Mortars testing
Mortars testing © Historic England
Mortars testing © Historic England

Materials and Techniques

Understanding the performance of materials, techniques and treatments for conserving heritage assets:

The aim of this research topic is to gain a better understanding of the factors that affect the long-term performance and behaviour of building materials. A further aspect of this area of research is assessing the suitability and efficacy of conservation materials, techniques and treatments, both conventional and innovative.


‘Nanolime’ is a recently developed treatment for consolidating friable surfaces of stonework. However, comparatively little is known about its effectiveness or long-term performance, and there is a lack of consistent guidance on its use. To address these issues, ‘nanolime’ is being evaluated in a programme of laboratory-based investigations and site trials. The aim of the project is to understand better the factors affecting its performance, its limitations, and the risks of any potentially harmful effects.

Participants: University of Bath; Odgers Conservation.

Read our guidance on Nanolime.

Hot-mixed mortars

Hot-mixed mortars are made by slaking quicklime and aggregates together. This method of making mortars and renders was widely used in England for many centuries. Interest in hot-mixed mortars has grown in recent years, and some practitioners have gained experience in their use.

There are countless examples of historic hot-mixed mortars which have lasted hundreds of years, and compelling anecdotal evidence that modern hot-mixed mortars perform extremely well. However, there is little scientific evidence to explain why this might be the case. Also, there are few test results that would give clients, specifiers and contractors the confidence to use them widely.

This research project aims to better understand the properties and performance of hot-mixed mortars, and to develop guidelines for best practice. This will be done by a combination of laboratory testing and monitored site trials. The current phase of work focusses on the role of steam in slaking during the hot-mixing process, and how this affects the properties of the resultant mortar.

Participants: University of Oxford; Colin Burns; Bill Revie (CMC Ltd)

Read more about hot-mixed mortars: Hot-mixed mortars: the new lime revival - article in volume 154 of Context, the Journal of the Institute of Historic Building Conservation, published by Cathedral Communications Ltd in May 2018.

Read more about the role of steam in lime slaking: Pesce, C., Godina, M.C., Henry, A. et al. Towards a better understanding of hot‑mixed mortars for the conservation of historic buildings: the role of water temperature and steam during lime slaking – paper in Heritage Science, volume 9, published June 2021.

Natural Hydraulic lime

Natural Hydraulic Lime (NHL) marketed in the UK complies with BS EN 459-1: 2010. Under this standard, NHL products are assigned to one of three classes (NHL 2; NHL 3.5; NHL 5) according to compressive strength after 28 days, as determined by standardised tests. However, the range of permissible properties for each class is wide and there is considerable overlap between them. This means that different products within the same class can vary widely. In addition, standardised tests (developed for cement-based products) give no indication of the properties a particular NHL product will attain in the long term. Therefore, the classification alone gives a practitioner little useful information when specifying a mortar for a particular application.

The aim of this project is to understand better the factors affecting properties and performance of mortars made with commercially available NHL products. A further objective is to develop a method for predicting the performance of NHL binders, based on their chemical and physical properties. This would allow new NHL products that come onto the market to be evaluated quickly.

Participants: Buildings Lime Forum; Cristiano Figueiredo, University of Bath.

Read about the results of this research:
Hydraulic lime production – coming full circle? - article in the Building Conservation Directory, published by Cathedral Communications in January 2018.

Earthen building materials

Buildings constructed with earthen materials are to be found in many parts of Britain. Examples in England include the cob buildings of southern and southwest England, the mud buildings of the Midlands, the clay dabbins of Cumbria, and the clay lump buildings of East Anglia. In addition, masonry buildings were frequently built using earthen mortar for bedding the stones or bricks, and earth-based plasters were also widely used.

Earthen building materials are often concealed behind a render, plaster skim or paint finish and so frequently go unrecognised. Often they only come to light during repair work, or when lack of maintenance has caused them to deteriorate to the point of collapse. Buildings incorporating earthen materials are often repaired in an inappropriate way because of a lack of understanding of their characteristics and performance. In some cases this has led to further deterioration and substantial harm to the heritage significance of the building.

This research topic comprises a number of individual research projects including developing appropriate repair materials for earthen mortars, understanding the factors affecting the stability of mass earth walls, and assessing the impact of remedial works and alterations on the stability of cob buildings.

Participants: University of Bath; Ridout Associates,Historic Environment Scotland.

View our 2020 webinar on Earthen mortars and plasters.
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Concrete repair

The aim of this project is to develop more sympathetic and visually acceptable methods for surface repairs to historic concrete structures. Tests and site trials are being carried out to evaluate the performance of surface repairs using materials that replicate original mixes, and stone masonry techniques. This is contrary to conventional practice - using polymer modified materials and hydrophobic coatings - which tend to be visually obtrusive. The performance of trial repairs carried out to the Tecton structures at Dudley Zoo have been monitored over a period of ten years. To date, no evidence of cracking, failure of repairs to adhere to host material or increased reinforcement corrosion adjacent to the repair has been observed.

Participants: Dudley Zoo; 20th Century Society; Rowan Technologies. Further phases of research are under discussion with Laboratoire de Recherche des Monuments Historiques and the Getty Conservation Institute.

The monitoring of patch repairs and other remedial treatments is continuing.

Paint durability

Weathering tests are being carried out to compare and evaluate the performance of commercially available exterior paints for woodwork, as an alternative to lead-based paints. The tests have been in progress initially at the Paint Research Association’s lab at Hampton, Middlesex and now at Melton Mowbray, Leicestershire. Thirty-four different paints have been applied to softwood panels fixed on to racks at 45° facing south; each year of exposure is equivalent to two years on a vertical surface. Weather conditions are monitored and observations made on loss of colour, surface finish, cracking and mould growth. Testing is continuing in order to provide data on the best performing paints after the equivalent of 10 years exposure. 

Weathering tests have been carried out since 2011. Exposure trials will continue and we expect the final report will be published in 2023.

Participants: Traditional Paint Forum, Historic Environment Scotland, National Trust, Paint Research Association.

Contact for all above projects: [email protected]