What mortar proportions (cement:lime:sand) are appropriate for pointing historic masonry structures? I have seen high-lime mortars recommended in several references, but these are seldom used in practice. Why?
When repairing older masonry structures, use the softest mortar possible that will still be durable. Mortar that is too hard may cause the older masonry around the repaired area to spall. A mortar with a lower compressive strength will not concentrate forces at the face of the wall and thus can prevent damage. In freeze/thaw environments, the mortar also must be durable. Type O and K mortars have very low compressive strengths. However, they are not very durable. Where freeze-thaw durability is a concern, use Type N mortars with the maximum lime and sand permitted by ASTM C 270.