A common repair done with a concrete block foundation is to replace loose or missing mortar with new material. The process requires safety equipment, hand tools, mortar mix and a sealant. Wear safety goggles for eye protection.
- Remove damaged mortar
Use a cold slim chisel and a hammer to remove the loose or damaged mortar from between the blocks. Chip away at the damaged areas until clearing a 1/2-inch deep space between the blocks in the damaged area. Brush the cleared mortar lines with a wire brush to remove any remaining loose debris.
- Mix the repair mortar
Mix a batch of repair mortar in a large bucket by combining one part masonry cement with three parts fine sand. Add water to the dry mix, while stirring it with a trowel, until the mixture is a thick paste about the consistency of oatmeal. Make the mixture slightly dry, so that it is easy to work with and sticks to the blocks well.
- Prepare the block joints
Wet down the joints between the bricks where the old mortar was removed using water and a brush to force the water onto all joint surfaces. This removes any remaining dust and keeps the existing mortar from drawing out the liquid contained in the repair mortar.
- Fill the joints
Use a pointing trowel to force the repair mortar into the joints between the blocks. Fill the joints completely, and then scrape a brick jointer along the top of the new mortar in the joints to remove enough excess mortar to make the new mortar level equal to the older, undamaged mortar surrounding the blocks. Begin with the vertical joints, filling them before moving on to the horizontal joints.
- Seal the mortar
Wait two weeks after applying the repair mortar, then spray a coat of high-quality masonry or acrylic sealant over the joints and the blocks. The sealant protects the new mortar from water damage.