In the majority of applications lime has significant performance and visual benefits over cement mortar, particularly in relation to traditional stonework.
Portland cement developed during the second half of the nineteenth century, largely replacing lime as the principal binder after the second world war.
The purpose of this is to inform the benefits of lime mortar in traditional stone construction. We believe there is a place for all mortar types during contemporary approaches to building, whereas historically, cement mortars have been used as an inappropriate application for traditional stonework.
Lime mortar
(An image of a historical wall laid in lime mortar, mostly still intact)
We use NHL (Natural Hydraulic Lime) as the binder when gauging our lime mortar mixes. Various types of sand are available for selection. We are generally guided by the colour of the mortar we are trying to match or our clients wish to choose to as the finish on a newly constructed wall. Lime, sand and water are blended to create the lime mortar mix.
We use various different lime products dependent on the suitability its purpose is for.
Natural Hydraulic limes are available in three categories:
NHL 2 is suitable for friable brick and natural stone.
NHL 3.5 Soft - medium brick and natural stone.
NHL 5 Most brick & sandstone, dense masonry backgrounds such as granite and basalt.
Hydraulic lime will cure in damp conditions and at lower temperatures dependant on the environment and conditions.
A general rule of thumb when selecting the appropriate materials for mortar mixes is the mortar should have a weaker set than the masonry unit.
Lime mortar can be re-worked creating a workable mix even the following day. Once the mortar mix is originally mixed, it can be left to stiffen and re-worked. This helps to avoid waste during the building process.
Lime mortar is vapour permeable when compared to cement mortar. Some lime mortars are more vapour permeable than others depending which binder is used when mixed.
Healthy lime mortars absorb and evaporate moisture freely. In less permeable stone walls such as granite, the function of lime mortar becomes even more important for the free passage of moisture.
Lime mortars have an inherent flexibility. Cement mortar does not. Once cracks form, moisture can enter freely and the stonework can fail at an accelerated rate. Impermeable cementitious mortar can also accelerate the decay of stone walls, particularly those formed of softer stone such as sandstone.
The return face of the same wall has been historically repaired using cement mortar. The inappropriate use of mortar shows the decay of stonework at an increased rate.
Comments