The short-term effects of mortar joints on salt movement in stone

Salt-induced decay processes are one of the most important reasons for the decay of many stone buildings and monuments. This paper investigates the role played by masonry mortars in the movement of salts within Baumberger sandstone and Portland limestone. The methodology used involved the monitoring of stone units containing mortar joints which were exposed to a city-centre environment. Rain-water run-off was collected from the stone/mortar units during the exposure period; the units were subsequently destructively analysed. The methods of run-off and destructive sample analysis used included ion chromatography and atomic absorption spectrophotometry; stone and mortar samples were also analysed petrographically. Mortars were found to act as sinks of environmentally derived ions and as sources of mortar-derived ions in well washed surface environments. Mortar enrichment by sulphate (SO4–), sodium (Na2+) and chloride (Cl−) ions, which may lead to salt damage of stone around joints, was observed. High levels of calcium (Ca2+) ions, in the form of reprecipitated calcite, were recorded in the stone, leadingto a reduction in the surface permeability of the stone, a common cause of delamination.