What are the disadvantages in using a high strength mortar to repair historic masonry?
Portland Cement, the key ingredient in newer high strength mortars came onto the market in 1871. The widespread use of portland cement in masonry construction did not propagate until many years after the introduction due to limitations of cost and availability. In the absence of portland cement lime was used for mortar in masonry construction which is a much softer and more flexible material. Repairing lime mortar joints with portland cement based mortar only creates a temporary repair and eventually leads to further deterioration of the original lime mortar. The bond between the two mortars is very weak and when it fails the weaker material (the lime mortar) is the substance that breaks. When portland cement based mortar is used to repair joints in a soft block construction such as terra cotta, there have been instances where the original block itself has failed. The best bond can be achieved when the compressive strength of the repair mortar is matched with the original mortar. Many
