Concrete and masonry building materials can absorb water through capillary action, drawing salt with it to the surface where the water evaporates, leaving efflorescence behind. As more water migrates and more salt accumulates at the surface through osmosis, incredibly strong hydrostatic pressure builds exceeding the psi structural strength of the building material, which can blow the surface off the material. This cracking and flaking of concrete, stone, brick, and block is known as spalling.