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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), are more often than not used in concrete to reduce the Portland cement content.

Portland cement (CEM I) may only be 10-15% of the concrete volume, but it has a high embodied carbon. Use of additions such as ggbs and fly ash significantly reduce the embodied carbon dioxide associated with the total cement content of a concrete.

The addition is either part of a factory made composite cement or a blended combination at a batching plant. Both materials take part in the hydration reaction alongside Portland 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 is typically used as a replacement for 35–70% of CEM I 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 CEM I in concrete. It is of similar fineness to cement and requires minimal processing before being combined with it. It can also be used as an alternative source of silica in cement manufacture, reducing the need for clay and/or sand.

Example of embodies CO2 reducition
Assuming a concrete containing 320kg/m3 of cementitious material, the embodied CO2 for a CEM I based concrete is 283kgCO2/m3 of concrete. Replacing 36% with ggbs reduces this to 198kgCO2/m3 and by 65%, 120kgCO2/m3. Similarly, using 36% fly ash, the embodied carbon reduces to 188kgCO2/m3. [ref: Burridge J, How to specify low carbon cement now, Concrete, March 2021]

The ICE has a database and carbon calculator for concrete.


Acknowledgement: The Concrete Society


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TR74 Cementitious materials