Gwynne Anthony 2020

ART506 Dissertation 2019. Anthony Gwynne . 109 D2. Hydraulic lime Consists of limestone containing a natural proportion of clay in addition to calcium and magnesium carbonates, which is burnt in a factory process to produce chemical compounds similar to Portland cement, which are stronger but less workable than non-hydraulic limes. It hardens by chemical reaction with water and by carbonation. The higher the percentage of natural clay and minerals in the lime - the higher the strength and initial set times but the poorer the permeability and flexibility. A ir can pass through the open pours in a lime binder which makes it vapour permeable in a way that doesn't happen with a denser material like cement that is non vapour permeable (www.lime.org.uk ). Hydraulic lime mortar mixes The lime used for re pointing and building has to be mixed with aggregates to match the existing mortar in the required ratio depending on the type of stone or brick and degree of exposure in accordance with the guidance table below. Mortar must not be allowed to dry out too quickly and surrounding masonry must be kept damp. Pointing should be kept moist for 7 days- the carbonation set can only complete in the presence of moisture. Building can be carried out at the same rate as Portland cement, depending on hydraulic lime used and weather conditions. Hydraulic lime is more appropriate where a strong rapid set is required. Lime mortars can take several months to fully cure and should be left to weathered naturally. The application of artificial weathering finishes may reduce the life of the mortar. Only breathable paints as detailed in this guidance should be applied to breathable walls and breathable buildings. Guidance Table 2: Typical hydraulic lime mortar mixes Hydraulic lime mortar mix (gauged) Type of material in wall Sheltered application Lime: Mortar Exposed application Lime: Mortar Stone/brick - poor durability 1 NHL2 : 3 or softer (use non hydraulic lime table for lime stones) 1 NHL2 : 3 or softer (use non hydraulic lime table for lime stones) Stone/brick - medium durability 1 NHL3.5 : 3 1 NHL3.5 : 2.5 Stone/brick - good durability 1 NHL5 : 3 1 NHL5 : 2.5 Fine joints (up to 3mm) Use non hydraulic lime table Use non hydraulic lime table Key: NHL2 Natural hydraulic lime containing up to 12% clay (slow set); NHL3.5 Natural hydraulic lime containing 12-18% clay (moderate set); NHL5 Natural hydraulic lime containing up to 25% clay( faster set). All are natural hydraulic lime. Notes: Above mortar mixes are only suggested mixes and the actual mortar mix is to be specified by a suitably qualified and experienced conservation specialist- suitable for the type of wall material and degree of exposure; The exact ratio will depend on the sand/aggregate used; The colour, texture and workability of the mortar is influenced by the sand/aggregate; The softer the stone/brick, the softer the mortar mix required.