Alteration textures in historic Scottish lime-mortars and the implications for practical mortar analysis

Evidence of dissolution and later precipitation of carbonate binder materials can be found in medieval lime-mortars from the west of Scotland. Examination of impregnated, polished thin sections by polarising microscopy and SEM reveal isopachus concentrically accreted calcite pore linings that delineate extensive areas of secondary porosity created by earlier dissolution of binder. Some pores show no precipitation and are potential sites of recent dissolution. Multiple isopachus pore linings exhibit several stages of growth. Innermost layers consist of acicular calcite growth with crystal axes perpendicular to the pore wall. Other areas show evidence for enhanced porosity, in the 10-20um range, that have sharp transitions with neighbouring zones of dense binder structure. However, the samples examined so far are from unprotected fallen masonry, where rain and ground water penetration is likely to be greater than in well-constrained positions within walls. It is uncertain at present whether the observed structures formed within a wall or after the masonry had fallen. The position of such structures within a wall will be important for identifying the source and pathways of calcite during dissolution and precipitation.

Significant secondary binder porosity within mortars could compromise analysis aimed at characterisation for matching correctly for restoration. Using uncorrected measurements of binder/aggregate proportions in such ancient mortars as a guide to specifying repair mortar would result in unrealistically high aggregate contents. Porosity, permeability and strength would be very difficult to match in these old and environmentally aged mortars. Despite the dissolution of material from the mortars they have high apparent durabilities, perhaps resulting from the coarsely crystalline re-precipitated calcite. Hence solution-reprecipitation processes are important factors in the maturation and enhanced durability of mortars.