Lime binders for the repair of historic buildings: considerations for CO₂ abatement

Lime binders are utilised worldwide and are associated with a considerable scale of production and corresponding CO₂ emissions. The relevance of this review is therefore international in scope, with production transcending geographical boundaries and construction practices. An holistic view of lime binders, considering their provenance, production and utilisation offers the potential for significant CO₂ savings urgently required globally. Importantly, the technical aspects of lime materials production is critical, but the alteration of behaviour of both specifier and those undertaking the construction processes is also essential in achieving meaningful CO₂ emission reduction. This review paper investigates the life cycle stages of lime binders in line with the features highlighted in the Scottish Building Alliance (SBA) ‘building life cycle stages model’. It attempts to determine what can be learnt from our understanding of the manufacture and use of binders from historic, current and future perspectives in the context of reconciling the production of historically authentic materials in a decarbonising environment. The production and use of such authentic historic lime binders initially appears highly carbon intensive through its utilisation of relatively inefficient kiln technologies and loss of economies of scale associated with larger operations. However, this review shows numerous benefits in the production of such binders; including their CO₂ sequestration capability, lower potential processing energy, and a reduction in excessive ‘carbon miles’ associated with transportation. Importantly, we show how historic production and on-site manufacture approaches with lime-based materials also offers exciting potential for carbon savings. Importantly, hot mixed lime mortars offer the promise of higher durability materials compared to their modern cold manufactured counterparts, thereby reducing the frequency of repeat maintenance interventions, again yielding CO₂ savings. We suggest hot mixing, whether in an innovative contemporary materials realm or within traditional contexts should be explored. In addition, hot mixed materials can concomitantly satisfy conservation requirements for repairs through better reflecting the aspired to building conservation requirement of like for like materials replacement reflected with historically produced binders. We suggest future paths for the industry that would simultaneously reconcile demands for authentic materials and production methods for the conservation sector with the necessity of achieving cleaner production in a decarbonizing world.