Building with lime mortar

BUILDING WITH LIME MORTAR

As a building material, lime is one of the oldest types of mortar known to man and dates back to at least the 4th century BC. It can be found in ancient buildings all over the world and there are good examples of its use in places such as Ancient Rome and Greece. Before lime came into use clay and gypsum mortars were in wide use.

The consistency of lime has changed over the centuries; and today, with the introduction of Portland cement, the use of lime in new constructions has declined drastically, but over recent years it has experienced a resurgence in popularity. The use of cement can be extremely important when there is a need for ease of use, quick setting and/or compressive strength; however, there are many advantages to working with lime, particularly when working with softer materials, such as stone or terracotta.

Lime mortar rendering

Lime mortar rendering

Initially lime has seen a great deal of use in the repair of older stone built or historical buildings; but as the use of lime increases, its effectiveness as a building material is becoming much better understood. Time-honoured practices, based on tradition, folklore and trade knowledge are being studied in detail and then put into practice, and that enthusiasm for building with lime has been greatly fuelled by the fact that so many older buildings that have been built with lime mortar are still standing.

LIME MORTAR

Lime mortar basically consists of lime (hydraulic or non-hydraulic), water and an aggregate such as sand. Lime is an extremely caustic material when wet and today’s health and safety standards will advocate the use of protective equipment when using the product. It is also recommended that a bucket of clear water is kept readily accessible to administer immediate first aid should there be exposure to the skin or eyes.

Non-hydraulic lime needs air to carbonate and set, and will be produced from high purity calcium lime stones. This type of lime would be blended with sand and water to be used as lime plaster. It is still produced today in a small number of kilns and is referred to as hydrated lime (not to be confused with hydraulic lime as hydrated refers to the form in which it is supplied). A traditional lime plaster that is used in restoration work may also contain horse hair or other organic fibres to reinforce the plaster. It is sold as powdered lime in bags, or can be obtained as lime putty, which is more suitable for pure lime applications. Lime putty will have been through a lengthy process to arrive at that state and will need to be ‘agitated’ to allow it to transform into a more liquid state. This is done just prior to use as the putty will revert from a thick liquid to a putty state if it is left to stand following agitation.

mixing lime mortar with horse hair

mixing lime mortar with horse hair

Once mixed and applied, non-hydraulic lime will set as it dries as it reacts to the CO2 in the air. This process is a lot slower than modern plastering and can limit its use as the lime may stay soft for months or even years. In fact, the slowing process of lime needs to be controlled or regulated at a steady rate to ensure a good final set. If it dries too rapidly this will result in a poor quality, low strength mortar which will possibly contain shrinkage cracks. To control the setting process the mortar may be covered with hessian and/or sprayed with water.

One of the reasons that a building constructed using lime mortar is more durable is that the mortar is self-healing when minor cracks appear due to movement. Whereas Portland cement will crack, the small cracks in lime mortar will re-crystallise, effectively repairing the bond. Building movement will also result in the weakest element cracking first. As lime mortar is normally softer than the main construction material, this can result in easier and cheaper repairs to the structure. If you are repairing a building constructed with lime mortar it is important to respect the building materials that have been used. If a cement based product is used to re-point brickwork or to render the building, this may result in that area of the building no longer moving in harmony with the rest of the structure. This will inevitably cause stress to the building and may even cause a localised build up of moisture. It is important to allow the building to function as it was designed to do, and to credit its builders with the respect they deserve for using a product that has been proven to be an effective, durable and flexible material.

Hydraulic lime is obtained from lime stone containing impurities, and the term ‘hydraulic’, when used in the context of lime means to ‘harden under water’. The limestone used will contain sufficient quantities of clay and/or silica to allow it to be used in this setting.

lime plaster on timber lath

lime plaster on timber lath

Why use lime mortar?

Why use a product that has been around for thousands of years, and can be found in almost every building constructed before 1910, and has allowed buildings to remain standing for hundreds of years? Why use a product that allows for the movement of a building, and works with nature rather than fighting it? The material speaks for itself.