Lime Mortar and Plastering
Lime plastering is a traditional method, used on solid walls that have no cavity. As lime mortar is permeable moisture can evaporate freely, reducing the risk of any further damage to plaster work and decay to surrounding timbers. It is important that lime is used on old buildings, in particular timber framed houses.
Lime can be either lime putty or a hydraulic lime NHL 2 or 3.5, or in the most exposed locations, NHL 5. (The letters stand for ‘natural hydraulic lime’ and the figures relate to the material’s compressive strength in N/mm2 at 28 days.) With soft or damaged bricks NHL 2 might be considered appropriate, while NHL 3.5 or 5 should be used in areas that are very exposed and wet or with harder brick types. Hydraulic lime can be mixed with sand and other aggregates in the ratio of 1:2:5.
Hydraulic, in this context, refers to the reaction of the lime with water to achieve at least a partial set. This develops within hours and the mortar then continues to harden once it has dried out through carbonation, a reaction between the lime and carbon dioxide in the air.