Joints in stone, brick, ceramic, concrete and other masonry are designed, not only to structurally bond units, but also as a sacrificial material to protect the masonry itself. An effective mortar wicks moisture and salts at a greater rate than the parent material and is softer so that freezing moisture and expanding salts will damage the mortar first. As mortars deteriorate, open joints allow water to penetrate between the units and promote biological growth, stone deterioration, and additional damage during freeze thaw cycles. Mortars with inappropriate water and water vapor transmission rates and excessive compressive strengths can contribute to significant long-term damage of the object or structure.
The first step toward repointing masonry is to assess and identify the original pointing, which might be lime based or cementitious mortar, lead, or a contemporary setting compound. Analysis of extracted mortar samples can determine historic mortar binder types, aggregate quantity and quality (sizes, shapes and colors), water absorption rates, and compressive strengths. Replacement mortars are designed to replicate these properties.
Successful pointing implementation requires careful attention to detail by skilled personnel during each site phase: protection of historic masonry by preventing cuts, scratches, smears and staining at all times; safe mortar removal to appropriate depths with squared ends to the back of the joints; cleaning loose mortar and dusts in joints and on masonry faces with compressed air, vacuum and water; appropriate mortar selection and mixing procedures; appropriate pre-moistening and mortar application by direction, pressure, lifts and profile; appropriate protection of newly installed mortar for proper cure of each mortar type; sensitive site cleanup; personnel and public safety.