Calcium loading of quartz sandstones during construction: Implications for future decay

Damage to quartz sandstones in polluted urban environments is frequently associated with the presence of calcium salts, especially gypsum. For this association to occur it is necessary to identify external sources of calcium. This paper describes an experiment in which the demolition of two sandstone test walls (Dumfries and Dunhouse sandstones) built with a lime mortar demonstrates the loading of the outer 1 - 2 cm of blocks with calcium. Although this calcium may eventually be mobilized into the interior of blocks, a significant initial effect is a marked reduction in permeability (identified by before and after measurement using a gas permeameter). It is suggested that that this sealing effect helps to constrain moisture and possibly salt cycling through the outer, exposed surface of blocks. Eventually this may contribute to the rapid, catastrophic decay of individual blocks, and is a factor that must be built into any future modelling of this decay process.