Specialist ‘restoration mortars’ for stone elements: a comparison of the physical properties of two stone repair materials

Background The perceived increasing use of ‘restoration mortars’ for the conservation of historic masonry is often viewed as controversial due to the long-standing contention associated with ‘plastic’ repairs in the heritage sector. Almost innumerable traditionally built sandstone structures exist worldwide and the wide scale use of restoration mortars for their repair is evident on a national and international level. Use of largely incompatible, impermeable and/or highly cementitious materials in the past is the cause of many problems, particularly associated with moisture entrapment. These issues, that continue several decades after a material’s use, raise the question of whether similar problems are likely to arise in the future due to the use of restoration mortars. Results The chemical and physical characteristics of two restoration mortars are determined by laboratory testing. This work revealed significant differences in the physical properties of two restoration mortar materials, but also between these and the physical properties of two natural sandstones. Material characterisation reveals differences between the composition of the binders and aggregates in the two restoration mortars tested. Significant differences in their strength and water diffusion properties were noted. Conclusions The results provide an indication of material performance and a platform for objective decision-making on the suitability and specification of these materials. This work is of particular importance in the heritage sector, but its applicability is not limited to historic buildings. Comparison of the materials properties presented in this paper with the characteristics of representative sandstone substrates can give an indication of material compatibility. This research is aimed at stimulating further testing into the interactions of these materials within the context of historic masonry substrates.