Spalling is a form of masonry deterioration in which the outer section of a stone, brick or mortar breaks away in a localized area. The term spalling sometimes can be a general one, refering to any type of coming away of the surface, such as exfoliation or delamination.
The cause can be either freezing and thawing cycles of moisture trapped within the masonry or salt crystal pressures. Another cause is mortar of excessive hardness. Any movement in the wall, from settlement, thermal expansion, or vibration, is deflected by the mortar, instead of being absorbed by it. This pressure then transmits back to the masonry, ultimately leading to spalling.