Lime

Introduction

Traditional mortars and renders have incorporated lime. There is now some clarity on the role it plays, and why owners of Victorian and Edwardian houses should insist on its use in restoring walls and avoid the indiscriminate use of cement.

What is Lime?

Lime is produced from limestone through these steps:

Limestone (calcium carbonate) is heated in a kiln or kiln pit until burnt, giving off carbon dioxide and leaving calcium oxide (quicklime).
Quicklime is mixed with water; this produces heat. If only a little water is added, the result is a dry powder called hydrated lime or lime hydrate. If a lot of water is added, this 'slaking' process forms calcium hydroxide, usually called 'slaked lime' or 'lime putty'. This is then left to mature for several weeks. This process is called 'hydration'.

Non-hydraulic lime (slaked lime) hardens by a slow process of drying and carbonation, reacting with atmospheric carbon dioxide to form calcium carbonate. This takes a period of some weeks.

Hydraulic lime, on the other hand, sets rapidly by reacting with water in a matter of hours.

A non-hydraulic lime can be made to set much more rapidly by the addition of an hydraulic or 'pozzolanic' additive. This practice is known as 'gauging'. Typical additives are finely crushed brick powder or cement. These contain highly reactive silica and/or alumina, which give a rapid chemical set by reaction with water. Of these, cement is by far the most widely used in the UK, and the cheapest. Typical proportions, commonly used, are 1:1:6 (cement:lime:sand) and 1:2:9. Use sharp sand with lime.

Cement

Portland cement was developed in 1824; it was used in the role of a hydraulic binder, which was very consistent and made the mix set rapidly. This rapid set was an advantage in cold or very wet conditions. Builders slowly abandoned lime mortar in favour of cement and sand mortars.

By the 1830's, the 1:1:6 mixture of cement, lime and sand ratio had been firmly established. Use soft sand with cement.

However, cement and sand mortars proved too strong for some applications and lacked some of the workability of lime mortar. By the late 1800's cement-lime mixes were again widely used where increased plasticity, workability and controlled strength was required.

Portland cement remained costly into the 20th century so a minimum was used. For example, the main brick mortar was lime and sand, but pointing included a little cement. Again, render generally used some cement.

Why Use Lime?

Experts in the restoration of old buildings argue that mortar should be softer than the bricks they bind. Because most Victorian and Edwardian bricks are soft inside a thin, harder outer shell, they are vulnerable to damage to the surface layer, such as that caused by hard cement pointing resisting movement due to settlement or temperature changes.

In the case of renders and plasters, lime has benefits because of its flexibility and its porosity; it helps moisture that has penetrated a wall to evaporate.

Lime is also less prone to crack than cement. Lime is even self-healing; fine cracks allow carbon dioxide to penetrate. This reacts with free lime to harden and close the crack.

There is an ongoing debate on the use of cement with lime, or lime in a secondary role to cement.

Using Lime with Cement

When added to a normal cement and sand mix, lime makes the mortar more cohesive and adhesive, and it makes the mortar 'fatty' and workable so that it spreads well.

When it has hardened, lime allows mortar to achieve optimum strength because it increases the flexibility of the mortar. Lime will give improved bond strength with the brick.

Lime also reduces water penetration by 'autogenous healing'. This is because there will be some free lime in the set mortar. This will re-carbonate over time, sealing any hairline cracks. This is particularly important with renders.

Using Cement with Lime

Cement can be used as a 'pozzolanic' additive; you can gauge a non-hydraulic mortar with cement to make it hydraulic.

The advantages are:

It imparts a chemical set which occurs before full shrinkage occurs, thereby reducing the risk of cracking.
Layers may be built up more rapidly, without the need to wait a long time for one to set fully before applying the next.
It hardens rapidly, thereby providing protection from rain before carbonation has been completed.
Being an artificial substance manufactured under closely controlled conditions, it is reliable and predictable in use.
It is available in a choice of colours, useful when it is necessary to match the colour of an existing mortar or render.

The disadvantages are:

The rapid setting time limits the time available to the user in which to work with the gauged mortar.
Some cements contain appreciable amounts of soluble salts, in particular potassium sulphate, which may cause salt damage to stonework.
The use of cement tends to lead to the user treating the gauged lime mortar as if it were a fully hydraulic lime or cement. Too much reliance on the initial chemical set leads to neglect of the importance of the longer term carbonation of the non hydraulic component present.
'Segregation' may occur, whereby the cement separates from the lime as the mortar dries and hardens, blocking the pores in the mortar, reducing the porosity and weakening the mortar.

The Smeaton Project, a research programme commenced by English Heritage, concluded that a 1:1:6 (cement:lime:sand) mix is unlikely to segregate, while a 1:2:9 mix will almost certainly fail. As the cement proportion is reduced further, the mortar will certainly fail.

Lime in Brickwork

The conclusion is that the best option for mortar and pointing is an un-gauged non hydraulic lime mortar using well-matured lime putty and sharp and well-graded aggregate. This does not require special skills. Use non-hydraulic lime and sand in a 1:5 ratio.

Lime in Render

The choices here are either a hydraulic lime, or non-hydraulic lime with some cement.

If you are patching render, you must use the same mix to avoid cracking.

There is more detailed guidance here.

Lime Plaster

Lime Plaster is the ideal finish for walls built using lime mortar; because it is flexible it will tolerate movement in the wall. Again, lime plaster is best painted with lime wash, rather than wallpaper or modern paints. Lime wash or distemper can be applied over lime plaster after a few days but any impermeable coating must not be used for several months as lime plaster needs air before it can harden.

Lime Wash

As with other lime products, lime wash is porous and therefore excellent for surfaces where the evaporation of moisture is important.

Limewash is a paint made from lime putty which has been matured for several months and then thinned with water to make limewash. Limewash is naturally white, matt, and slightly chalky but can be coloured with pigments.

Lime wash can be used internally or externally. Because it works by absorbtion into the surface so it is best used on porous surfaces such as sandstone, lime plaster, lime render and soft bricks.

When lime wash is used externally, raw linseed oil or tallow mixed into the lime wash help shed rainwater.

On materials including cement or modern plaster, casein should be added to the lime wash to help it bond.

For companies specialising in this area, see the Products and Services Directory.