the diffusion coefficient and retardancy (retardant ability) of tracer gas SF6 were studied. The results showed that the diffusion coefficient increases with increasing silicon dioxide

but decreases with increasing activated carbon. Addition of activated carbon to bentonite can enhance its adsorptivity

thus resulting in a better retardancy. On the other hand

addition of silicon dioxide will lead to a reduction of retardancy though some physical characteristics can be improved. The optimum volume fraction of activated carbon and silicon dioxide was 5% each.