Defoamers (anti-foam additives) can control or prevent frothing, foaming and air entrainment in cement slurries that may occur during the mixing process. Excessive foaming can cause an underestimation of slurry density downhole.  This is due to the entrapped air being compressed in downhole pressure conditions and increasing the density of the slurry over the lb/gal weight measurement gathered at the surface.  Additionally, excessive foam can lead to cavitation during mixing, which in turn may lead to equipment damage and/or a loss in hydrostatic pressure.