Understanding Earth & Lime Mortars

“Durability over strength”

Most pre-20th century stone buildings were built with either earth or lime mortars, or often both. Lime has been used for thousands of years, but has always been relatively expensive (requiring fuel to burn the limestone and labour to process and transport the material). Earth mortars, made from sub-soils of clay, silt and sand, were significantly cheaper – often being found on site or nearby and needing much less processing.

Higher status or more structurally demanding buildings, like castles, were often built entirely in lime mortar, for both the core structure and for pointing, rendering and limewashing. More modest domestic structures typically used earth mortars for the wall structure, with lime pointing as a weatherproofing element. The unique properties of clay are such that many buildings still stand after more than 300 hundred years.

Better understanding of building conservation, and the work of groups like the Building Limes Forum and Earth Building UK & Ireland, are helping ‘rediscover’ the benefits of these once ubiquitous building materials.

The Chemistry of Lime

The raw material of lime is limestone or chalk, in the form of calcium carbonate (CaCO3). When burnt in a lime kiln it releases carbon dioxide (CO2) to form quicklime (CaO). With the addition of water when mixing to make a mortar, quicklime reacts to produce calcium hydroxide (Ca(OH)2) and releases considerable heat in the process. During setting, calcium hydroxide reabsorbs carbon dioxide from the atmosphere and releases water to reform calcium carbonate. Pure lime is also known as air lime.

Hydraulic limes (commonly sold as NHL 2, NHL 3,5 and NHL 5) consist of lime as described above, plus the addition of burnt clay minerals, called silicates. The most common of these is ‘belite’ or di-calcium silicate (C2S). These react with lime in the presence of water and set harder and quicker than lime alone. Hydraulic limes are so called because they can set underwater, wereas air limes cannot.

Modern cements contain a wide range of minerals, but the principles are fundamentally similar to hydraulic limes. The chemical process in cement is the reaction of another silicate, known as ‘alite’ or tri-calcium silicate (C3S), which produces a yet hard set than belite.

Clay as a building material

Earth mortars work in much the same ways as any other mortar, with the clay acting like a binder around larger particles of silts and sands. The proportions of clay/silt/sand vary considerably depending on the local materials available, but the clay component will normally be between 10-20%; higher clay quantities are likely to result in excessive shrinkage and cracking.

As the load bearing requirements of a mortar in typical buildings are very low, the lower strength of earth mortars is of little concern, whilst its ability to absorb movement in older building is unsurpassed. Earth mortars also have excellent breathability, and are remarkably good at keeping out rainwater, thanks to the way clay particles close up when exposed to water.

Small amounts of quicklime can be, and often were, added to earth mortars to increase its load bearing properties. Modern road construction projects routinely use this approach for soil stabilization.

Eco-credentials

Cement production accounts for around 8% of global CO2 emissions, of which around half is released in the production of quicklime, and half from the energy requirements of the firing and production processes. Pure air lime mortars require a lower firing temperature, and reabsorb most of the CO2 lost in burning as the mortar sets. The total emissions of pure lime mortars is therefore significantly lower than that of cement equivalents.

Hydraulic limes sit somewhere between the two. The lime component in an NHL which reacts with silicates won’t reabsorb CO2. A typical NHL 3,5 might reabsorb around 40% of the CO2 originally lost in firing.

It’s also worth noting that in the context of CO2 reabsorption, there is growing awareness of the importance of short-term cycles as we attempt to deal with the growing urgency of climate change. Thus, a softwood tree will re-grow much quicker and recapture lost carbon sooner than a slower growing hardwood. Lime mortar set deep within a wall may take many decades to reabsorb carbon dioxide – in fact, there are examples of very thick walls several thousand years old that have still not fully carbonated.

In this context lime pointing tends to carbonate relatively quickly, and when used in conjunction with earth-mortared core – which has very little CO2 impact – can be a very eco-friendly building method.

Lime & Earth Mortar Specifications

Given the range of different buildings, it is difficult to give an all-purpose specification for earth and lime mortars. However, drawing on experience, a growing body of scientific research, and support from groups like the Building Limes Forum, we can nearly always recommend a suitable material for your property. Please visit our services page for more information.