University of West of Scotland and Laing Traditional Masonry

Mortars for use with granite masonry

Damage to masonry and interior dampness, related to water ingress and retention highlights the correct choice of mortar when pointing walls. Compatible mortar composition is important where masonry units are impermeable, as in the case of load-bearing granite, because the mortar controls the ingress and egress of moisture. It is common to find water penetration which can be traced to incorrect specification of mortar; inappropriate mixes trap moisture inside walls, causing problems with frost heave and bedding mortar decay. The development of appropriate mortars for structural granite walls is an area which has received little investigation.

Previous studies have emphasised the use of cement-based mortars for repointing of granite masonry, as do standard specifications for masonry walling. Some studies have acknowledged that deformability (to absorb movement) is a desirable property, and have attempted to minimise the content of soluble calcium as a source of damaging salts. This resulted in the formulation of very lean cement mortars (1:10). The experience of several practitioners and testing laboratories has also been to focus on the use of cement in mortars in combination with hydrated lime, and the single use of hydraulic lime binders to ensure early set and adapt the properties to match that of granite, which is a very hard stone. The quality of the bond between mortar and stone, particularly in the fresh state and its later influence on adhesion in addition to shrinkage of mortars leading to de-bonding, is repeatedly raised by practitioners questioned for this study.

In the UK, the use of cement is frowned upon for repointing, repair and maintenance of older and historic stone buildings, even for granite. Traditional methods of repair demand the use of lime-based mortars to ensure authenticity and compatibility. Current practice is increasingly to use hot-lime mortar mixes, those that use quicklime mixed directly with sand and water, which slakes intimately with the aggregate releasing a considerable amount of heat. These mortars are anecdotally suggested to improve the properties of mortar, and historic mortars exhibit evidence for hot-mixing in almost all examples, further reason to demand hot-mixing for repair mortars used on historic buildings. This traditional practice is being rediscovered currently, but there have been very few studies into the properties of mortar made this way. In this study we aimed to reflect real practice, and study and compare the properties of mortars mixed hot with quicklime, and consider what properties that could be measured to best evaluate performance for use with granite masonry, to improve the moisture handling characteristics of mortar.

Four mortar mixes, three containing different types and proportions of lime and one cement-quicklime mix, were prepared using current site methods (volume mixing) and prepared into standard specimens for later testing, as well as being applied to small blocks of granite to asses ease of working and the bond between the mortar and stone. Fresh properties of workability and water retention were measured just after mixing. After a period of curing (setting and hardening) the samples were subjected to compressive and flexural strength testing and capillarity measurements (capillarity refers to the ability of the mortar to draw water into its structure). The mixing and curing process was relatively unconstrained, but was faithful to site practice.

The results reveal that the cement-lime mortar has the greatest strength and lowest capillarity (ability to absorb water). The lime-based mixes have lower strength, and a higher capillarity (up to 4 times that for cement). The bond properties appear satisfactory under qualitative visual inspection, and shrinkage was lowest for the cement-lime mix, the other mixes showing a shrinkage of a maximum of 1.2%. There is a predictable inverse relationship between strength and capillarity.

There is much still to resolve- for example the drying behaviour may be critical in the performance of these mortars, and counter the tendency to view the lower absorption cement mortars as preferable to prevent water ingress. Further porosity, water vapour permeability, bond and durability testing would be desirable to understand the effects of mortar composition on compatibility and moisture handling properties of granite masonry.