Historic mortars: current issues and future work

Understanding mortars in historic buildings is a specialised area of study, that attracts the interest of a multidisciplinary range of stakeholders in engineering, construction, conservation, heritage science, materials science, earth science and archaeology. Materials of historic value can be defined as anything existing, from the very old to the very recent, including materials used for repairs, where they contribute to the historic, age, function or aesthetic value of the building under consideration. Nevertheless, the distinction is commonly made between contemporaneous, original, materials found in old structures, from new materials formulated and applied to old structures for their repair and conservation.

In application, the field can be divided into overlapping areas of enquiry; forensic materials characterisation and materials and structures engineering. Forensic analysis includes work to understand the composition and behaviours of historic materials, but also in the condition assessment of specific components and materials that comprise structures, entire structures and the behaviour of the composite that is masonry. Engineering involves understanding the chemical, physical and mechanical properties of newly formulated materials, and the control of these parameters using different binders, additives, aggregates, mixing and curing methods, and their behaviour in buildings.

There is a developed body of literature covering these forensic and engineering issues in regards to mortars in historic buildings, but we argue that there is a need for research into:
- meta-analysis of existing data of material characteristics to generalise properties and to constrain the development of repair materials.
- the overlap between forensics and engineering around the concept of compatibility, to ensure that materials do not cause damage to each other. There is a paucity of attention given to understanding compatibility, in practice.
- the assessment of material repairs guided by conservation philosophy. To what extent do science and engineering work in isolation in the area of heritage, and should they be constrained by value systems and considerations of authenticity and integrity?
- equilibria of materials repair in the context of shifting environmental equilibria due to climate change.
- the development of new laboratory testing methods for repair mortars with slow strength-developing binders (pozzolanas, limes).
- the design of modelling techniques, with which the development of damage
mechanisms in historic structures can be described and predicted.
- How scientific understanding of historic raw materials, traditional production methods and processing of lime and lime-based mortars can help to re-identify forgotten qualities, properties and latent values in relation to protection of cultural heritage and also for the production of new and more holistically-sustainable binders.