Lime is not much good for tower blocks or supermarkets, as it doesn’t have the strength or speed required; but it’s ideal for the domestic scale, especially self-build, and is essential when it comes to repairing old buildings, where cement should never be used. Cement doesn’t have the following properties that lime has:
Hygroscopic
- Because lime is hygroscopic (can absorb and dispel water) it can prevent condensation and therefore mould.
- With cement mortars, the only way water can evaporate from the building is through the bricks, and as soluble salts are often drawn to the surface, crystals form which make the bricks crumble.
- Salts can come to the surface through lime renders and be brushed off. With cement renders, the salts move through the underlying stone or brick to the interface with the cement, where they build up and can eventually dislodge the cement render.
- Soluble salts move easily through lime; lime mortars and renders allow salts to come to the surface with the moisture, where they crystallize (especially at ground level). The surface crumbles, and erodes sacrificially, but can easily be repaired. The structure of the building is not damaged. Lime render can be re-applied and mortars pointed.
- The same is true of ice crystals – it’s better for the mortar to erode than the bricks / stones, as it can easily be re-pointed; also, soft, flexible lime mortars can give with the expansion of ice or salt crystals unlike cement mortars.
- Old buildings need a ‘skin’, which can erode sacrificially, and then be replenished. Even stone buildings crumble, which is why it’s a good idea to limewash old buildings, even stone ones. Because stone is so attractive though, there is often local opposition to the limewashing of attractive stone public buildings. They would almost definitely have been limewashed originally, however.
Cement render remains proud as the softer stone crumbles.
pic: Mike Wye
Flexible
- Buildings (especially old buildings) can move, either because of expansion and contraction due to temperature changes, or slight subsidence. Inflexible materials can crack and let water in, which then cannot escape if materials are not hygroscopic. A small crack in an inflexible material can allow water in, which then freezes and causes a bigger crack, and so on. Many repairs on old buildings are due to the past use of cement, which stops flexibility and breathability. Old buildings move a lot – both over the long term, and on a daily basis due to changes in temperature and humidity.
- Cement is inflexible, and often stronger than the surrounding bricks – so any movement will mean that the bricks will crack before the mortar.
- Building limes form platelets that can move slightly over each other. This gives lime its flexibility. Cement forms much tighter bonds (tri-calcites) due to its higher burning temperatures. This produces a very hard, but brittle material.
- Castles were built with lime mortar between the stones; this meant that when walls were hit by cannon-balls, they would ‘give’ a bit and not crack.
- Lime is more than strong enough for most uses. There are Roman domes made from limecrete that are still standing. The dome at Assisi that collapsed after a small earthquake had been repaired using cement. If lime had been used instead, because of its flexibility, it would probably have survived.
Self-‘healing’
- If lime cracks due to movement, it will tend to be a network of tiny cracks and not the large cracks that can appear in cement. In these cases, water will enter and dissolve small amounts of calcium carbonate within the mortar. When water evaporates, it will deposit the calcium carbonate at the surface, eventually filling the cracks. Lime can automatically repair itself!
Fire resistant
- Lime has fire retardant properties which mean that it is good for old or natural buildings that incorporate flammable materials such as timber, thatch or straw-bales.
Mildly disinfectant
- Lime is a natural disinfectant, and so inhibits the growth of moulds.
Other benefits
- Lime is cheap and easy to produce in many parts of the developing world, where locals may not be able to afford cement.
- Using lime means that you don’t have to use expansion joints. An expansion joint is used in a long stretch of wall where bricks are put together with sand and cement. It is a long vertical gap filled with something flexible like a polymer membrane, which allows for thermal expansion in the wall; otherwise the wall might crack, as the mortar isn’t flexible. It’s extra work, requires the use of more impermeable, manufactured, non-biodegradable stuff on your building, and doesn’t look too pretty either. It isn’t necessary with lime, as it is flexible, and will expand and contract with temperature changes without cracking.