Low-energy drying technique for lime mortar
A low-energy drying technique for aggregates in hydraulic lime mortar has been developed by a UK consortium. The research aims to make lime mortars commercially viable for new build projects, as they have the ability to make bricks easier to recycle and contain 40% less embodied CO2 than cement mortars.
‘Bricks are one of the most durable building materials and can be recycled when the building has come to the end of its use. However, when bonded with cement-based mortars the bricks cannot be removed, without causing damage,’ explains Peter Walker, Professor of Innovative Construction Materials at the University of Bath, one of three academic partners including the Universities of Bradford and Bristol. ‘This is not the case with hydraulic lime mortar, which is strong enough for modern construction, but sufficiently soft to be removed from bricks, allowing them to be recycled.’
Hydraulic lime mortar has greater water vapour permeability and capacity to accommodate movement, as well as lower embodied energy content. However, fast setting stronger cement-based mortar has largely displaced lime mortar since its predominance before the Second World War.
‘This is because, traditionally, the sand used in hydraulic lime mortar must be kiln dried prior to blending with hydraulic lime. This is a time and energy intensive process,’ explains Walker. ‘Our approach relies upon the chemical combination of water and quicklime calcium oxide to yield calcium hydroxide, a chemical reaction that produces heat. Drying is a function of hydration of the lime to calcium hydroxide, and evaporation due to the exothermic reaction.’
Using quicklime to dry aggregates is not a new idea, but because calcium oxide is such an unstable material, it is important that the mixing process is carefully controlled. ‘The amount of quicklime required will vary with sand moisture content and desired mortar mix,’ explains Professor David Hughes from the Department of Environmental and Infrastructure Engineering Research at the University of Bradford. ‘As the mortar mix designs become leaner, increased quantities of quicklime will be required to dry sand, with an associated reduction in the hydraulic lime component.’
To accurately measure the quantity of calcium oxide needed, Hughes calculated the percentage evaporation required against the percentage stoichiometry of the quicklime. As percentage evaporation reduces, more calcium oxide needs to be added. For this reason mixing cannot be done onsite. Instead, controlled mass batching of all the materials is achieved in a silo and a wet mix is extruded onsite.
The process is as safe and cost effective as working with cement and traditional lime mortars, claims the team. Lime Technology, based in Maidstone, (one of the many industrial partners that also include the Brick Development Association and Ibstock Brick) is taking forward a patent for the controlling and mixing process.