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, fly ash and limestone fines 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. These materials take part in the hydration reaction alongside Portland cement.

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.

Limestone fines is extracted from a quarry and ground into fine powder. It can replace upto 20% of the Portland cement clinker with a CEM II/A-LL

Example of embodies CO₂ reduction
Based on a concrete containing 320kg/m³ of cementitious material, it is assumed the embodied CO₂ for a CEM II/A-LL at 15% is 231kgCO₂/m³ of concrete. Replacing 50% with ggbs reduces this to 156kgCO₂/m³ and by 70%, 110kgCO₂/m³. Similarly, using 30% fly ash, the embodied carbon reduces to 189kgCO₂/m³. [ref: MPACement Fact Sheet No.18, April 2025]

The ICE has a database and carbon calculator for concrete.