Cinder blocks, commonly used to build foundation walls, are usually hollow inside. When a cinder block within a wall or other structure breaks, you must replace the block with a new one to ensure the structural integrity of your wall. Although this may seem like a daunting task that would require you to dismantle the wall, the process is relatively straightforward and easy to accomplish. You will not have to dismantle the wall.
Position the tip of a cold chisel between the mortar joint of the cracked block and the adjoining blocks, and tap the back of the chisel with a hammer to crack the mortar around the block. Work your way around the cracked block until you break the mortar joint completely around the block. This will allow you to remove the broken cinder block without damaging the adjoining blocks.
Break out the remaining pieces of the cracked cinder block with the cold chisel and hammer. Remove any leftover mortar from the hole with the chisel and hammer.
Sweep out all of the debris from the hole with a small nylon brush.
Place a layer of mortar along the bottom edge of the opening with a mortar trowel. Keep the layer of mortar at the same thickness as the mortar used on the rest of the wall.
Slide the block into the wall and place it so the face of the block rests flush with the face of the wall and the block is centered between the two adjoining blocks on each side.
Place mortar onto the trowel and slide the mortar into the gaps around the replacement cinder block. Wipe the excess mortar off the face of the wall with a moist rag.