centraltowerAs bizarre as it sounds, burning a bit of rock produces one of the most efficient and versatile building materials in the world. Lime, in all its forms, can be used to construct foundations, walls, floors, roofs and vaults, and also to finish them, with plasters, renders, paints, and decorative work such as architectural mouldings and hand-modelled stucco.

Lime is produced by burning limestone, one of the worlds most abundant materials, normally locally sourced (how many villages have a Lime Kiln or Lime Pits Lane?). This stone is heated to around 900°C in a coal- or wood-fired kiln, driving the carbon dioxide out of the limestones calcium carbonate. This gives calcium oxide, or quicklime which, when combined with water results in the production of calcium hydroxide. If the quicklime is “slaked” with a little more water than the reaction requires, then any excess is driven off by the heat of reaction, leaving a dry powder, known as hydrated or bagged lime. If a considerable excess of water is used, then a milky suspension of lime is produced that, when placed in a pit, will settle to form a cohesive mass of lime putty, above which is a clear, saturated solution of lime, known as lime water. These slaked, or “fat”, limes produce calcium carbonate (and water) on reaction with atmospheric carbon dioxide. These processes are known collectively as the lime cycle.

This simple chemical process, the burning of stone, was realized and developed as much as 10 000 years ago, the earliest surviving example being an 8000-year-old floor in Turkey. This technology, of using lime in mortars and plasters, was brought to the British Isles by the Romans. However, pure limes, from stone predominantly composed of calcium carbonate, will not set under water, so for building in wet conditions, especially for engineering works such as bridges and canals, special materials are needed. The properties of the lime produced are influenced largely by the chemical composition of the limestone burnt. If the limestone burnt contains clay minerals, such as aluminates and silicates, these can combine with water to give a chemical set, in addition to the carbonation of uncombined (or free) lime. These limes are known as hydraulic (having the ability to set in water). Where the proportion of clay in the stone is small, the hydraulic set is minimal and normal carbonation can take place. However, where the proportion of clay is very high, so much so that normal slaking cannot take place and the lime must be ground before the addition of water, natural cements are produced. These binders were developed extensively, and patented eventually in 1824 as Portland cement. This material, with its setting ability, most importantly in marine and aquatic environments, meant the slow decline of the use of lime until its virtual disappearance in the twentieth century.

Portland cement has shown itself to be a fantastic material for mass concrete and engineering structures because it is strong, rigid and impermeable. This, however, brings some disadvantages, and it has been recognized that hard, cement-rich mortars are unsuitable when used in conjunction with relatively soft and porous materials such as stone, brick, timber and earth the predominant construction materials of older buildings. The cementitious pointing is too hard to accommodate settlement or movement within the wall, and so when movement occurs, it is the stone or soft brick which spalls and cracks, not the mortar. To compound this, when a wall is damp, either through moisture within the building, or just through the action of rain water, the mortar is not sufficiently permeable to allow this moisture to pass to the surface of the wall via the joints. The water passes through the face of the brick or stone, drawing any soluble salts with it, which are allowed to crystallize on the surface of the wall, causing crumbling and decay. Worse still, if a render is made with cement, any moisture from the inside of the building cannot pass through the face of the wall, and so will pass back through the wall causing damage to internal finishes. Although each lime is different, and some perform very distinct and separate tasks, there are certain characteristics which are typical of most limes and set them apart from other products, such as the cements, gypsum plasters and plastic paints, most notably flexibility and permeability.

When buildings made with lime are subjected to small movements they are more likely to develop small cracks within the joints, as opposed to the large cracks more normal with cement mortars. Water ingress into these cracks can dissolve any free lime, and bring it to the surface. On evaporation the lime is left to carbonate, effectively healing the cracks. This is how old buildings, on poor foundations, distort rather than break and fail.

The high porosity of lime allows any trapped moisture to evaporate through the joint, therefore damaging the mortar and not the stone, brick etc. This allows for easier and, fundamentally, cheaper repair and maintenance: only a programme of mortar renewal is needed as opposed to new stone etc.

The advantages of the use of lime in mortars and renders, either as remedial work to old buildings, or in new construction, are well known. The use of lime and lime-based products as a repair medium for the conservation of stonework is less so. One of the great success stories of the last quarter century in the world of stone restoration and conservation was the work carried out on the west front of Wells Cathedral, during which a great deal was discovered and much pioneering work was done in the particularly sensitive field of the consolidation, repair and protection of stone. These techniques, involving the use of slaked lime, developed by Professor Robert Baker at Wells in the 1970s and 1980s, are collectively known as the lime technique.

Consolidation is undertaken with limewater: water rich in dissolved calcium hydroxide, such as the saturated solution above lime putty, of which at least thirty coats are applied. This reacts, producing calcium carbonate, the basic constituent of limestone.

Repairs to fine cracks can be made using a traditional lime mortar, gauged with various sands, dusts etc. to gain a good match for both texture and colour. This mortar can also be utilized for more substantial repair, such as lost detail to architectural mouldings, weathering to stones. Although these are made using the fat limes, owing to their high calcium carbonate content, in areas where a slightly harder set is required, certain materials that set in water can be added, conferring on the lime ‘hydraulic’ properties. This phenomenon seems to have been appreciated first in the Mediterranean, under the influence of Rome, where there is a profusion of volcanic materials, the largest source of pozzolans the name given to this group of materials (after Pozzuoli the region near Naples where there was an abundance of volcanic ash). These materials contain active silicates, which rapidly harden the lime. Modern pozzolans are brick powders and pulverized fuel ash (a by-product of power stations).

A sacrificial coat, known as a shelter coat, is also used is to provide protection to surfaces that suffer unduly from the effects of erosion from wind, rain and pollutants or to protect and consolidate freshly cleaned historic masonry. A shelter coat is a thin sacrificial layer of lime mortar, mixed to match the stone to which it is applied. It serves to protect the surface of delicate stone by physically covering it with a layer which must weather away before the actual stone is attacked once more. Generally they are used as a last resort in the knowledge that they offer the only solution, even a short-term one, to particularly problematic surfaces on historically significant buildings. In some circumstances, friable surfaces can be consolidated by several applications of a shelter coat.

Whether it is for the conservation and repair of historic stonework, or new construction, the properties of lime make it a sympathetic binder. In addition, with the production of cement being the third largest cause of human-produced CO2 in the world (contributing some 10% of total emissions), lime is also sympathetic to the environment. During the manufacturing process, the firing temperature of cement is significantly higher (1200 – 1500°C) and hence more energy is required to produce a tonne of cement than a tonne of hydraulic lime, thereby increasing CO2 emissions; both materials release CO2 on heating but, in a quasi-sustainable way, lime also reabsorbs CO2 on its setting. In a climate where carbon emissions need to be kerbed, lime offers a greener building product.

Simon Armstrong PGCArchCon
Wells Cathedral Stonemasons,
Brunel Stoneworks,
Station Road,
Cheddar, Somerset
01934 743544