Masonry walls (whether brick, blockwork, or stone) constructed using cement mortar must allow for horizontal expansion and contraction of the wall—hence the incorporation of vertical expansion joints, which are generally seen as lines running up the wall surface. Lime mortar is more flexible than the cement variety and consequently it permits a greater degree of movement within a masonry structure; except in earthquake zones—most “old” (i.e. pre-twentieth-century) walls do not have expansion joints. The structure of lime-mortar walls is more “forgiving” than that of their modern counterparts.
Expansion and contraction in masonry walls occurs owing to: