What is Lime & why use it?

History of lime

Lime plasters, renders and mortars were common place in Ireland before the mass production and widespread availability of products such as concrete, cement based renders, plasters and chemical paints. Most buildings constructed before c. 1915 incorporate lime mortars and pointing in brick and stonework with lime plasters and lime washes. Lime plastering, rendering and pointing can be found in buildings from high status cathedrals, castles & townhouses to the relatively humble cottages and farms which characterise many rural landscapes. Our specialty lies in the sympathetic restoration, conversion and maintenance of such traditional buildings.

Lime remains in use across the world, particularly by communities dependent on locally produced traditional building materials. Lime is ecologically friendly, has a lower carbon footprint than cement, and is well suited to low impact sustainable buildings (old and new) such as those made from clay, stone, bricks and straw bales.

What is lime?

Lime mortars and plasters are made from solid limestone crushed and burned (traditionally carried out in lime kilns that can still be seen in limestone areas) to form ‘quicklime’. Following this it is put through a process called ‘slaking’, where it is mixed with water to form either lime putty, or dry hydrates. These are then mixed with sand and water to produce lime mortars of different consistencies. After application in the form of plaster, render, mortar or lime wash, the lime cures and hardens, returning as close as possible to its natural form. This process is often referred to as the ‘natural lime cycle’ or the carbonisation.

Types of lime (strength)

The principal difference between lime putty, (hydrated lime) and Natural Hydraulic Limes, (NHLs) is that NHLs would set underwater by chemical reaction hence the hydraulic in the name, but hydrated lime will only harden (carbonate) in the presence of air.

There are three different types of NHLs based on the percentage of silica and alumina present and hydrated lime:

NHL2: Internal and external plastering on soft walls (clay walls, straw bales, soft stones and bricks, building soft stone or bricks).
NHL3.5: This is the most commonly used for plastering internally and externally, repointing, and general building work.
NHL5: Plastering exposed area such as chimney, modern hard stone building or bricklaying (This lime tends to be slightly darker than the other ones)

Materials that have been blended with external substances will be sold as NHL-Z. They are often call lime as well but would not be as breathable or natural due to usually some sort of cement additives. Numbers 2 / 3.5 and 5 after the NHL label are the minimal compression strength at 28 days in Mpa/mm2, for information cement resistance is at 32.5 Mpa or more.

So why use lime?



It allows walls to breath; I always describe cement plastering as being like glass, applied on an old stone building, it would look good and flat for many years but as it is non-flexible an non breathable (moisture permeable), it would be likely to crack and let water in . As it is none breathable (moisture permeable) the moisture trapped behind the plaster would never dry creating large problems after a few years .The frost would make it swell and damage the plaster, stones and brick walls.
It is more flexible and allow for light structural movements.
It will show the colour of the sand by transparency and / or can be dyed.
It can be easily removed at any time without damaging the material or wall it’s used with.

Lime is very good at its job and has been for centuries across the world, but it requires a bit of knowledge and understanding. Although some conventional builders / plasterers, and even architects might tell you so, there’s absolutely NO NEED to mix it with cement, and there’s absolutely NO NEED for a ‘base layer’ of chicken wire, or any ‘special coatings’ such as PVA. Such practices make a mockery not only of using a traditional material but also of lime’s inherent breathability and environmentally friendly credentials.

Why lime has become a specialist job:

Cement is a bit cheaper, much easier to use and need less organization and precautions. For lime mortar, the choice of sand is important, the drying time between coats as to be respected, it should not be applied on thick coats and it has to dry slowly in order to properly carbonate and last and therefore needs protection against wind, sun and rain for a few days after application. Unlike cement which once mixed always sets within a few hours lime takes ambient air to dry and times can vary a lot. If it dries too fast it will lose all of its resistance. Because lime is a traditional material, its application outside is dependent on seasonal conditions. Whilst inside plastering and painting can take place all year round, rendering and pointing are best done between frosts, (April, September, and October) and in the dry.

Lime has a wide range of uses:

Mixed with sand to build with stone & bricks.
Lime based concrete floor (aggregates can be stone, sand, hemp), steel reinforcement should never be used with lime mortars.
Mixed with hemp for insulation purposes it’s used as insulated plaster on walls that need to breathe, be moisture permeable.
Mixed with sand for internal or external plastering.
Mixed with water as lime washing or paint.
Mixed with marble dust, gypsum, mud etc. for stucco.

There are endless possibilities if the basics are respected.