An Introduction to Lime in Construction by Ryan Stojic General Manager at Mike Wye & Associates Ltd
When we look at our built environment, there’s clear evidence that lime mortars and plasters have been around in this country for hundreds of years. Portland cement, by comparison, has only been in common use domestically since the 1920s but is now ubiquitous. However, attitudes are changing as we've come to a better understanding of the benefits of more sympathetic materials; particularly when used on traditional buildings.
As a nation, we have often been seen to lead the way in technological advances, but in the world of lime we were some way behind. Early examples of the use of lime in construction and decoration can be traced back thousands of years to ancient China, Egypt and Rome. In fact, it was the Romans who introduced lime to Britain many centuries after the early forms of kilns were busy burning lime in Mesopotamia!
Lime putty is manufactured from pure limestone sources including chalk, seashells and bones (calcium carbonate). The limestone is burnt at temperatures varying from 600°C to 900°C which releases carbon dioxide and produces quicklime (calcium oxide). Controlled amounts of quicklime are then added to clean water in a process called slaking and left to mature for a minimum of 12 weeks resulting in lime putty (calcium dihydroxide).
This natural, rich and fatty binder is ready to be mixed with carefully selected sands to create mortars, renders and plasters or diluted with water to make limewash.
Lime putty hardens by re-absorbing carbon dioxide from the air (carbonation); returning to calcium carbonate and therefore lowering its carbon footprint. This is known as The Lime Cycle.
In more recent years, there has been a revival in the use of ‘hot lime’ mixes. The production of hot lime omits the slaking stage and involves mixing quicklime directly with sand and water. The term ‘hot lime’ comes from the heat given off in an exothermic reaction between water and quicklime. This reaction can be very aggressive and is supported by the hazardous classification of quicklime. Although care should be taken when handling any form of lime, it is only quicklime that is classified in this way and therefore knowledge and safety are essential before considering going down this route.
However, there is historic evidence supporting the use of both hot lime mixes and lime putty mixes through the writings of the Roman architect and engineer, Vitruvius, during the 1st century BC. Some lime specialists are keen to recreate historic mortars and plasters in this way based on recent research and evidence supporting their use in the UK.
Both lime putty and hot lime mixes gain strength through carbonation which continues over a period of months after the initial drying phase. Evidence suggests that the ancient Minoan civilisation, hailing from the Aegean Islands, had discovered that adding forms of burnt clay (such as crushed pottery shards and brick dust) would achieve a quicker, harder set before carbonation kicks in. The burnt clay reacts with lime producing a ‘hydraulic’ set meaning that it will begin to harden in the presence of water; much like cement does.
Romans used a pumice from volcanic regions in the same way and we take from their discovery the term ‘pozzolan’ derived from the city of Pozzuoli, Naples. The Romans referred to a similar hydraulic building material of lime and pozzolan as ‘cementum’ and it was used in major building projects such as the construction of the Parthenon and Roman aqueducts. It’s clear that builders and engineers of the day must have had extensive practical knowledge of the strengths of the pozzolans they were using - something that we’ve had to rediscover in the last few decades with the use of manufactured alternatives.
Some limestone belts contain natural seams of clay which, once burnt in a kiln, produce limes which are naturally hydraulic without the use of additives. The different limestone quarries were investigated by John Smeaton (1724 -1792) who established that the hydraulicity of the limestone was directly related to its clay content. Smeaton was a civil engineer who specialised in the design of bridges, canals and lighthouses. He needed to find a quick-setting mortar that would harden in the 12 hours between the times of high water for his most famous project: the third Eddystone Lighthouse.
Smeaton’s research led to the development of natural hydraulic lime (NHL). These are categorised into three main strengths: NHL2 (feebly hydraulic), NHL3.5 (moderately hydraulic) and NHL5 (eminently hydraulic).
One thing that is often misunderstood about these designations is that they are minimum strengths after 28 days under laboratory testing. The range within each category is actually very large. For example, to qualify as an NHL3.5 the compressive strength of the lime must test between 3.5 N/mm² and 10 N/mm². Therefore, an NHL3.5 from one brand may be significantly stronger or weaker than that of a brand from another brand.
The qualifying bands also overlap one another: NHL2, for example, must test between 2 N/mm² and 7 N/mm² at 28 days. This means one brand of NHL2 could in theory test higher than another brand of NHL3.5…misleading huh?
Following on from the pioneering attempts of Smeaton, Joseph Aspdin patented Portland Cement in 1824 which was so called because he thought that it looked like Portland stone. It was a mixture of limestone and clay fired in a kiln at high temperature. Initial take-up was slow but the advantages of a consistently performing, fast setting, easily transportable and strong binder were obvious – a building material for the modern world. Lime putty mortars fell out of favour from that time up until the lime revival in the 1980s.
It’s now clear how inappropriate and damaging cement mixes can be to older buildings. Cement is a useful material for many construction applications but there are fundamental differences between traditional, solid wall constructions and those built with a cavity wall as standard from 1919 onwards - including the materials they need to perform efficiently. Looking particularly at old stone and cob buildings which have been cement rendered externally and painted throughout with modern, acrylic paints, it is obvious that modern building techniques can seriously damage their health and structural integrity.
First and foremost, lime mortars and plasters (whether lime putty or NHL) are more vapour permeable and allow walls to “breathe”. They should always be coupled with a breathable paint such as silicate paint, clay paint or a breathable emulsion to work as part of an effective system. Managing dampness in traditional buildings is not only important for the building itself but also for the health of the occupants when asthma and allergies are common complaints nowadays.
Secondly, having a reduced strength mortar is a distinct advantage for many applications which require sympathetic consideration. Lime putty mortars are more flexible than mixes containing cement or hydraulic lime – a property which can be essential in minimising cracking from slight movements found in older buildings over time.
Selecting a strong mortar on the perception of durability can be damaging to masonry and contribute to trapped moisture, condensation, mould growth and reduced thermal efficiency.
There are some basic points you should consider before selecting lime for your project:
Are you in a Conservation Area or is the building listed? You may need to seek advice from the local Conservation Officer before carrying out any works.
Is the application internal or external? External applications may need the addition of a pozzolan.
Are you bedding stone/brick, laying a floor screed, pointing, rendering or plastering? You may need to match historic mortars like-for-like whereas the strength of a hydraulic lime is essential for lime floors.
What sort of environmental conditions will the substrate and mortar be exposed to? A chimney breast in the west coast on top of a hill will need a stronger mix than a sheltered garden wall in the south east.
Do you need to match an existing colour? Carefully selected aggregates are used to replicate local mixes and blend in with the surroundings.
Thankfully the use of traditional lime mortar mixes in renovation hasn't become a lost art but has been rejuvenated through the efforts of enlightened organisations, lime specialists and conscientious homeowners.
The use of sympathetic materials has itself moved on, with increasing interest in natural insulation materials such as cork and wood fibre which are finished with lime plasters and renders on both traditional and modern homes.
This perfect partnership helps to improve energy efficiency without sacrificing function and therefore forms a new chapter as part of this fascinating history.
For free advice and guidance on the selection and application of lime mortars, renders and plasters and all other aspects of traditional building maintenance please visit www.mikewye.co.uk.