Microstructures in historic Scottish lime mortars

Detrimental interactions between stone and cement mortars and failures of new lime mortars have focused attention on the microstructures that determine mass transfers that in turn influence interactions within masonry and between masonry and the environment, and thus control rates of decay. The analysis of historic lime mortars is necessary so that compatible replacements can be devised. Calcium carbonate microstructures in selected historic mortars from 12th-16th century structures in the east and west of Scotland are varied, but can be generalised into four sub-types within a wide continuum; anhedral angular fragments, oval plate-like fragments, dense crystalline masses and amorphous paste structures. The controls on the formation of these structures are not understood, though may relate to variations in carbonation mechanisms and interactions with climate and stone in masonry. There is a clear regional variation in quality, for the mortars sampled from the east and west coasts of Scotland in terms of apparent durability and microstructures, suggesting climatic and masonry lithology type interactions as a control on microstructure formation. Post-carbonation carbonate crystallisation is seen in west coast mortars, creating hard durable mortars. The influencing factors on the performance of stone masonry are: age, environment, stone-mortar interactions, binder-aggregate interactions and all working and production practices.