The Building Conservation Directory 2020

PROTECTION & REMEDIAL TREATMENT 4.1 133 C AT H E D R A L C O MM U N I C AT I O N S T H E B U I L D I N G C O N S E R VAT I O N D I R E C T O R Y 2 0 2 0 REACTIONS, SETTING AND CURING Quicklime is calcium oxide; the most commonly available in the UK is CL90 calcium oxide – the 90 refers to the fact that it has a calcium oxide content of 90 per cent or higher. When water is added the quicklime slakes, releasing a large amount of heat and forming a CL90 calcium hydroxide. ‘Air lime’ is the term commonly used to describe a CL90 calcium hydroxide, but the material is also known as ‘putty lime’, ‘non-hydraulic lime’ and ‘fat lime’. When used in a mortar this binder stiffens after placement through loss of water, it does not ‘set’ or ‘cure’ and only hardens by carbonation The table shows hydraulicity increasing with increasing clay (and/or silica) content, from air limes on the left (up to 5%) to eminently hydraulic limes and cements on the right. The ‘lime limit’ is where any increase in the proportion of silica/clay begins to have little or no effect on hydraulicity because there is no more lime to combine: everything to the right of this is therefore a natural cement. (Table reproduced by kind permission of the Louis Vicat Foundation) TERMINOLOGY LIME a word with multiple meanings; in construction it refers to calcium or magnesium salts, including carbonates, oxides and hydroxides. It is often used contextually or with additional descriptive words to differentiate between them, such as hydraulic lime, slaked lime or quicklime. CALCINED STONE a stone which has been chemically altered by heat often with a limited supply of oxygen. In the case of limestone, water and carbon dioxide are driven off to form oxides, and hydraulic compounds can be formed once the right temperature is reached. HYDRAULICITY the capacity that certain binders or their ingredients have for hardening under water NATURAL HYDRAULIC LIME (NHL) a binder with hydraulicity derived entirely from silica/clay minerals naturally present in the original limestone POZZOLAN a predominantly silcacious material, sometimes containing aluminous reactants, which have no binding power by themselves but react with calcium hydroxide to form hydraulic compounds. CEMENT any hydraulic component can create a ‘cementing’ material but that does not make a binder ‘cement’. A cement is an entirely hydraulic binder; one that sets with water principally and that does not require slaking after manufacture. THE MAIN CONSTITUENTS OF LIME AFTER CALCINATION UNBURNT STONE Mostly limestone or silica, raw stone which hasn’t reacted or calcined in the kiln REACTED LIME Calcium oxide: on mixing with water this should slake over time to produce calcium hydroxide DICALCIUM SILICATE Also known as C2S or Belite, this is the main hydraulic component in natural hydraulic limes (NHLs) and natural cements TRICALCIUM SILICATE Also known as C3S or Alite, this is the main hydraulic component in Portland cements TRICALCIUM ALUMINATE Also known as C3A, this is typically considered undesirable in hydraulic binders as it can lead to flash setting, high heat generation after initial hydration and sulphate attack TETRACALCIUM ALUMINOFERRITE Also known as C4AF or Brownmillerite, has very little effect on binder performance by itself, but can give a weak early set and helps accelerate other parts of the binder changes in trace elements can have significant impacts on final hydration products and crystallography. The main constituents of lime (including hydraulic limes) after calcination are shown in the table above. The proportions may vary, but natural hydraulic limes will usually contain everything in this table. However, most of the commercially available limes in Europe at the moment contain insignificant quantities of C3S, C3A and C4AF. The strength of the lime predominantly depends on the ratio between belite (C2S) and calcium hydroxide along with both the particle size and the amount of water required to make a workable mortar. Natural cements are essentially NHLs with very little or no calcium hydroxide remaining; the binders are predominantly belite. Ordinary Portland cements (OPCs) have changed over many years and are now predominantly alitic (C3S) binders with a proportion of belite. Historic OPCs burnt in vertical shaft kilns had different crystal structures to modern OPCs and higher belite contents, causing historic cements to be dramatically weaker. Consequently, they created significantly less dense mortars than their modern counterparts. To put this in perspective; modern Portland cements can have a density between 1.4–2Kg/L, whereas historic Portland cements would have been closer to modern natural cements, like VICAT’s Prompt which has a density of 1Kg/L. Density differences influence the weight of binder per cubic metre as mortars are mixed by volume. 1700 1500 1300 1100 900 700 500 0 5 10 15 20 25 30 35 Hydraulicity index ((silicates + aluminates)/lime) burning temperature (°C) % clay content 0.1 0.31 0.65 0.5 air lime moderately hydraulic limes hydraulic limes eminently hydraulic limes ordinary Portland cement slightly hydraulic limes natural prompt cement natural Portland cement natural cement lime limit Compressive strength development in pastes of pure cement compounds over 100 days (Table: Sydney Mindess et al 2003)