This indicates that the concrete used for your foundations will have 75% portland cement with 25% fly ash or furnace slag as its "cementitious" components. The use of such "waste substitute" can substantially reduce the embodied CO2 of the cement with relatively little impact on the physical characteristics of the concrete.
Click to open the following link in a new window (see above):
http://www.greenspec.co.uk/cement-substitutes.php