In 2000, a stone decay study assessed 61 mortared stone monuments considered representative of the most common and important stone types in Ireland. 12 of these monuments, including 5 medieval buildings and a 19th century fortification were repointed with lime-based repair mortars soon after the 2000 study. The study was repeated in 2010 and again in 2020 to understand how these monuments had changed over time, deterioration rates, how effective repair works (including repointing) were, and what happened when no conservation works were carried out. The findings of this research are being published in a series of scientific papers.
The re-assessment of stone monuments at intervals was found to be a powerful tool to understand the long-term nature of stone decay processes, and also provided insights into the weathering of historic lime-based mortars and of the performance of modern lime-based repair mortars over a twenty year period of performance. The study also highlighted the stark contrast between the diversity of historic building materials and the relatively small palette of modern materials; the key role of the masonry-mortar system in understanding the failure and success of lime-based mortar repairs; and the potential value of follow-up case studies to improve conservation practice and research.
From the perspective of a practitioner, there is a relatively small range of modern lime materials available in the EU to ‘match’ problems in historic buildings and monuments built with a diverse range of stone types and mortars. Due to regional diversity, there are challenges in adopting the solutions and approaches taken by other researchers as environmental conditions and weathering stresses are not uniform across Europe. A medieval church in a hilltop town in Italy, in the heart of Athens, on an island in Sweden or on the Atlantic coast of Ireland will experience different environmental conditions, and lime-based repairs to those monuments will experience different intensities of natural wet/dry, freeze/thaw cycling and salt action. In addition, arguably most historic building materials were sourced from the local area or region, and reflect the diversity of the local geology and building traditions. In Ireland, while there are a number of relatively ‘pure’ limestones; a great many Irish limestones contain impurities, and when burned produce naturally hydraulic lime. In addition, historic lime in Ireland was also made by burning magnesian limestones, marbles and marine shell in coastal areas; with brick pozzolan and ash commonly added to mortars from 1700 onwards. The often complex mix of local stones and locally produced historic lime mortars found in historic buildings and monuments is an important component of the regional diversity of European cultural heritage; but also poses significant challenges for scientific researchers asked to provide universal solutions for lime-based repair mortars which can be used for in-situ application by practitioners focused on site- and case-specific issues. There is an unfortunate potential for significant divergence between the work or researchers and practitioners; but there is also potential for better synergies if suitable common ground and areas of dialogue can be identified. In particular, it would be useful to identify what practitioners are most concerned about, to consider is research addressing these areas, or if existing research outcomes can already be used to address current practical concerns.
A general finding of the assessment of 61 mortared stone monuments over a twenty year period was that modern lime-based repairs show no evidence or indications of long-term incompatibility with historic porous stones; unlike the incompatibility between Portland Cement repair mortars and porous building stones widely reported in the professional literature. In the short-term, lime-based repair mortars do not appear to commonly fail because of an inherent incompatibility or failure of the material itself. Instead, short-term failures of modern lime-based repair mortars were found to be uncommon. Where failure occurred, it could often be attributed to external factors such as workmanship, the failure of other elements of the building fabric, or inattention to the dynamics between a mortar-masonry system and the weathering stresses experienced at the building.
Recent Publication: Bolton, J. (2023) “Long-term performance of lime-based repairs in Ireland: Seapoint Martello Tower and the Romanesque doorway of Clonfert Cathedral, 2000-2020”, in Valek, J, et al [eds]
RILEM TC 277-LHS Workshop Lime-based repairs for repairing historic structures, 3-4 Feb 2022, Aristotle University, Thessaloniki, Greece , Pp.157-164
