Which of the following is usually added to cement grout to reduce shrinkage?

• A. Silica
• B. Bentonite
• C. Fly ash
• D. Sand

Answer: (B)

Bentonite is used to reduce shrinkage in cement grout because it acts as a water-retaining thickener. When hydrated, bentonite swells and increases the grout’s viscosity, which slows water bleed and minimizes settlement as the grout cures. That retention of water helps control drying shrinkage and crack formation, keeping the grout more cohesive and stable in the borehole or annulus.

Other materials don’t target shrinkage in the same way. Silica is mainly used to densify the cement paste and reduce permeability, not primarily to curb shrinkage. Fly ash can help with workability and may modestly affect shrinkage through the pozzolanic reaction, but it isn’t the typical additive chosen specifically to reduce shrinkage. Sand, as an inert aggregate, tends to increase shrinkage and cracking potential if used in grout, so it’s not used to achieve shrinkage reduction.