The Building Conservation Directory 2020

134 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 C AT H E D R A L C O MM U N I C AT I O N S over time. In this process carbon dioxide is absorbed from the atmosphere, and the lime reacts with it in the presence of water to form calcium carbonate. This will not occur without air or water, and deep within a wall it can take years to react. Alite and belite on the other hand, form a chemical set on addition of water and must be damp-cured to develop strength. Both are varied in chemistry due to the minor components found in the source rocks, as well as calcining temperature and residence time in the kiln, but belite is typically simplified to di-calcium silicate, which reacts as per the following simplified equation; 2 (CaO) 2 ·SiO 2 + 8.6 H 2 O  3.4 (CaO)·2(SiO 2 ) 2 ·8(H 2 O) + 0.6 Ca(OH) 2 Belite+ water  calcium silicate hydrate + calcium hydroxide Alite is typically simplified to tri-calcium • compacting the mortar as it begins to stiffen increases the density and final strength once carbonated, and will also enhance other final mortar performance metrics. On the downside, air lime mortars need more water than other binders, which will increase plastic shrinkage through water loss. However, if plastic shrinkage is managed through tempering or reworking, larger visible cracks shouldn’t occur. Air lime mortars should be kept slightly damp in their infancy to promote carbonation as both water and carbon dioxide is required as part of the chemical reaction, but not too wet as the water can impede gas diffusion, slowing carbonation. Mortars made with binders which are primarily blends of lime, belite or alite have a hardened structure which is dependent on the water/binder ratio when the material sets. Binder/aggregate ratio and aggregate properties also have an effect; a higher binder content increases the strength and decreases the porosity, the level of decrease is dependent on the belite/alite ratio with higher alite contents being much less permeable. The aggregate minerology and grading can have varying impacts on strength, water/binder ratio and shrinkage. Belite takes a long time to develop strength; two to three years is a realistic timescale to achieve close to the final strength. Belitic mortars should be kept damp in the early stages to ensure full hydration of the hydraulic components and to aid in carbonation of any lime content. Alite develops strength quite quickly, it’s typically accepted that 28 days of damp curing silicate which reacts as per the following simplified equation; 2 (CaO) 3 ·SiO 2 + 10.6 H 2 O  3.4(CaO)·2(SiO 2 )·8(H 2 O) + 2.6 Ca(OH) 2 Alite + water  calcium silicate hydrate + calcium hydroxide Both belite and alite form the same resultant compounds; calcium silicate hydrate and calcium hydroxide. If you compare the equations you can see alite produces more calcium hydroxide than belite. When adjusting for density, alite produces approximately 3.2x the calcium hydroxide produced by belite. Over time calcium silicate hydrate, the predominant compound from alite and belite reactions, will carbonate to form calcium carbonate and silica gel. This means that samples which were hydraulic limes used hundreds of years ago may now be interpreted as non-hydraulic or feebly hydraulic mortars through analysis. COMPARING BINDER COMPONENTS Air lime mortars are the lowest density and the weakest, and will give the highest vapour permeability – often desirable characteristics when used in the repair of a fragile historic masonry. Their hardened structure is relatively independent from water/binder ratio as they first stiffen through loss of water before slowly carbonating. Binder/aggregate ratio, aggregate properties and compaction dictate mortar performance: • higher binder contents giving a more porous material • limestone aggregates have been shown to increase both porosity and physical strength Above; high level repairs are invariably exposed to a greater degree of attrition from wind and rain and benefit from a hydraulic set, as here at St Michael’s Mount. Below; Huer’s Hut, a lookout above Newquay, Cornwall was finished with a lime shelter coat. (Both photos: Cornish Lime Company)