Gwynne Anthony 2020

ART506 Dissertation 2019. Anthony Gwynne . 108 Appendix D: Lime mortar, plaster and render (Source: Gwynne, 2013.) Types of lime mortars, lime renders/ plasters and decorative finishes suitable for breathable (vapour permeable) buildings There are two main types of lime binder used in mortars, renders and plasters , non hydraulic and hydraulic lime as detailed below:. D1. Non- hydraulic lime (known as lime putty or fat lime) Consists of pure limestone, burnt in a factory process to drive off carbon dioxide, an excess of water is added to slake the resulting quick lime into a lime putty. It hardens by exposure to the air, in the presence of water, in order to carbonate, and over a long period of time it reverts to a limestone. 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 ). Non-hydraulic lime mortar mixes The lime putty to be pre-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. Turn, beat and ram the mortar as necessary to make it more plastic without the addition of water in most cases. For walls to be rendered, leave the pointing finished 6mm back from the stone/brick face to provide a key. Non-hydraulic lime may be more appropriate for use on historic buildings where a slower set and soft mortar is required to maximum permeability and flexibility of the wall structure. Lime mortars can take several months to a year to cure and should be left to weathered naturally without the application of any artificial weathering which may damage the mortar. Only breathable paints as detailed in this guidance should be applied to breathable walls and breathable buildings. Pozzolanic materials can be added to the non-hydraulic mortar mix to increase initial set times where specified/required and carried out in strict consultation with an experienced conservation specialist detail. Note: Hydrated or bagged lime is normally as a plasticizer and is added to a cement mortar mix, it can be used as a mortar but not always with good results. Guidance Table 1: Typical non hydraulic lime putty mortar mixes Non hydraulic lime putty mortar mix (gauged) Type of material in wall Sheltered application Lime putty: Mortar Exposed application Lime putty: Mortar Stone/brick - poor durability 1 : 3 1 : 2 Stone/brick - medium durability 1 : 3 (use hydraulic lime table for sand stone) Use hydraulic lime table Stone/brick - good durability 1 : 3 (use hydraulic lime table for sand stone) Use hydraulic lime table Fine joints (up to 3mm) 1 : 1 1 : 1 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.