1. Hydraulic cement
- You can patch cracks and small holes in a concrete wall with hydraulic cement — a remarkable compound, sold at hardware stores, that hardens even under water. Because it expands when it dries, hydraulic cement bonds firmly with the surrounding concrete.
2. Enlarge the crack
- Wearing gloves and safety goggles, enlarge the crack (or hole) with a chisel and hammer, angling the chisel to make the opening larger inside than out so the cement will be locked in.
- Don’t cut deeper than two centimetres (3/4 inch) into a concrete-block wall or you may break through to the hollow core.
3. Clean the opening, the moisten
- Clean the opening with a stiff brush, then vacuum out any remaining debris.
- Moisten the opening slightly with a wet sponge.
- Mix a batch of hydraulic cement with cold water to the consistency of soft modelling clay. It starts to set very quickly so mix only as much as you can use in three minutes.
4. Fill a crack
- Fill the crack with the cement, using a trowel, to within one centimetre (1/2 inch) of the surface.
- Wait 10 minutes; then fill the crack with more cement, holding it in by pressing the trowel against it until it hardens. Trowel the surface smooth.
5. Fill a hole
- To fill a hole, form the prepared cement into a cone about 10 centimetres (four inches) long. The base of the cone should be about 2.5 centimetres (one inch) wider than the hole.
- Push the cone into the hole and hold it there until the cement hardens. Trowel the surface smooth.