Lime mortar: The 21st Century technology that is 1,000 years old

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Media captionAn old technology could help Hobkirk parish church survive into the future

Hobkirk parish church stands just outside Bonchester Bridge in the Scottish Borders. It is a matter of yards from the Rule Water, a matter of miles from the Border.

The minister here, the Reverend Douglas Nicol, says Hobkirk's story is a long one.

"We're on a site which goes back to the eighth or ninth century," he says.

"We've evidence of two earlier buildings on this site, including a Celtic one.

"But the building we're in was built 150 years ago and we're celebrating that anniversary in a month or so's time."

Now the church is an unlikely testbed for leading edge science - a new project involving Historic Scotland and Heriot-Watt university to ensure that it and other historic structures remain standing for centuries to come.

With funding from the innovation agency Knowledge Transfer Partnerships they're working to turn a 1,000 year old recipe into a 21st Century building material.

It's called lime mortar. It holds together most of Scotland's historic buildings and 20% of our housing stock.

And it has properties that make it ideal for repairing and maintaining older buildings.

The Romans

The moment humans put up a building, nature starts trying to take it back down again. Wind, rain and frost eat at the stones and the stuff that binds them.

Lime mortar's advantages were its flexibility and breathability. It provides a more forgiving environment for old stones and timbers. It allows old structures to let water evaporate.

The Romans brought it with them when they passed this way and it has been in continuous use in Scotland for around 1,000 years.

But in the wake of WWI lime mortar went out of fashion. Many of the old skills had been lost in the conflict and builders started using cement mortar instead.

Cement mortar is stronger, making it ideal for big civil engineering jobs. But it is also inflexible. It doesn't let the building breathe, and after a couple of decades it can start to crack and fall away. In the meantime the fabric of the building is suffering.

As a result lime mortar has been making a comeback for some years now. This new collaboration is aiming to take it further by putting it on a scientific footing.

The project will test various lime mortar mixes for their strength, how they handle moistures and by how much they resist deterioration. The findings will be made available to the building and conservation industries.

It will also investigate one of its most intriguing properties: as it sets, lime mortar pulls carbon out of the atmosphere.

'Environmental agenda'

According to Dr Alan Forster of Heriot-Watt's school of the built environment, "all lime mortars have the ability to sequester - or take in - carbon dioxide as part of their setting process.

"A lime mortar will not really set hard without the reintroduction of carbon dioxide.

"And carbon dioxide sequestration is obviously on everybody's environmental agenda at the moment."

The project supervisor is Historic Scotland's technical research manager Roger Curtis.

He showed me some fresh lime mortar pointing between Hobkirk's old stones. It is pale pink and looks much more at home than some of the hard and cracked cement mortar nearby.

"This is going to be good," he says, "because we're going to be able to put quite a lot more science behind the intuition by quantifying more accurately the embodied carbon that goes into the manufacture of the product, the amount of carbon sequestration that the curing of the lime will do, and by the longer lifespan of the material in the wall.

"The research with Heriot-Watt will quantify that a lot more."

It is work which could see as many as 500,000 Scottish buildings repaired using new versions of old methods. And it is likely to pay its way.

It is reckoned that a successful completion of this collaboration between science and conservation could see Historic Scotland start saving on its repair and maintenance bills in as little as three years.

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