How to Repair Brick Mortar Inside Fireplaces

Remove the mortar in and around the damaged area with a carbide-tipped grout saw or electric grinder. Clear the horizontal joints first and dig into the mortar at least 3/4 inch deep.

Clear the debris out of the joints by sweeping them with a stiff wire brush and vacuuming.

Soak the bricks where you're patching the mortar with water and allow the water to dry overnight. The bricks will absorb the water you're applying and will prevent them from drawing water out of the mortar, which can weaken it.

Prepare a batch of mortar, following the manufacturer's instructions.

Apply water to the bricks again, using just enough to get the surface damp.

Fill the horizontal joints with mortar, pushing the mortar completely into the joint with a trowel. Apply the mortar to the horizontal joints after the vertical joints have been filled.

Run the trowel or a joint tool over the mortar to smooth the surface and wipe off any mortar that got onto the bricks with the trowel.

Wipe a brush across the mortar in a diagonal direction after it has begun to firm. This is done to remove any dry bits of mortar from the joints.

Spray the bricks and mortar with water for the next three days to keep them damp while the mortar sets.