Can cracks in a wood-burning fireplace be repaired with a concrete product?
Refractory cement is usually used to repair the the joints of the fire brick. If the old cement is cracking it may be a good idea to remove all the loose material and repoint or fill the damaged areas. There is also fire clay but this not normally used above the fire box area that takes the most abuse from the fire (I have seen fire clay used many times but believe refractory cement is better).