How to Seal Holes in Cinder Block Foundations in Basements

Remove any loose material surrounding the hole in the cinder block with a chisel and hammer. Cut away the loose material at its base where it meets the block.

Enlarge the hole slightly with the chisel and hammer. Try to even out the edges as much as possible, forming a dovetail cut on the cinder block where the bottom of the chipped edge flares out slightly from the top, creating an undercut that the patched material can hold onto.

Clean out the chiseled block with a nylon brush, sweeping out all of the concrete dust and debris you created.

Mix the mortar for patching the cement in a large bucket with an electric drill and a paddle bit. Pour one part Portland cement with two parts fine masonry sand into the bucket, and then add water to the mix to create a stiff mortar the holds together when formed by hand.

Pack the cinder block full with the mortar mix, using a trowel to help you get the mortar into all corners of the hole, making sure not to leave any voids in the block. Continue to place the mortar into the block until it slightly overfills with the patch mix. Smooth the surface with the flat of the trowel and then allow the patch to dry for about seven days.