Combination cements and environment
Cementitious materials produced from by-products of other industries, for example, ground granulated blastfurnace slag (ggbs) and fly ash, also known as pulverised-fuel ash (pfa), can be used to supplement the Portland cement clinker. This helps reduce the amount of carbon dioxide emitted per tonne of combined cement manufactured. Both materials take part in the hydration reaction alongside cement, and contribute to strength development.
Ggbs is a by-product of iron manufacture, mainly consists of calcium, silicon and aluminium oxides. It is produced by water-quenching the molten slag from a blastfurnace and grinding it to a fine powder. In this form, it may be used as a substitute for 40–70% of Portland cement in concrete.
Fly ash, the residue from burning pulverised coal in power stations, contains silicon and aluminium oxides and can replace 20–55% of the Portland cement in concrete. It is of similar fineness to cement and requires minimal processing before being combined with it. Pfa. It can also be used as an alternative source of silica in cement manufacture, reducing the need for clay and/or sand.
Use of these materials significantly reduces the total carbon dioxide emitted during concrete production by reducing the requirement for cement. (Figures for 1997 indicate that the use of pfa and ggbs in the UK reduced annual greenhouse gas emissions by 1.2 million tonnes of CO2 equivalent and reduced primary energy use by 1.94 million megawatt hours.)For further information see Concrete and the Environment, published in CONCRETE in September 2001, pp39–46. Copies are available as a free download from the Members Area of the Concrete Society web site.