Impact of calcitic aggregates on air lime mortar properties

Abstract

In recent years, the importance of aggregate type on the properties of mortars has become increasingly recognised. In the context of restoration, it is particularly important to achieve the optimum properties that provide the best compatibility between the repair mortar and the existing masonry. With that in mind, the properties of the aggregate, in addition to binder type, should be given priority when designing the repair mortar mix. A critical analysis of the current state of the art is presented, identifying the areas of research that have not yet been explored thoroughly. The role of calcitic aggregates in mortar is one such area, and the research presented here examines the notion that calcitic aggregates cause an increase in the strength of lime mortar. Little is known about the reason for the higher strengths observed, and therefore this research aimed to establish possible causes. This research analysed flexural/compressive strengths of a range of mortar mixes containing different types of aggregate, and confirms previous studies that use of limestone aggregates rather than silicate can lead to higher strengths in air lime mortar. Furthermore, the depth of carbonation in the samples was measured by phenolphthalein staining and is not found to relate directly to the mechanical strength of the mortars. A range of samples were subsequently analysed using TGA, MIP and SEM. TGA has shown that the quantity of CaCO3 varies with the different aggregates used, with Ham Hill having around have as much mass loss as Portland and Stoke Ground respectively. It was found that the pore structure of the limestone mortars varied somewhat from the silicate sand mortars; aggregate pore structure was also found to differ significantly. The porosity of silicate sand was 5.81% while Portland was 49.9%. Detailed examination of the SEM images shows significant micro-structural differences between the different mortar mixes, with calcite crystals appearing on the surface of the limestone aggregates, and evidence of a more cohesive bond between binder and aggregate evident in the limestone aggregate mortars than the silicate sand mortars.

Date of Award	7 Nov 2017
Original language	English
Awarding Institution	University of Bath
Supervisor	Robert Lawrence (Supervisor) & Pete Walker (Supervisor)