The interior face of masonry walls in a clock tower near Chicago are spalling badly. When the spall was removed, the back face was covered with a white powdery substance. Has this substance caused the spalling or has it entered the crack after the spall occurred? What can be done to stop this from occurring?
The white powdery substance is efflorescence. Soluble compounds leached from either the brick or the mortar are deposited on or near the back face of the masonry. Although such spalling typically is attributed to "salt crystallization," the distress mechanisms may involve recrystallization, osmotic pressures, and chemical attack. Such efflorescence and spalling are a particular problem in older structures with common brick on the interior face. The problem seems worse in unheated structures, such as bell towers and church steeples. The longer it takes the masonry walls to dry out, the worse the problem becomes. The water most likely is coming from the exterior walls. Examine the exterior masonry. If the mortar joints have many voids and separations or are badly deteriorated, the walls should be repointed or surface grouted. Either will significantly reduce the amount of water entering the walls. I also recommend heating the clock tower interior during the winter to help the walls dry out faster.