A prophet is without honour in his own land. In parts of France the widespread use of natural lime in construction is not celebrated, but taken for granted. They never stopped. Contrast the uk, where advocates see lime as the true path, combating the demon opc. And while they claim to seek its popularisation, they take an apparent pleasure in belonging to a relatively closed community with its own arcane secrets, a modern lime guild.
To see this contrast, a small group was recently taken to the Perigord region by the St Astier company based there. St Astier is Europe's largest manufacturer of natural hydraulic limes, working on an industrial scale with an annual capacity of 100,000T. It is this approach, of producing lime as an industrial commodity, that could overcome some of the uk construction industry's objections to using lime more widely:
it becomes available in quantities that can fit in with tender programmes large as well as small
prebagged as lime or as mixes for mortar and render, it can fit more easily into current construction practice and skill levels
through industrialisation of production, lime emerges in a range of standard products with predictable performance, compensating for the current lack of British Standards, and encouraging its engineering use. (There is a proposed EuroNorm on natural hydraulic lime, pren 459.1/2/3.)
Burning limestone a tonne or two at a time (maybe even slaking it on site), as in the uk, will never achieve these.
It has to be said that St Astier is helped here by a massive underground limestone layer of unusually pure natural limestone. The current cut is 8m high, which can readily be extended to 12m. But the limestone layer is several hundred metres thick. It allows drive-in mining without despoiling the landscape. The limestone is blasted, trucked to an underground crusher, then hoisted to the surface kiln hoppers and burned, slaked and batched on site. Burning takes around two days, as does slaking in silos. It is mainly the time for burning that creates the three grades of lime produced; the raw material content is the same.
The most obvious difference between St Astier's three grades of lime is increasing strength and hardness and the accompanying faster setting. St Astier describes them as feebly, moderately and eminently hydraulic. Though using a uk classification developed by Stafford Holmes and Michael Wingate*, they prefer to be classified as moderately hydraulic (xhn 30) eminently hydraulic (xhn 60) and natural cement, such as Roman cement (xhn 100).
If these materials are relatively stronger in uk terms, they retain many of the advantages of using lime, notably the ability to build large areas without movement joints. For both masonry and rendering, this has become an increasing motivation for uk architects to use hydraulic limes.
For some of the prebagged mixes, St Astier also adds up to 15 per cent opc, which builders want to increase further the speed of set. Alternatively, some builders include cement in their lime mortar mixes on site.
More difficult to transfer to the uk in some ways is the need to use a well-graded but relatively coarse, sharp sand. uk tradesmen find this both unfamiliar and needing extra mixing of the mortar.
Lime in new work
One reason for the historical continuity in use of lime through many areas of France has been the continued popularity of rendered walling rather than brickwork. The underlying structure has modernised; it is often now single-leaf concrete blockwork. On one site of low(ish)-cost housing for a gendarmerie we visited, concrete blocks were being laid at normal speeds in a 1 cement: 3 lime (xhn 60): 9 coarse sand mix. The whole was later to be finished in a lime-based render, spray-applied in one 12+mm coat, trowel-finished.
Rendering is often one-coat, sometimes two. The use of small-scale spray rendering is an important part of the economics of the process. It is, however, unforgiving of poor workmanship; spray renderers are typically specialist subcontractors. St Astier reckons that four men can spray a 200m2 plan house in a day.
Several recent new buildings and refurbishments we saw also made effective use of mineral and earth pigments to create body colours. Pigmented limewashes are also available.
It remains the case that lime-based mortars and renders cannot compete with sand:cement mixes solely on price. But lime-based materials do have distinct uses. Industrialisation of manufacture and the ensuing standardisation of products should give the architect greater confidence to take up these options.
St Astier in the uk can be contacted through Ugo Spano, tel: 01372 465779.
Building with Lime. Stafford Holmes and Michael Wingate. Intermediate Technology Publications (tel: 0171 436 9761), 1977