Barry Hunt considers the use of limestone as a building stone as he continues a series of articles exploring the main categories of natural stone used in construction, following his introduction to the series in last month’s Natural Stone Specialist.
We begin our journey through building stone with a look at limestone, probably the first widely used building material when you consider that most pre-historic caves are found in limestone massifs (a massif being a defined section of the Earth’s crust).
Some of the most famous building stones in the UK are limestones – Portland and Bath probably being the best known by the general public.
Portland has been used to build much of London since the Great Fire of 1666, including Christopher Wren’s St Paul’s Cathedral. And, of course, Bath stone has been extensively used in the city of the same name. But there are many more limestones available, both indigenous and imported.
Limestone is, ideally, easy to extract from the ground and to shape in the workshop. In latter years, diesel and electric power, in conjunction with hydraulics and pneumatics and, most recently, computerised controls have supplemented muscle and brain power. This increases the efficiency of extraction, processing, transporting and fixing stone, helping to keep its price down and make it competitive with other building materials – especially when the whole life cost is considered. Stone has a particularly low maintenance long life.
Some of the country’s limestone cathedrals might need constant attention, but they were built perhaps 1,000 years ago and much of the stone in them dates from that time, even if some of the carvings and mouldings have been re-worked from time to time.
Some of the oldest stone buildings in the world are limestone – the pyramids in Egypt, and structures found on Gozo and Malta that date from 6,000 years ago.
Limestone is most easily worked and weathers better when its properties are roughly equal in all directions, when it may be termed ‘freestone’. When masons are preparing limestones they are concerned about the ease of working them, their ability to hold a good edge (arris) and that there be no obvious splitting planes.
Historically, people have built with whatever stone was available locally, which has resulted in a wide variety of stone being used. Much of it is no longer available, which can make building conservation difficult when the ideal solution is to replace like-with-like.
The heritage sector is always seeking to determine the true character of the original masonry of buildings and the mortars used with it in order to ensure any repairs are sympathetic.
Fitness for purpose
It is not only warm cream and honey toned limestones, such as the Cotswold limestones, that are appreciated for building, but also the crisp whiteness and light greys of limestones that are livened by subtle natural variations in shell content, grain size and constituents other than the Calcium Carbonate (CaCO3) that constitutes pure limestone.
Limestone is ideal for large and persistent features (cornices, string courses, friezes, columns, plinths) that adorn buildings. It is also occasionally used for external paving. There was a lot of the local limestone used for the paving on the docks on the Dorest Island of Portland for the Olympic sailing events that were based there last year. It is more commonly used for interiors, although there can be issues of slip resistance, especially with imported polished hard limestones and if the floor becomes wet or greasy.
Limestone is absorbent and can become quickly stained, which is why floors, especially in commercial applications, are normally sealed thoroughly (and there is no shortage of sealers to choose from) and subject to regular maintenance to keep them sealed and cleaned.
The building engineer is usually largely concerned with the strength of stone in relation to the structure it is being used for. The best building limestones do have relatively high strength… but there is a limit.
This may seem an odd statement until you realize that many of the best building limestones have been geologically weathered over millions of years. This has removed most of the potentially adverse constituents within them but also a little of their strength.
Stronger but less geologically weathered stones, such as the blue Lincolnshire limestones, may hide compounds such as pyrite that can decay rapidly once the stone is removed from the ground and they are exposed to the atmosphere. In some circumstances, the reaction can physically tear the stone apart.
Portland and Bath stone generally exhibit compressive strengths of between 30 and 50MPa (megaPascals), which is similar to good quality concrete and more than sufficient for most purposes.
Limestone is typically impressively strong under compression, like concrete, but exhibits flexural strengths of around one tenth of the compressive strength. This limits its use in construction.
The massive limestone cathedrals of the UK and elsewhere across Europe have certainly survived many centuries but they rely on their sheer size and the use of arches and buttresses to transfer lateral loads into compressive loads for their success.
This way of building changed in Georgian times with the arrival of ashlar. As ashlar, stone provides a smart exterior without having to support the building superstructure. Ashlar is typically at least 70mm thick. More recently, even thinner stone has been used, with anything below 70mm being regarded as cladding.
Modern stone cladding panels generally need to be supported individually with fixings tied back to the building frame. The cladding panels will be subject to wind buffeting that will exert forces that pull the stone panels away from the building as well as pushing them on to it, especially higher up on a building. That clearly places a flexural strain on the panels.
When limestone is used for cladding it is vital that both its properties and those of the fixings being used are carefully assessed. The whole system’s ability to resist wind loading and thermal movement must be carefully calculated and a considerable margin of error allowed for.
Reducing panel sizes or increasing the fixing support will often resolve any problems but will increase the cost and might run contrary to the architect’s aesthetic, with increasingly large cladding panels being specified.
Working practices and designs have changed a lot over the years. Stone and our expectations of its performance have not.
For some, buildings are not supposed to weather. They want buildings to look the same 20 years on as they did the day they were first opened. This is unrealistic for most building products and certainly will not be the case with limestone.
Not only is the hope that limestone will remain the same unrealistic, it also fails to tap into the real beauty of using stone – that it improves with age.
The durability of limestone
Limestone is undoubtedly durable – most of it is millions of years old before it leaves the ground. But from the moment we extract it and stand it up in the atmosphere it starts deteriorating. This is compounded by our wish to use stones in environments that previous generations would have considered unsuitable. Now, more than ever, we have to consider potential durability.
Limestone is just a heap of calcium carbonate crystals that will readily dissolve in acidic environments. But what makes one limestone fall apart in a frost while another remains unaffected?
Geologists know the answer: the difference in grain size, shape, texture… in fact, all those natural features that are so familiar to a geologist.
But if it is so easy to understand, why is it we still cannot accurately predict the behaviour of limestone in use – even though there are many tests available for assessing the stone?
The simple answer is that tests rarely mimic real life. Or, if they do, they only assess one property of real life rather than the whole environment.
Some limestones, such as Portland, have performed for hundreds of years in the harsh London environment, yet when they are subjected to a freeze-thaw cycle test they can fall apart half way through.
The stone world is saddled with inappropriate tests that have become standard because no better alternative has yet been devised. Unfortunately, architects and engineers are not always aware of this problem and seek easy answers that are not always readily available for stone.
Stripping limestone back to its basics, it is difficult to go wrong with determining the basic properties such as strength, density and absorption.
Determining the absorption under vacuum gives us an understanding of the degree of microporosity, which allows us to assess how much of a buffer a limestone has when it freezes while saturated.
Water expands as it freezes, which compresses as yet unfrozen water remaining in the stone. The only way to release this pressure might be for the water to migrate into the micropores not filled under normal saturation conditions.
If a limestone has no micropores available for the water to migrate into it could be suspect, although this will also depend on the strength of the stone to resist bursting.
Further complexity arises from how the pores are connected and the ability of water to move through them and drain out from the stone.
In simple conclusion, limestone can be used anywhere… as long as it does not become saturated and is then subjected to frost attack.
Does this sound too simple? Of course it is. But it is a pretty good principal to start with and is the main reason why granite and other more durable stones are used for plinths, where the stone is more likely to be subjected to moisture rising from the ground and splashed on to it by passing traffic.
The more microporous a limestone is, the longer it will hold water, which means it might be more susceptible to the deposition of dirt, which tends to stick more easily to wet, rather than dry, surfaces.
The other major problem facing limestone is salt. On exteriors, this might be from pollution, sea-spray and surrounding materials such as concrete and brick. On interiors it could be from cleaning materials or even simply from respiration if lots of people congregate in the area (over many years, people breathing can give rise to high concentrations of salts).
Salt damage requires a specific concentration to produce crystal growth, so a design should try to limit areas where concentration can occur.
Salts are usually first deposited on exposed surfaces. The salts are then concentrated by moisture flow, either down the stone face or through the stone.
Studies have shown that salt concentrations increase lower down buildings because moisture generally is flowing downwards under gravity.
However, salts also concentrate in sheltered areas where they are not washed from the face. Salt laden moisture can be sucked into these sheltered, drier zones. Cornices, string courses and other protruding features are designed to throw water from a building’s vertical surfaces, providing shelter, but in doing so become the focus of salt attack.
Salts can cause discoloration problems as certain sodium and potassium based alkali hydroxides can dissolve simple organic materials, such as tannins. While there are only traces of them, if they become concentrated at the stone surface they can lead to dark brown or even almost black discolorations.
Portland cement can be a source of sodium and potassium alkali hydroxides and limestones should never be used in combination with Portland Cement or where backing or substrate concrete materials that contain Portland cement could leach into the limestone.
Often, when stone is left alone it finds a happy medium of its own, although sometimes it can benefit from a little help along the way to reaching that point.
Limestones in Britain
Armed with all these facts, what are the good building limestones of the UK?
Well, just take a look at a geological map of the UK and search along the Jurassic band stretching from Dorset up to the Wash and then on up to Tyneside.
The list of building limestones in the UK is extensive. It includes the already mentioned Portland and Bath stones that are so well known outside of the stone industry as well as within it, but also many other great stones. There is Ancaster, Chilmark, Clipsham, Cotswold, Doulting, Ketton, Purbeck…
Running parallel to the northern part of this band are the Permo-Triassic Magnesian limestones that include Cadeby and White Mansfield. We must not forget the likes of Hopton Wood, one of the few Carboniferous limestones, which has proved good enough for the Geological Museum (now the Earth Galleries of the Natural History Museum) in London.
You can find the limestones currently in commercial production listed with contact details in the Natural Stone Directory, which can be bought at www.naturalstonespecialist
It is no coincidence that many of these stones are oolitic or have an underlying even grain size, making them easy to work and pleasing on the eye.
The only real exception in building stone terms is Chalk, which in its hardest form is used as a building stone and colloquially is referred to as ‘Clunch’.
Clunch is softer than many other limestones but its different characteristics provide it with a reasonable resistance to weathering when it has been allowed to season, although allowing stone to season is now unusual in the stone industry because of the commercial pressures. Clunch misleadingly falls apart in the various durability tests.
The Table (left) lists some of the standard tests that are applied to limestones and gives an idea of performance for the results obtained. The table is not a comprehensive guide and some stones may fare better or worse than the results shown indicate.
In conclusion, limestone is a good all-round building stone with many advantages that make it possibly the most user-friendly stone available.
Aesthetically it provides a warming, natural feeling that is able to grace the greatest structures but is not out of place around the home. It will last for centuries when selected and treated appropriately.
Next time Barry Hunt turns his attention to sandstone.