Bentonite grout and neat cement grout both are used to plug abandoned wells as part of a decommissioning process. Because of its swelling properties, bentonite is sometimes added to cement grout. Because these grouts have to be pumped down holes, they are more liquid or fluid than grouts used for laying tiles and other building uses. Grouting wells is a job best left to qualified professionals; consult your local health department for recommendations.

Hand Grouting Tile
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Close-up of man grouting tiles with cement.

Bentonite Grout

Dry sodium bentonite, obtained from clay, absorbs several times its mass in water. This swelling ability makes bentonite grout useful for plugging old wells. Industries and governments use it for sealing underground metal pollutants and containers of spent nuclear fuel. Bentonite grout is commonly made of 50 pounds of powdered bentonite to 34 gallons of water to which up to 50 pounds of washed sand or granular bentonite are sometimes added.

Bentonite Grout Use

Bentonite grout needs to be pumped at higher pressures than cement grout. It is mixed with a paddle mixer and pumped while it is still viscous and has not started to swell and set. To delay rapid setting, granular sodium bentonite, chemical drying additives or calcium bentonite may be added to the grout mix. Calcium bentonite expands less than sodium bentonite.

Neat Cement Grout

Portland cement, the hardening ingredient of neat grout, is composed of powdered calcium silicate obtained from limestone and shale that hardens when mixed with water. Neat cement grout sets quickly and consists of one sack (94 pounds) of rapid-drying Type III Portland cement and no more than 6 gallons of water. Using less water, such as 5.4 gallons of water instead of 6 gallons, can make neat cement grout less permeable and reduce shrinkage. Up to 4.5 pounds of bentonite can be added to increase the bulk of neat cement grout as it dries.

Neat Cement Use

Neat cement grout swells less and shrinks more than does bentonite grout, but it hardens more quickly and solidly. Neat cement grout is better than bentonite grout for sealing small openings and the space around rocks and drill casings. It is also better for sealing artesian wells. The Minnesota Department Health recommends against using neat cement grout with plastic casing because of the heat produced by the drying cement.