Properly pointed masonry will effectively drain moisture from the joint and thus minimize situations where water could collect and freeze. Since freezing and thawing should not be a serious problem, air-entraining additives should not be necessary. Entraining air in mortar will make it more plastic, but will also cause a reduction in its strength. Mortar properties Repointing mortar should be soft and flexible rather than hard and rigid. Lime mortars are ideal for this purpose because the lime produces limited but adequate strength such that the resulting mortar will be soft.
Lime mortars are also flexible.
Rigid mortars that are harder than the surrounding masonry can cause damage to the masonry if the materials expand. In a wall subjected to high temperature, the masonry units will expand, and if the mortar is rigid and hard, there is the possibility that the mortar will chip edges and corners of the masonry unit, particularly if the masonry unit is soft. In cold weather, components of a masonry wall will contract, and a rigid mortar will not move with the contraction. In this situation, the mortar will crack, generally at the interface of the masonry and mortar, and this crack will now be a pathway for water to get into the structure. The exact mixture proportions for mortar for repointing masonry will vary depending on the type of structure, the material properties of the masonry and the mortar, the importance of historical accuracy, and the availability of materials.