Heritage Retrofit

BCD SPECIAL REPORT ON HERITAGE RETROFIT FIRST ANNUAL EDITION 31 HOME AND DRY Developing a non-hydraulic setting air lime for the insulation and repair of traditional buildings HARRY CURSHAM A LMOST ONE in four buildings in the UK are traditionally constructed using lime rather than cement. If well maintained, solid walls of brick or stone set in a lime mortar work well, drying rapidly after a rain shower so damp never penetrates far into the wall. Condensation inside the building also dries quickly, so the walls act as a buffer for both humidity and heat, moderating extremes in the building. By the end of the first world war lime technology had largely been abandoned in favour of faster-setting mortars. It was only in the late 20th century that conservationists began to realise that these cementitious mortars were actually damaging traditionally constructed buildings. Traditional mortars were softer and tolerated the natural expansion and contraction of solid masonry without failing. And they were highly permeable. It was discovered that problems occurred when old masonry was repointed, as this introduced just a thin layer of hard cement at the surface alone. As the core remained flexible, even modest thermal movement could cause the surface to spall, as pressure is exerted across the face of the wall. Being relatively impermeable, cement also prevented the mortar from wicking moisture to the surface. In particular, the cement renders used tended to trap moisture, and if cracked, more moisture is drawn in by capillarity, making the walls cold and damp. Over the course of 60 years or so prior to the lime revival, traditional methods of making and using mortars were forgotten. Text book descriptions were often ambiguous, and a new generation of conservators had to rely on trial and error and on the analysis of old mortars. Today, new discoveries are still being made. For the retrofit sector these developments are important because damp walls are known to leak up to 30 per cent more heat than dry walls, and in some cases the actual figure can be far higher. Simply by getting all our traditionally constructed buildings up to a sound condition would help reduce the UK’s carbon emissions substantially. The types of lime used generally fall into two categories: non-hydraulic or ‘air’ limes which set very slowly by a chemical reaction with carbon dioxide alone; and hydraulic limes which stiffen more quickly due to a partial crystallisation set. One area of great interest is in the development of non-hydraulic hot-mixed mortars, because they seem to be producing mortars which are much closer in nature to those found historically and there is a growing consensus that these mortars can provide the best performance in use. While their slow set means that air limes require more care and skill in use, they tend to be more permeable, and some hydraulic limes have been shown to become almost as hard and as impermeable as cementitious mortars when aged. Mortars made with air limes also offer lower conductivity and are therefore a good insulant, all of which make these mortars ideally suitable for older solid wall buildings. These properties are also significantly enhanced when used with appropriate aggregates, and the final result is akin to most pre- industrial mortars found in the UK and on the continent. This article looks at one proprietary product which has recently been developed. Although a form of non- hydraulic calcium hydroxide, it is supplied as a dry powder and when water is added the mix stiffens to provide a preliminary set without any addition of setting agent or pozzolan. The set is referred to by its manufacturer as a ‘Vivus’ set, which results from the way that the quicklime is manufactured and slaked. There are no clay impurities in the limestone used to make it, and none are added, so in essence it remains a pure air-lime and within the normal ‘lime cycle’. Carbonation is a secondary setting process in that it is unnecessary for construction work to proceed, but adds strength in the long term. Like a hydraulic lime, it will continue to carbonate over the following months and years, depending on the depth of material. In the process carbon dioxide is absorbed from the air, completing the lime cycle (see Figure 1). This non-hydraulic setting mortar is able to perform in the application stage A traditional roughcast lime render on a solid masonry wall