Concrete forms like a sponge and will have capillaries and voids throughout the slab. Crazing (a network of fine cracks or fissures on the surface or mortar caused by shrinkage of the surface layer) is also common and while they have no effect on the structural soundness of the concrete, they can affect clarity and reflectivity in polished concrete. Grout coating concrete during the polishing process can help reduce the effects of some of these surface imperfections. There are two methods of grout coating: clear resinous (epoxy) grouting and scavenging grouts. Grout coating can add as much as $1-$3 per square foot depending on the method depending upon the severity of imperfections and methods used.
Clear Resinous (epoxy) Grouting – is typically done by applying a clear, low viscosity, high solids resin (often 100% solids epoxy or polyaspartic) to the slab after the first heavy cut with metal bonded diamonds. The material is allowed to cure and then the surface layer is removed with the subsequent grinding steps leaving the resinous material to fill the surface imperfections.
Scavenging Grouts – use silicates or polymers during grinding to scavenge ground material and fill the surface imperfections.