Binder microstructure in lime mortars: implications for the interpretation of analysis results

Many historic mortars are composed of a silicate aggregate and a lime-based (calcitic) binder. Thin section examination suggests that binder structure and composition are complex and that binder quantity can vary significantly during the lifetime of a mortar. Lime mortar analysis for conservation purposes commonly involves the quantification of binder: aggregate ratio by weight. Many such analyses of historic mortars have generated results suggesting compositions of 1 part lime binder: 1 part aggregate, in contrast to the 1:3 ratio commonly specified in current building work. The high binder content can, in part, be attributed to the presence of unmixed binder. Fragments of lime that have not been fully mixed with the aggregate do not, therefore, play the part of a binder during set. There is also evidence for dissolution and re-precipitation of binder in mortars, leading to changes to binder morphology as well as net gains or losses in binder content. The presence of both lime inclusions and dissolution/re-precipitation textures within a mortar has implications for any mortar analysis method which relates the quantity of calcium carbonate to original binder content. Examination of a mortar, preferably using a petrological microscope, is essential if any meaningful quantitative data are to be generated.