It is a common practice to send samples from historic buildings to a lab for testing to choose the correct mortar for repairs, though not always necessary. If the mortar is harder than the brick or stone, damage will occur when the building expands and contracts. Mortar and brick can separate, having no bond at all anymore, and the faces of the brick or stone can flake off, this is called spalling. It is extremely important to use a soft mortar for re-pointing historic brick and stone.
Solid Restoration & Masonry, Inc.
Historic Masonry and Stone Repair