Waste materials which may be recycled as cementitious binders or aggregates in concrete are described. Cementitious binders, which are blended with Portland cement, include pulverised-fuel ash ( pfa ), a by-product of coal-fired power stations, and the properties of which are specified in BS EN 450: 1995. Additions of 25-35 wt% are used. Ground granulated blast furnace slag, as specified in BS 6699: 1986, is added at levels of 40-70 wt%. Additions of fume silica, which is extremely fine and reactive, are in the range 5-10 wt%. Up to 20 wt% limestone fines may be mixed with cement clinker ( BS EN 197-1: 2000 ). The use of burnt oil shale is uncommon in the UK, but is used elsewhere in Europe. Waste, in the form of recycled concrete aggregate, or manufactured material, may also be incorporated into concrete as aggregate. Pfa and blast furnace slag ( BS 1047: 1983 ) may be used to manufacture aggregate. By-products from the timber and paper-making industries are also used as concrete additives. 14 refs.