Hydraulic lime
Durability Guaranteed - Pulhamite Rockwork - Its conservation and repair, published by Historic England in 2008, defines hydraulic lime as: ‘A form of lime which will set and harden under water, primarily through chemical reaction with the water (in contrast to non-hydraulic limes, which harden by reaction with carbon dioxide in the air; see Non-hydraulic lime). Hydraulic limes contain varying amounts of calcium silicates, calcium aluminates and calcium hydroxide, produced either by the burning of clay-rich (argillaceous) limestones or by the addition of various hydraulic materials.’
Short Guide: Traditional Scottish Brickwork, published, on 1 March 2014 by Historic Environment Scotland, defines hydraulic limes as: ‘Limes which give a chemical set that is quicker and harder than the carbonation of pure limes.’
The Society for the Protection of Ancient Buildings (SPAB) in their online Glossary define hydraulic or water lime as: '(i) Natural hydraulic limes are prepared from limestone with reactive silica and alumina impurities. The lime reacts with the impurities in the presence of water to form calcium silicates and aluminates (cf non-hydraulic lime), in addition to the calcium carbonate that contributes to the hardening process when exposed to the air. They are classified under BS EN 459: Part 1: 2001 as NHL 2, 3.5 or 5, in order of increasing strength.
(ii) Natural hydraulic limes with an additive are classified as NHL-Z, while HL denotes an artificial hydraulic lime. Since manufacturers do not have to state whether the additives include Portland cement or what the percentages are, these products should be avoided in conservation work.'
See also: Non-hydraulic lime and Natural hydraulic lime.
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