An admixture is defined by the American Concrete Institute (ACI) as ‘a material other than water, aggregates, hydraulic cement and fibre rein­ forcement, used as an ingredient of concrete or mortar, and added to the batch immediately before or during its mixing’ (ACI Committee 212, 1989). The use of admixtures in concrete is certainly not new, in Roman times blood or eggs were added to concrete to improve its properties and in the thirteenth-century linseed oil was added to make the concrete water tight (Aitcin and Baalbaki, 1995).