Here is an image of the 3 types of lime that we’ve used over the years. They are all readily available from all the builder’s merchants in our region, although the brands and specs will vary slightly in different regions of France. By the way, CHAUX is the french for lime and is pronounced like “SHOW”
1. CHAUX RABOT : This is the standard hydraulic lime in our area. “HYDRAULIC” simply means that the lime is not pure CaO, but has other silicates and carbonates added to render it hydraulic—i.e., when mixed with aggregates (sand/gravel) and water, the mix will “dry” or cure even in the presence of water. In fact it is water that initiates the curing process. NON-HYDRAULIC lime by contrast is a purer lime, no added silicates etc, and “dries” or cures by the action of carbon dioxide in the air. So this materials will NOT set under water, unlike, cement or strong hydraulic limes. The strength of hydraulic limes are rated on a scale of 1 to 5, from weak to strong, so the Chaux Rabot shown above is rated NHL 5 — strong. We use this lime for all general stone and block laying, (never use cement mortars) and also for basic first coat wall pointing and renders.
2. CHAUX BLANCHE : This is a weaker hydraulic lime, pure white in colour and therefore useful if you want to pigment the mortar or render—it’ll take the colour better than Chaux Rabot, which has a warm pale grey colour. Being weaker, it is better used in wall pointing and render, rather than structural work, and being more expensive, is best restricted to more specific interior finishes and decorate work.
3. CHAUX AERIENNE : Traditionally this lime has been used for final finishes and lime washes. When using lime mortars for external stone pointing, the best practise is to first point up the brushed and cleaned out joints between each stone with a medium to stong hydraulic lime mortar, then brush that back after a few hours before it sets too hard, then apply a further two layers as smoothing renders, the latter being ‘weaker’ than the former. Chaux aérienne would be used for this last coat. But this is labour intensive and quite time consuming so nowadays, is retricted to special renovation work. We tend to point the jointing first and then point again with the same (hydraulic) mix, wire brushing back as soon as the cure is dry enough. (‘ll link this later to a post on Pointing). Denise however, makes good use of this lime for interior wall finishes—it’s very versatile and she’s already used it on top of clay renders.