Choosing the Correct Lime

The need to use lime mortars for the conservation and repair of our historic structures has been re-proven over the last 30 years. A great proportion of our building stock; our city centers, our farm houses, our traditional cottages are over a century old and lime was the principle binder used in their construction. During the second half of the 20th century Portland cement almost completely replaced lime as building material for both new build and repair of existing buildings. The results of repairs to these historic buildings using Portland cement have been seen to be detrimental, both to the structure itself and the environment of those inhabiting the building. Rather than protect and weatherproof the building as intended cement mortars have accelerated the decay of masonry fabric and created an unhealthy accumulation of moisture. Traditional mass masonry buildings, built with lime mortar without cavity walls or dpc foundations operate on the principle of allowing free evaporation of any moisture in the wall. The comparatively more flexible mortar also allows more movement of the structure than cement constructed buildings. The use of cement finishes in this case can have numerous deleterious effects;

·     Facilitating the accumulation of water (rising damp) in the building by restricting the passage of air and moisture out through the masonry.

·      Damage to softer stone/brick in a more flexible wall when coated with a hard, rigid cement.

·      Increase in masonry decay of cement pointed walls as moisture transfer is concentrated in the fabric immediately adjacent to mortar joints.

·      Salt crystallization and damage following the introduction of sulphate rich cement exacerbated by the concentration of moisture movement through the masonry.

Damage to historic brickwork caused by cement repairs

The identification and recognition of these problems has led to the understanding that for much of the conservation, maintenance and restoration of our historic building stock Portland cement is an unsuitable material. Mortar and plaster repairs to traditional mass masonry buildings must use a material that is sympathetic (not stronger or more rigid) to the masonry itself and vapour permeable, allowing moisture travelling through the fabric to escape.

The past twenty years have seen the re-introduction of lime binders as a building material in Ireland, first limited to lime puttys then soon afterwards natural hydraulic limes (NHLs). Lime putty mortars set by carbonation only (the absorption of carbon dioxide from the air). They produce the softest, most flexible, finest of lime mortars, excellent for smooth finish plasters and sympathetic to the softest, most friable building materials. The restrictions of lime putty mortars are their slow setting time, carbonation only occurs above 10oC (limiting their use in the winter months), and low natural water repellency. NHL’s harden by both a hydraulic set (on the addition of water similar to cement) and carbonation. They are produced in a range of strengths; NHL2, slower setting and most flexible, NHL3.5 and NHL5, faster setting and stronger. NHL’s have the property of faster setting times than lime puttys and increased natural water repellency while remaining highly flexible and vapour permeable (breathable). The most appropriate lime binder must be determined by the fabric of the wall (ranging from cob to brick to granite) and the environment (how exposed the building is, the ambient climate etc.).

Care must be taken when choosing or specifying a Natural Hydraulic Lime for conservation work. NHLs are manufactured without the addition of any materials beyond rock and fuel. Unlike the regulations for Ordinary Portland Cement which ensures a standard product from every manufacturer the European standard which regulate NHL production (EN459) allows for significant variation in properties, including but not limited to; colour, bulk density (value for money), sulphate content (important in saline environments) and strength. Indeed the standards allow overlap between classifications, a NHL2 from one manufacturer may be stronger than a NHL5 from a different manufacturer once fully cured. You should therefore always ensure you have the technical datasheet from the manufacturers showing the expected performance after a period of two years. For example NHL from a single manufacturer is now sold in Britain under numerous labels, the technical information supplied from each do not necessarily correspond to the original manufacturer. Therefore we always recommend only to purchase NHL supplied in the originals bag with the original technical datasheets.

Vincent Coller