Mortar between bricks can be replaced by using a bonding agent. Bonding agents can be used to fill in the missing mortar, and they come in both powder and liquid form. The easiest way to use a bonding agent is to mix it with water to create a slurry. The slurry can then be used to fill in the gaps between the bricks. Another option is to apply the bonding agent directly to the brick surfaces before inserting the new mortar.

Mortar between bricks is usually replaced by either repointing or rebuilding the entire section of wall. Repointing is the process of removing the old mortar and replacing it with new mortar. Rebuilding is the process of removing the old bricks and replacing them with new bricks. If you are only missing a small amount of mortar, you can use a brick chisel to remove the old mortar and then use a brick trowel to apply new mortar. Make sure that you brush off any excess mortar from the surface of the bricks before it dries.