To fix a broken chimney flue, first clean the chimney flue to avoid being covered by soot and dust when moving the bricks. Examine the chimney to determine the extent of the damage. If there are serious flaws on the inside of the chimney, consider replacing the entire structure. If you can repair the damage, gather a trowel, chisel, mortar, hammer and caulking to perform the task.
Using a hammer and a chisel, knock down the damaged bricks, and place them to the side. Replace the damaged bricks with new ones that match the color of the original bricks. Use a hand trowel to mix masonry mortar in a 5-gallon bucket. Use the cement mortar to position the bricks tightly together, keeping in mind that gases should not be allowed to escape via any gaps.
After finishing the brickwork, cover the area with a layer of waterproof caulking. The caulking protects the chimney from water damage and prevents gases and heat from escaping via the gaps in the side. Allow the area to dry for six to eight hours, and check the new bricks to ensure they are set correctly.
Before completing the task, inspect the rest of the brickwork to be sure that there is no more damaged mortar in the surrounding area. Look out for old mortar that crumbles under the pressure of your fingers. Apply a layer of caulking to prevent any old mortar from chipping or crumbling.