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Certain types of concrete masonry can be left unfinished while others will readily accept an applied finish, provided it is suitable for application to an alkaline background and permits a degree of moisture vapour movement through the construction.
Paint: A suitable paint for application to concrete masonry should be specified. Some blocks can and are intended to receive paint directly. Others may require the filling of small blemishes if a smooth appearance is required.
Plaster and render: Gypsum or sand /cement based plaster can be applied to internal concrete masonry. External use would require the application of sand/cement render. A mechanical and/or chemical key will be required to enable good adhesion between the plaster/render and blockwork background. Many standard textured aggregate blocks have such a mechanical key. Some preparation may be necessary with low suction and smooth backgrounds, e.g. raking of the joints or the application of a bonding coat. Also, adjustment to the consistence of the mix may be required with high suction backgrounds to prevent rapid loss of workability. Unless a proprietary system is used, renders are normally applied in two coats. The finishing coat should be thinner and/or weaker than the undercoat. The finish required would dictate the render mix but strong mixes should be avoided with low strength backgrounds.
Acknowledgement: Concrete Society