A Beginners Guide to Lime
by Neil Quinn
The answer is lime – what’s the question? This is not as flippant as it sounds. Lime is the perfect building material. It’s carbon neutral, recyclable, aesthetically pleasing and much, much better for buildings than its most commonly used alternative, cement – even in modern construction. Using lime can only help the long-term health of a building.
So why isn’t lime used more often? Probably because it’s more expensive and requires a level of understanding of its capabilities that isn’t as common as it used to be. However many in the construction industry are now rediscovering the virtues of this natural material.
The Romans introduced the technology of lime to Britain over 2000 years ago, but it was used across the ancient world for thousands of years before that. Before cement was invented in the mid 19th century, most houses were built using limestone mortars and plasters, but during a time of rapid construction cement was cheaper to buy and set more quickly, allowing buildings to be constructed faster.
In the early days of building restoration and preservation, cement was often used but it didn’t take long to discover how damaging this was. Cement is dense and fragile, and when it’s used to repair an old building it won’t absorb moisture, resulting in cracks. There is now a significant legacy of damage caused by inappropriate use of cement in old buildings by previous generations. However, the use of lime shouldn’t be confined to the repair of old buildings. Heritage architects now recommend lime in the building of new houses too because of its qualities. It takes longer to set and needs specialist lime plasterers, but it improves the life and appearance of a building. No expansion joints are needed and it’s self-repairing to a degree – when cracks appear, tiny granules of lime will trickle down to lodge themselves in the cracks below.
Whether a limestone house is old or new, there’s no point applying modern paint on top of lime mortar or plaster. It’s like wrapping a building up in cling film – moisture has nowhere to go and will build up in the limestone causing cracking. There are many ‘breathable’ paints available, as well as traditional limewash or distemper with their subtle colours and beautiful finish.
Lime is produced by heating limestone, chalk or shell (calcium carbonate) to a temperature above 900 degrees centigrade. Carbon dioxide is given off and the material becomes calcium oxide (quicklime). Lime is created by adding water, known as slaking. If it contains impurities (such as aluminium or magnesium silicates), hydraulic lime is produced.
There are two main types of lime: Hydraulic and non hydraulic. Additives present in hydraulic lime (whether naturally occurring or subsequently added) will strengthen the mortar. Non-hydraulic is pure slaked lime. Modern hydraulic limes are increasingly being used in the repair of old buildings as they help strengthen a building while maintaining its porosity and flexibility. A good supplier [such as Rose of Jericho] will analyse a sample from an existing wall and produce a suitable match for repair.
Lime mortar is lime mixed with an aggregate such as sand and animal hair (horse or goat for example), to strengthen and bind it. Lime plaster will also have animal hair added, though no aggregate.
The beauty of lime is that the longer lasts, the harder it gets and the more carbon dioxide it absorbs as it returns to its original state of calcium carbonate, making it the perfect material for our time. For me, there is no question about it: For 21st century building and restoration, the answer is lime.