Before specifying mortar for a project or using it in the field, it’s important to understand why mortar performance is vital to the successful performance of masonry and how the ingredients in mortar affect its performance. Mortar is the material that binds masonry units and binds joint reinforcement and connectors to the units. It’s also used as a spacer between units and as a means of leveling and plumbing the units. More important, mortar plays a crucial role in resisting water penetration. In a typical clay masonry wall (or in a water-repellent-treated concrete masonry wall), water does not significantly permeate the masonry units or the mortar. However, it may penetrate the interface between mortar and masonry unit if the bond between