According to this company they call it tuckpointing. Water damage and age can cause the mortar that holds your chimney together to crack and crumble. Without supporting mortar, bricks begin to sag and loosen. Unless old mortar is replaced, you could be in for falling bricks and pricey repairs down the road. Tuckpointing your chimney, is a process in which they replace deteriorated mortar and with fresh, new mortar.

You have to see this web site. Extremely well done.