The use of lime based mortars and renders can reduce damp problems, prolong the life and enhance the appearance, of historic buildings. Unfortunately, all too many have been repointed or rendered using unsuitable cement based materials, leading to considerable problems and sometimes to accelerated stone decay as shown below.

The story of modern cement really begins with John Smeaton’s construction of the Eddystone Lighthouse between 1757-9. Smeaton’s Narrative on the Construction of the Eddystone Lighthouse describes his experiments to discover a suitable ‘water lime’, a cement that would set chemically in wet conditions. Many followed Smeaton, including James Parker, who in 1794 patented 'Roman Cement', a natural hydraulic lime produced by firing 'marl', or limestones rich in clay, which were thought to resemble Roman mortar. With its characteristic reddish colour and brittle texture, many local versions of Roman cement were produced around 1800. In 1824 Joseph Aspdin of Leeds obtained the first patent for an artificial cement he claimed to have invented in 1811. Made out of fired clays and crushed limestone, he called it 'Portland Cement' because of the popularity of white Portland limestone among his contemporaries. Despite bearing little resemblance to Portland stone, the name stuck, and OPC (Ordinary Portland Cement) made to a standard recipe is now specified all over the world. Commonly available from around 1845, it was simple to use and mass concrete soon replaced clay and rammed earth structures, with reinforced concrete being patented in 1855. The main material it replaced was lime, which had been the basis for all historic mortars in stone and brick walling, and in renders, though clay, earth, dung and even bitumen had also been used. Lime plasters have been found in 1500BC remains at Knossos and lime mortar can be found in 2000-year-old Roman structures.

For a century after its invention, cement was thought by many to be the modern answer to crumbling and discoloured lime mortars and renders. With a 28 day strength in excess of 25 N/mm2, it also seemed just what the building boom of the 19th century needed. It was ideal for structural innovation and its predictability made it the subject of one of the first British Standards, BS 12 in 1904. With hindsight, however, there have been problems. High alumina cement concrete, sulphate attack, chlorides, carbonation and rusting reinforcement are a few of the problems that have received publicity. We now know that in many cases well intentioned repointing or rendering with cement mortar also led directly to accelerated decay of historic brick and stone masonry, with immense loss to our built heritage. In some cases a grid of hard cement mortar can be observed projecting from the masonry which is rapdily eroding, to the point where total rebuilding may be required. In others, cracking of the impervious cement mortar or render allows water to become trapped in the interior of the walls, leading to rot outbreaks in building timbers.

The solution to such problems is likely to begin with removing the inappropriate cement, where this can be carried out without damaging the masonry even more, and repointing in softer lime based mortars. Unfortunately, traditional methods of preparing and using lime mortars and renders had largely been forgotten by the time the problems of cement were identified. Nevertheless, most conservation professionals now appreciate that historic buildings must be repaired using compatible traditional materials and techniques, and in recent years there has been a resurgence of experimental work which has provided a firm basis for the reintroduction of lime mortar.

But what is so bad about cement mortar?

Oddly enough it is the strength of cement that is its main failing as far as historic buildings are concerned. Sophisticated manufacturing processes ensure cement does not simply "dry", but rather "sets" by a complex chemical reaction producing chain molecules of tri-calcium hydrosilicates. The result is that at 28 days a 1:3 mix of sand and cement mortar can be about twice the strength of the same mix using natural hydraulic lime, and over 20 time the strength of a non-hydraulic lime mortar. This makes it much more brittle, so that it is liable to crack, and it is also more dense and less permeable. In addition the chemical set can and often does produce calcium sulphate and sodium salts. Water that finds its way into the wall cannot dry out through the mortar joints. Instead it is concentrated, along with the soluble salts, in the adjacent stones or bricks. Crystallisation stresses and freezing result in the stone rather than the mortar being eroded, especially older, softer masonry and brickwork. Instead of merely repointing the wall every century or so, you may be faced with replacing whole masonry units, or even complete reconstruction.

The advantages of lime mortars

By contrast, traditional lime mortars are based on the fact that limestone, or calcium carbonate (CaCO3), can be transformed by a simple process of burning, slaking and mixing with sand, into a highly workable, long life mortar based on calcium hydroxide (CaOH). This simply "dries" back into limestone again by reacting with the carbon dioxide in the air, progressively increasing in strength over time as this carbonation proceeds. The simple chemistry of the 'Lime Cycle' is one of the most important "secrets" of traditional building technology. BS 890:1972, which defines building limes, has in the past been more often used to specify dry hydrate, used as a plasticiser in cement mortar, but class CL90 in the standard is now also used to describe high calcium putty limes, which can be used in the most sensitive conservation work. This standard has been supplemented by ENV 459, describing hydraulic limes, giving conservation professionals a wide range of options to cement mortars. Three main grades of hydraulic lime are currently used in many conservation projects: NHL2, NHL3.5 (most commonly used) and NHL5 (the hardest, almost equivalent to 1:1:6 cement mortar).

Lime mortar can be extremely plastic, making it easier to apply and eliminating the need for movement joints. It has a higher thermal resistance, though thermal insulation calculations do not yet recognise this. It produces less CO2 and has a lower energy input in production than cement because it does not require grinding and mixing and is fired at a lower temperature. Perhaps most important of all, it is permeable: a "breathing material", which permits the passage of moisture vapour while acting as a sacrificial sink for salts and resisting water penetration. By attracting damaging salts and water that would otherwise accumulate in the stones or bricks, traditional lime based mortars can protect soft bricks and stones and reduce dampness and condensation. This means that traditional mortars and renders will gradually erode and eventually need replacing as a normal part of maintaining traditional buildings. Those "crumbling" historic mortars had probably been doing a good job after all!

Despite the growing knowledge base available to them, many builders remain wary of lime mortars and renders. This mainly due to bad experiences as a result of inadequate knowledge and training. Lime mortars do need more mixing, more careful application, and more protection from the elements than cement mortars. Cement mortars undoubtedly have their place, and historic cement structures, especially reinforced concrete structures from World War 2 are certainly providing some interesting problems for conservation specialists, but we need to learn from the past if we are to safeguard the heritage of the future.

The process of repointing

Raking out or picking-off existing cement mortars needs to be carried out with great care being taken not to damage the stones or bricks. The use of air chisels, saws and angle grinders is usually best avoided because of the danger of overruns and widening joints. Where hard cement or hydraulic lime mortar joints are very thin or deep, it can sometimes prove impossible to remove the cement without undue damage to the stones or bricks, and other approaches may have to be considered. To ensure sufficient 'key', as a general rule, joints should be raked out to a depth of at least 25mm or twice their width, and flushed out with fresh water immediately before repointing.

While failed mortar, organic material, earth and roots should be removed, particular care should be taken to preserve any surviving historic mortar or patches of render. Where such original lime mortar remains, some suppliers can analyse samples and advise on compatible sands and probable original mix proportions but this is an imprecise art, as the composition of historic mortars can change considerably over centuries of weathering, due to leeching of free lime and recarbonation. Historic corework, pointing and renders may also have been carried out using different mortar mixes. The overriding principle of specifying lime mortars for repointing is that they should always be slightly softer than the stone or brick of which the wall is built. This will ensure that the mortar decays as it is designed to, rather than the stone or brick. Generally tubs or bags of hydrated lime putty, obtainable from specialist suppliers should be used rather than dry bagged lime hydrate commonly available in builders merchants, which may have only a low proportion of reactive lime. Ready mixed lime mortars, or 'coarse stuff' are also available from some specialists, but care should be taken to ensure that the sand used is a proper match to the lime mortar originally used. Historic buildings were invariably constructed using easily available local sands and these will give a distinctive colour to the mortar. Mortar mixes are usually in the proportion of 1:3, binder:aggregate. This is because the free space in a well graded sand (ie with grain sizes distributed between c.5mm and 5microns) is approximately a quarter of its volume, so that if this is filled with lime, the sand will be well bound together. Additives, such as colourants, plasticizers and retardants should generally be avoided in traditional mortar mixes, as they may alter the qualities of the lime and in the case of colours, will soon wash out. If the mortar needs to be specially durable, for example on high chimneys or rooftop features, which are subject to heavy frosts, pozzolans, such as ground brick or tile may be used to give the lime an element of chemical set, but hydraulic limes are probably a more traditional approach, ie lime mortars that have a natural chemical set, like the ‘water limes’ Smeaton experimented with. Sometimes, white cement can be mixed with lime mortar to provide a strong chemical set without affecting the natural colour of the sand, but should be used in a proportion at least equal to 50% of the lime content to ensure that chemical bonding can take place.

It is always wise to undertake some test panels to assess the appearance of the proposed repointing, ideally leaving the test in place for a winter to ensure frost durability, thought this is seldom practical. It is essential to protect lime based mortars after pointing or rendering, from excess dampness or drying too fast, to enable carbonation to take place gradually. To help this, lime based mortar are usually applied in layers no more than c.25mm thick, pressing the new mortar to the back of the joint. For narrow or deeply eroded joints, specially shaped pointing key or tamping rods may have to be used. Where joints are wide, small stones called pinnings or galletting may have been used to reduce the risk of shrinkage cracking in the mortar and it is helpful to restore these where this is the case. The surface of the mortar is usually stugged, beaten or brushed once it has started to harden, using a stiff bristle brush to roughen it and increase the drying surface area. Damp sacking, geotextile fleece and plastic sheets or tarpaulins can be used to prevent over-rapid drying, and these can sometimes be soaked in water and hung like curtains on the scaffolding c.100mm from the face of the wall: If they touch the wall, they can lead to staining. Wall heads dry particularly fast and need covering with damp fleece or sacking, but care should be taken to ensure that water does not run down the wall face and wash out lime, as this can lead to disfiguring lime stains which are hard to remove.

Many publications are readily available describing the correct use of lime mortars, but considerable care is still required in selecting a contractor competent to undertake such work, as opposed to those who just say they can. There is a growing number of suppliers offering a range of lime based products, from hydraulic and non-hydraulic limes to pre-mixed mortars and plasters, lime washes and distempers, and care is also required in specification. Repointing or re-rendering can alter the character and appearance of listed buildings and while consent is always required for repointing scheduled ancient monuments, listed building consent, conservation area consent or other approvals may also be required; the local authority conservation officer should be able to advise as to what consents are required. As conservation architects with wide experience of traditional buildings, we are always glad to advice in projects where lime is used.

ROBIN KENT

This article is based on published articles in The Building Engineer, Oct 1995 and Building Products, Sept 1995.

NOTE | This article is copyright. No responsibility is accepted for errors or omissions. It provides pointers to general principles and should not be viewed as a comprehensive guide. Each historic building will need separate consideration.

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