Role of mortars in the decay of granite

Conservation work carried out at Trinity College, Dublin, highlighted problems of decay in granite which is widely regarded as a durable stone. Visual condition assessment of stone work identified an association between decay of granite and the presence of salts. Salt build-up was related to the presence of sources of soluble salts. Mortar joints and limestone features in particular were identified as potential sources of damaging salts. Petrographic analysis of thin sections taken from high and low salt surface environments revealed decay processes typical of granite weathering. Fracturing and chemical alteration of minerals were evident. Decay processes in the sample from the high salt location were more extreme. Analysis of calcium and sulphate in these locations revealed an association between disruption of the stone and high levels of salts. The physical role of mortar in controlling the water movement through the stone is also important in determining where damaging salts are deposited. A new repointing mortar was developed and tested. This mortar has high permeability and flexibility while having a low soluble salt content.