Designing and evaluating repointing lime mortar for the conservation of historic buildings in highly exposed environments

Many historic buildings are under threat due to their severe exposure to wind, rain and high humidity. One of the main conservation challenges for historic and traditional buildings is to ensure water ingress is mitigated and moisture that has entered a wall evaporates, contributing to drier and less damp conditions. These are some of the main roles of mortar joints in a masonry: absorbing moisture from the surrounding stones and contributing to its evaporation. When mortar deteriorates or when inappropriate mortar, that traps moisture inside the wall and encourages evaporation through the porous stone, has been used, it has to be replaced by a new mortar called repointing mortar. This thesis focuses on repointing in a common UK situation of high exposure to wind and rain and masonry with low permeability, as found in the south-west of England. It aims to investigate the role of repointing mortars in the conservation of historic buildings in exposed environments: What would be the most effective mortar to repoint dense stone masonry in exposed locations? How do the environmental conditions found on-site affect these mortars? Can these repointing mortars mitigate driving-rain ingress? This thesis addresses these research questions with a multi-method and multi-scale methodological approach, from laboratory samples to test walls, to optimise, evaluate and validate twenty-three compositions of lime mortars. This work is therefore relevant to other high humidity environments and to a multitude of structures. Key scientific findings are made on the effect of specific materials in lime mortars, such as the use of wood ash, porous aggregates and quicklime, qualifying them as suitable materials to optimise repointing mortar. Wood ash contributes to increase the capillary absorption capacity of mortar while giving a potential pozzolanic activity, a positive outcome when applied under humid conditions. The thesis brings further understanding of the impact of humid environmental conditions on the properties of lime mortars, and showed that laboratory evaluation should be made on samples cured under realistic conditions if information on the early to medium-term (up to 90 days) characteristics of NHL mortar is required. Findings from experiments on test walls demonstrate that repointing mortar, by being more compacted, helps mitigate rain ingress after a short intense rainfall event. The novel pilot scoring system developed could have considerable implications for more effective and durable repointing interventions. Together, these findings have significant implications for decision-making on repair interventions for the conservation of historic buildings, especially in building resilience in a context of climate change.