This indicates that the concrete used for your foundations will have at most 60% portland cement, and at least 40% fly ash, and/or furnace slag as its only "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. It's possible to reduce the OPC content to as little as 10%, consult the help link and your concrete supplier for more information.
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