a grouting composition comprised of 84 - 95 % by weight of granular water - swellable sodium - based bentonite , 1 - 8 % by weight of finely divided salt - water hydratable sepiolite , and 4 - 8 % by weight of a clay stabilizer . the grout composition which contains no organic polymeric substances is mixed with fresh water to provide a pumpable , rehydratable , and easily applied grouting slurry . the slurry admixture will form a solid seal with low hydraulic conductivity and high structural stability . the grouting composition is useful for sealing the annular space around a well casing and plugging abandoned wells in practically all types of formations . a preferred grouting compostion comprises 88 - 90 % by weight of granular sodium bentonite , 6 - 7 % by weight of sepiolite , and 4 - 5 % by weight of granular ammonium sulfate . in use , 50 pounds of the preferred grouting compostion is mixed with 13 - 14 gallons of fresh water directly to provide a grouting slurry with a total of 30 - 32 % by weight of active solids and a density of 10 . 1 - 10 . 3 pounds per gallon . the slurry admixture is a completely inorganic , non - toxic , and environmentally safe grouting system .