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

Abstract

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.

Introduction

The sensitive repair of historic buildings invariably requires an understanding of indigenous materials that compose their fabric in order to ensure greater levels of compatibility and authenticity upon intervention (Clifton-Taylor, 1987, Gibbons, 2003, Hughes and Válek, 2003). Lime binder technologies play a key role in many building conservation projects given their international prominence in the communication of traditional architecture. Such binders are commonly utilised in a wide range of construction technologies within historic buildings, and include concretes, mortar, plasters, renders, lime washes, and grouts (Forster and Carter, 2011, Bras and Faria, 2017). The scale of their traditional use is today reflected in the cost of repair and maintenance of historic and traditional buildings which is considerable. Indeed, in Scotland alone, ECORYS (2012) estimated that 0.5 million pre 1919 traditional buildings exist ostensibly constructed in lime based materials (5.5 million UK wide), and the estimated spend on repair and maintenance is in the order of £4 Billion in Scotland. Regrettably, as with other binders, lime based materials have been traditionally associated with high environmental impact given the nature of their production. Yet, within today’s decarbonizing world it is essential that the sustainable production, manufacture and supply of repair materials are capable of satisfying the societal, economic and environmental demands placed upon them (Brundtland, 1987). Societal demands relate to the retention and support of cultural heritage through compatibility of repair materials, with aspirations to replace on a like for like basis (BS 7913, 2013, Bell, 1997, Jokilehto, 1998). Economic demands are contextualised around costs associated with procurement, manufacture, initial and life cycle construction efficiencies, and ultimately deconstruction (Eagan, 1998, Latham, 1994). Environmental demands relate to the aim of producing low carbon materials solutions that attain compatible and high durability fabric repairs (RIBA, 2011, Forster et al., 2015, Forster et al., 2011, Kayan et al., 2016).

Yet the nature of such demands can differ greatly, and reconciling them is extremely challenging, with conservation aspirations to retain and support like for like authentic historical materials difficult to achieve within the environmental, and also the economic, constraints associated with current binder technologies. Although modern methods of lime binder ‘production’ may be more environmentally and economically efficient in terms of their reduced CO₂ production during calcination, the nature of the lime they produce may not adequately reflect those historically encountered, with current limes lacking critical characteristics such as regional mineralogical specificity. Consequently, the sector is seemingly caught between an aspiration to promote ‘traditional’ lime binders and their associated technologies on authenticity grounds, whilst struggling to adjust to global austerity economics (Meegan et al., 2014) and increasing environmental regulation and decarbonization (EU ETS, 2018, EU Paris Agreement, 2018, Stork et al., 2014).

The importance of this cannot be understated, given that the cement industry contributes 5% of global anthropogenic CO₂ (Ishak and Hashim, 2015). Contextualising emissions for the broader construction sector, current figures for the degree of carbon expended in the UK can be divided into significant stages. Prominently, in contemporary practice the ‘before use stages’ of construction highlights that; Design expends 1.3 CO₂ (Mt); Manufacture expends 45 CO₂ (Mt); Distribution expends 2.8 CO₂ (Mt); and Operation on site expends 2.6 CO₂ (Mt) (Lawrence, 2015). Attempting to calculate lime’s carbon impact, broad frameworks such as the Publically Available Specification (PAS) 2050 (PAS, 2011) attempt to specifically evaluate and establish the main parameters and boundary conditions. Whilst the ‘UK Building Black book: capital cost and embodied CO₂ guide’ (Hutchins, 2010) gives more specific CO₂ data on materials, components, and associated construction elements. Furthermore, Hammond and Jones (2008) provide a materials focused inventory of carbon and energy. However, although these sources are recognised as being helpful for objective measurement, they are currently insufficiently nuanced, and fail to include specialised, specific data on lime binders.

Such specific data is key, as lime production in Europe satisfies many applications, with approximately 20% of all lime produced being utilised in construction and civil engineering applications (Stork et al., 2014) and, logically, it proportionately negatively contributes to emissions. Although modern lime production both appears to, and does, offer advantages, here we argue that in fact, additional CO₂ savings can be attained by evaluating historic manufacture and construction practice. This review therefore attempts to determine what can be learnt from understanding the production and use of historic lime binders. The review highlights possibilities that may reconcile the conflicting societal, economic, and environmental demands whilst simultaneously drawing on methods of production and construction satisfying the authenticity required in historic repair. To achieve this, the review evaluates the carbon associated with lime binders on a life cycle basis in line with the features highlighted in the accepted stages of the Sustainable Building Alliance (SBA, 2009) model. Importantly, in order to view lime binders and their potential for CO₂ reduction holistically, the review considers these stages in past (historic), contemporary (current), and potential (future) contexts. For each of these contexts, the review utilises these accepted stages to create a narrative broadly reflecting, the ‘before use’ (Provenance, production and construction stages), ‘use’ (Building operation performance, maintenance and refurbishment) and ‘end of use’ stages (Deconstruction, disposal and recycling) of the materials.