Lime crumble

What links St Paul's, Lincoln Cathedral, Oxbridge colleges and many historical buildings across Western Europe?

Give up? All were constructed with limestone, a material whose long-term weaknesses could ultimately reduce all these buildings to piles of rubble. Already, the walls of Worcester College, Oxford are showing a patchy limestone display.

As you may already know, limestone can ravage a building to a state of disrepair. In fact, if you look around your own town, chances are you have already stumbled past decaying limestone.

Big clues include hard black crusts, scaling or cavities munching into churches, castles and cathedrals.

Not only is it unattractive to look at and touch (it will crumble in your hands), but also costly to restore. More alarmingly, if untreated, a building could easily fall apart.

So why does it happen? Limestone has been used as an effective building material for hundreds of years, but it is severely affected by pollution and climate change.

Researchers know that chemicals such as sulphur dioxide and nitrogen oxides from air pollution react with the stone to make it dissolve. Bad weather such as rain or snow causes problems and equally harmful is road salt, which eats into limestone. But could there be other causes?

Researchers also understand the problem is not isolated to certain areas of the country or one type of stone. But it remains a pressing problem, and any possible solutions would have a positive impact in preserving the country's heritage for future generations.

Cracking the problem

So what can be done to save some of the country's much loved buildings?

Well, a crack team of academics, civil and electrical engineers from Oxford, City and Queen's (Belfast) universities are spearheading a study that hopes to do just that. Their groundbreaking research aims to understand why limestone decays and pave the way for possible solutions.

This project is examining oolitic limestones that are formed from small spherical grains of rock embedded in a matrix. The mineral contains calcite, which is highly prone to chemical attack by acid rain.

Although basic knowledge exists on why limestone causes damage, the stone appears to act unpredictably. For example, a limestone building may show accelerated signs of crumbling in some of its blocks, but not in others. This suggests that there may well be something in the limestone itself that causes this to happen, says team member Professor Ken Grattan of City University.

This research is vital to enabled action TO be taken before decay spirals out of control

Heather Viles, Oxford uni

"The question is why?" he says. "Can we learn something from the way that the limestone has been cut, the source of the limestone and the other impurities and features of the limestone?"

Dr Heather Viles of Oxford University Centre for the Environment, who is also working on the project, believes the research is crucial.

"This understanding is vital to enable action to be taken before decay spirals out of control, but also to ensure conservation decisions don't lead to unnecessary block replacement and expense.

"As well as informing anti-decay strategies, our research will generate knowledge about types of limestone best suited to particular environmental conditions, and so will benefit renovation and new-build projects."

City University is developing compact fibre-optic sensors that will monitor how limestone is affected by a number of factors including traffic pollution, road salt, temperature, humidity and wetness. The technology will also be able to detect subtle changes in the stone caused by changing moisture levels and salt movement.

The sensors will be inserted into a boundary wall at Worcester College, and possibly other limestone structures around the country, so comparisons can be drawn.

Information will be fed by the sensors, via fibre-optic cable, to data loggers and analysed to see how decay correlates with environmental factors and with the limestone's physical, chemical and mineralogical characteristics.

The three-year project will cost just over £500,000 and is being funded by the Engineering and Physical Sciences Research Council (EPSRC). Work is already under way.

And its significance cannot be overstated. Grattan says: "If we know what the problem is, then at least we can begin to see if we can find a solution. Clearly the key thing is to be able to conserve these blocks - to ensure that any damage done can either be reversed or at least arrested."

As yet the solution is unknown. One idea being mooted is that the blocks could be subjected to some kind of chemical that would react and stabilise the erosion.

Whatever the real solution may be, the team hope they can get results.

"A good achievement would be to produce the right sort of information to understand what is really happening. As a result of that information we could enable our civil engineers and geographers to take corrective action so that this problem could be solved and the difficulties that arise could be alleviated," adds Grattan.