The impact of lime mortar mix design on its environmental resilience and durability for use in conservation

: (Alternative Format Thesis)

Abstract

This study contributes to the knowledge of mortar design and specification for conserving significant heritage structures. Lime mortars in heritage assets are exposed to environmental stressors including freeze-thaw events, salt deposition, embodied steelwork, driven rain, and air pollution. The increasing demands from these stressors due to factors such as climate change raises the importance of understanding how lime mortar durability can be achieved. When specifying repair mortars, it is essential to understand the implication of mix design on the interaction between the mortar, the environment, and the surrounding historic material, and how this influences the longevity of the structure. This PhD provides insight into the relationship between mix design and resulting mortar durability and resilience, which was found to be closely linked to the pore structure and moisture transport properties. This topic was addressed through laboratory assessment of lime-based mortars supported with in-situ assessment of mortars from conservation settings.

The hot-mixing production method was investigated to establish the extent of aggregate etching due to lime slaking with aggregate. Imaging techniques (scanning electron microscopy (SEM) and a stereo microscope) were used to identify grain surface features that could be attributed to hot-mix etching across four different aggregates (siliceous Chard stock sand, calcitic limestone gravel, oolitic limestone sand, and quartz sand). There was no significant evidence to support the hypothesis that hot-mixing etches aggregate. It was concluded that hot-mix etching is not likely to contribute towards the durability of lime-based mortars.

Mortars from an internal mosaic were analysed to establish the impact of the design requirements on the mix design and internal environmental stressors on the mortar resilience. Mosaic failure was attributed to oxide jacking of embedded steelwork, which cracked the mortar. For these mortars the top layer (‘thin-set’ and ‘grout’) had to be adhesive yet workable, which was achieved through use of a 1:1:0.5 sand : cement : lime mix. A highly porous structure (foamed mortar) was identified through SEM which was possibly designed to create a curved profile without contributing excessive weight to the roof. Crack healing was a marker of mortar resilience that was not present in the mortar specimens from the mosaic (circa 85-year-old ‘bedding coat’ and ‘foamed mortar’) but was positively identified on hydraulic mortars exposed to 26 years of external environmental conditions (hydraulic mortar mixes used Blue Lias lime, Unilit B Fluid X (Unilit), or Unilit with Chards non-hydraulic putty with flint aggregate). Autogenous healing and secondary calcite precipitation occurred due to a combination of three factors: available free lime in the hydraulic mortar mixes, available moisture in the external environment, and a matrix of capillary pores in the 0.2 µm to > 0.3 µm region.

Pore analysis of 26-year-old weathered and unweathered samples (Blue Lias lime, Unilit, or Unilit with Chards non-hydraulic putty with flint aggregate) showed that in-situ application of mortar altered moisture movement properties by significantly altering the material pore distribution via porosity reduction and pore refinement. This phenomenon was also observed in further pore comparisons of laboratory cured lime mortars (NHL5, CL90Q quicklime, and Vicat Prompt binders with Quartz sand, Oolitic sand, or Oolitic sand and Coal Ash blends) with in-situ counterparts.

The impact of binder and aggregate selection on the frost durability, salt durability, and carbonation rates of mortars was tested on pointing repair mortar mixes produced in-situ at Bennerley Viaduct (UK, 52.98970 latitude, -1.29780 longitude) and in the laboratory. Including hot-mix non-hydraulic lime (CL90Q) binder or coal ash aggregate reduced the frost and salt durability due to the generation of pores of diameter >10 µm (which reduced frost durability) and >1 µm (which reduced salt durability). Mixes containing CL90Q also had reduced carbonation rates.

This research advanced the knowledge of lime mortar durability and resilience and supports lime mortars specification for conservation applications. Where lime mortars require increased durability, the mix design should seek a resulting pore structure with a reduction in pore diameters >10 µm (frost durability) and >1 µm (salt durability). Where resilience through crack healing is desirable, the specified mortar should include free lime and pores in the 0.2 µm to > 0.3 µm region. Trial panels in-situ are essential for evaluating representative expected pore distributions.

Date of Award	25 Jun 2025
Original language	English
Awarding Institution	University of Bath
Supervisor	Stephen Emmitt (Supervisor), Richard Ball (Supervisor) & Kevin M. Briggs (Supervisor)