The purpose is to render a porous surface impermeable with a view to
both preventing fluid loss through percolation and preventing caving of wall rock during
excavation and drilling. Examples are sealing of water tanks, dams, canals, water wells etc.
So, sodium bentonite, being highly impermeable, is preferred. The Indian industries specify
some parameters, the important ones of which are: swelling capacity 16-20 ml; gel time
instant to 5-7 minutes; gel index 35-45; base-exchange capacity 65-70 milli-equivalents per
100 gm; green compressive strength 8-9.5 psi.
However, bentonite is not suitable where the water level is fluctuating. As long as it is
submerged under water, it absorbs water, swells and plugs the pores. But as soon as it is out
of water, it begins to dry up, shrink and crack.