Spalled Brick

Spalled brick is typically the result of water entering porous brick on the exterior of the chimney and then the temperature goes below freezing and the trapped water in the brick freezes and the brick defaces. What you are seeing is the superficial damage, but keep in mind that it is likely that most of the damage could be underlying and not on the surface. If the damage is minimal you could just replace the damaged brick, but if the damage is substantial then you may either need to rebuild that portion of the chimney or parge it with a bonding cement.