Part One. Why use lime mortar?

Mortar has a threefold purpose. It bonds masonry together, ensures that loads are spread evenly and fills the gaps between the bricks or stones. The latter function helps makes the wall weatherproof and thereby excludes damp. This is essential in traditionally built houses with solid walls. Unlike modern cavity or timber-framed walls, which separate the outer and inner layers of the construction, solid walls fulfil their weatherproof function by their ability to absorb and release water.

Limestone is porous; it can absorb large amounts of moisture, for example from driven rain, and in dry conditions needs to release equivalent volumes of water through surface evaporation. Therefore, it is important that the ‘breathability’ of the stone is not inhibited and that the correct type of mortar is used to carry out repairs, including repointing. Dense cement-based mortar severely limits evaporation from the mortar joints and causes moisture retention. Dampness can lead to rot in built-in and internal timbers, frost damage to stonework, poor thermal performance and surface erosion of the stone due to salt crystallisation.

Photograph 1

Salts are present in the soil, most historic stone structures were built with earth mortars, and lime mortar, a relatively expensive commodity, was only used at the exposed joints. Moisture will mobilise salts and as evaporation takes place, salts are left behind at the surface (efflorescence), or within the pores as crystals. During crystallisation, the salts expand in volume from their dissolved state. Erosion of the stone can take place when crystallisation occurs inside the stone. In stone with large pores, the crystals can form without causing any damage but finely pored oolitic limestone, such as that commonly found in buildings in the Stroud valleys, is susceptible to this form of weathering.

The combination of hard cement mortars, frost damage and salt crystallisation is the cause of the ‘honeycomb’ weathering illustrated photograph 1. A certain degree of damp is inevitable in stone walls but it is clearly better that the salts crystallise in the mortar, which is easily replaced, rather than the stone. Moisture will tend to move from a dense material into one with higher porosity and evaporation will not take place from a material with impermeable characteristics, i.e. a cement mortar. It is a general principle that the mortar should have greater porosity than the brick or stone with which the wall is built. In this event, the mortar will act as a wick extracting moisture from the masonry. These characteristics needed are met by lime-based mortars.