Lime-based mortars are flexible and pervious, allowing for shifts in older foundations and structures and facilitating the movement of moisture in and out walls. This malleability is paramount to maintaining the integrity of older structures. When repairing masonry in homes built prior to 1910, cement products should be avoided. Making and applying traditional lime-sand mortars is an art form often learned through trial and error.
Step 1: A lot of expertise and research can go into identifying and matching the specific aggregate and lime-to-sand ratios of historic mortars. More often than not, standard mixtures consisted of three parts sand to one part lime. Make traditional mortar by filling three buckets with sand. Fill a fourth bucket with hydrated lime.
Step 2: Pour the three buckets of sand onto a large sheet of plywood or into a wheelbarrow or mortar pan. Hollow out the center of the sand, like a volcano, and pour the powered lime into the center of the sand pile. Do not inhale the lime dust.
Step 3: Lime mortar can be slaked (cooked) overnight or for up to two years to increase its strength. To slake the mortar mix, slowly pour a third of a bucket of water over the lime. Use a shovel and cover the lime-water mixture with the sand from the outer edges of the sand pile. Heat generated from the lime will dry the sand, forming cracks along the surface area.
Step 4: Once the slaked mixture has cooled, thoroughly mix the lime and sand together to form a putty-like mortar. Smooth the mixture by pounding it with a large wooden mallet or stick.
Step 5: If slaking is not desired, thoroughly mix the dry lime and sand together until a uniform color is achieved.
Step 6: Slowly add water to the dry mix, turning and stirring the mortar until a uniform, stiff putty texture is achieved. The mixing process should take a minimum of 15 minutes. Use as little water as possible, less than half a bucket, to ensure the strength of the mortar mix.
Step 7: Mortar mixes can be stored covered with a damp tarp or in a sealed bucket for several weeks.
Step 8: Once applied, facilitate the curing process by keeping the walls and mortar moist for several days or weeks. Do not allow uncured mortar to freeze.