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Lime from limeys

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Hydraulic Lias Limes (still known to some as Blue Lias Limes) has just received an Agrement Certficate for a hydraulic lime*, called hl2, the first uk company to achieve this. It recently held an open day to announce this - and also because it was a little stung when the aj pointed out that to get standardised, commodity hydraulic lime you had (then) to go to suppliers like St Astier in France (aj 7.1.99). A British-cum- Euro standard is on the way. In the meantime an Agrement Certificate will do nicely.

One of Hydraulic Lias Limes' reasons for this move is the belief that although much of the craft knowledge of using lime has been lost, lime mortars and renders have significant advantages today. We just need the confidence to use these materials. Traditional lime-use methods do still have a place, for example in making lime putty in conservation work. But this is hardly a robust enough technology to survive the likes of a d&b contract. To spread lime use to new-build and refurbishment generally, a standardised material is needed that can be part of today's calculated, engineered buildings and yet is sufficiently like a bag of cement to be able to survive in our often low-skilled industry. Research is starting at Bristol University to formalise the engineering knowledge further. Constructionally it can be done, as the Plymouth building by Form Design illustrates. Agrement certification also addresses some of nhbc's concerns.

It is true that standardising and widely distributing a particular hydraulic lime will overwrite some of the historic local textures of regional limes. But that is a small loss compared to the switch to cement-based materials. You could have made the same argument about slates when coasters, canals and railways opened up a national slate market. But today, slate's ubiquity is one accepted part of our Glorious National Heritage.

So what do you get with lime? The main benefits for many architects will be the accommodation of movement and consequent lack of cracking compared to cement-based materials, allowing large areas of masonry and rendered work to be built without movement joints. There are cost benefits here to be set against the fact that, bucket-for-bucket, lime mortar cost is around twice that for cement mortar.

Using sharp enough sands is important in both performance and surface appearance. Sourcing sharp sands is a problem in some areas, and tradesmen can react against the unfamiliar feel. The appearance, though, speaks for itself.

Detailing and mix proportions need reconsideration. Minimum working temperatures are a little higher than for cement-based mortars. And the slower set of lime-based materials can be an issue, though one that is often overrated. Broadly, the more hydraulic the lime, the faster the set. Some uk research looks likely to show that the common French contractors' practice of adding a little cement to a lime-based mix so as to speed up set risks reducing longer-term performance.

Will lime rejoin the mainstream? This certification can only help.

hl2 Blue Lias Hydraulic Lime. Agrement Certificate 99/3581. British Board of Agrement, tel 01923 665300; email bba@btinternet.com

For Hydraulic Lias Limes, tel 01935 817220; email hl2@limesolve.demon.co.uk

PIC CREDIT ERIC THOMPSON

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