The primary cause of brick spalling is the use of mortar with excessive compressive strength. Mortar is typically made up of three dry components: a binder, an aggregate and lime. This is usually Portland cement, sand and hydrated lime. It’s the ratio that determines the strength, or ASTM classification, of the mortar.
Brick is a reservoir material, meaning that it can absorb and store significant amounts of water. If, while brick is full of water the outside temperature drops and the stored water turns to ice, brick will expand.
Under these conditions, softer, lime-based mortar will compress, accommodating the expansion without visible damage. Harder, portland cement-based mortar will not compress enough to accommodate this compression and the developing stress will cause the brick face to spall.
If the mortar joint is flush with the brick face, spalling brick will lose only its corners. If the joints are raked, so that the brick face extends out past the mortar termination, the brick face may shear off along a plane flush with the face of the mortar, affecting all brick walls built with the same mortar that havre high moisture content.
The general term for this condition is “constrained expansion”, and it may also be caused by concentrated (point) loads.
A good mason should be able to mix a re-pointing mortar that will not jeopardize masonry. If the color or texture are more challenging, there are firms available online that will custom match mortar samples for under $200.