What to Spray on Cement Block Walls to Repair Them

Repair Sealant

• This sealant is largely made from a sodium silicate compound and water. It reacts on contact with the pores of the cement. Upon contact, the material expands to fill all cracks and crevices of the concrete, no matter how minute. The result is a much stronger concrete that is not totally impervious to water or radon leakage.

Application

• The silicate sealant is sprayed on a concrete foundation wall with a low-pressure sprayer no more powerful than 40 pound of pressure per square inch. The normal 5-gallon container will cover about 500 square feet of very porous cement. Poured cement, which is normally less porous than blocks, requires about half the amount to fill the same area. Usually, three or four applications are necessary for concrete block, while two or three are necessary for poured concrete. This is not mean to fill large cracks -- cracks of 2 inches or more -- but only seals and strengthens the wall. The concrete upon which the sealant is sprayed is about 20 percent stronger, depending on the starting density.

Polyurethane Fill

• Polyurethane fill is injected into a crack. This substance can be used first to fill large cracks, and then the silicate sealant can be added on top of that for added strength and reinforcement. This material is a polyurethane foam that is injected into a crack. Upon contact, it will foam, which fills every nook and cranny of the damaged cement. A paste must be applied to the crack first before the fill is used. Once the foam hardens, it becomes as strong as the concrete it is replacing and bonds to it instantly.

Applications and Uses

• Both sealants are for repairs, and both are sprayed on. The polyurethane spray is highly localized and is pushed into the hole. The silicate spray is meant for the broad coat of the porous concrete. Once the silicate is sprayed, the concrete is no longer porous. All the pores of the concrete — including any internal air pockets up to 4 inches deep in the wall — are now totally filled with the substance. Filling cracks is the first priority, then the whole wall. The addition of the silicate spray will fill in any gaps that the polyurethane injection might have missed. Using both substances can solve all crack problems in a concrete wall and reinforce the entire structure. In both cases, the addition of the material is permanent, and bonds totally to the cement, becoming, as it were, cement itself. Using both of these together creates a wall stronger than when it was new.