A lot of normal building brick is pretty porous, if not protected from getting really wet just before a freeze you get spalling, where the face of the brick fla

A lot of normal building brick is pretty porous, if not protected from getting really wet just before a freeze you get spalling, where the face of the brick flakes off. Brick that is too non-porous on the other hand, like sewer brick, forms relatively weak mortar bonds, which is why it is rarely used in buildings.