Supplementary cementitious materials (SCM)

Supplementary cementitious materials, abbreviated SCM, are fine materials used in place of part of the Portland clinker in a cement-based mix. The common ones are fly ash, ground granulated blast-furnace slag (GGBS), silica fume, calcined clay and limestone filler. Some react pozzolanically or hydraulically and contribute strength; limestone filler acts mainly as a filler and nucleation site, although it also reacts with aluminates.

In practice

SCMs change far more than the binder bill. Fine SCMs raise water demand and stickiness, slag and fly ash slow early strength, and silica fume sharply increases cohesion, so any substitution has to be re-tuned against yield stress, open time and buildability rather than assumed to be neutral. Supply is also shifting: fly ash depends on coal-fired power generation and GGBS on primary steelmaking, and both capacities are declining in Europe, so availability is tightening and quality varies more between sources. Industry roadmaps, including the GCCA one, treat clinker substitution as a major lever for lowering the carbon intensity of concrete, but the actual reduction depends on the replacement level, the SCM and the comparison baseline.

Related terms

• LC3 — a cement combining calcined clay and limestone with a reduced clinker content.
• Geopolymer — a binder formed by alkali activation rather than clinker hydration.
• Printable mortar — the fine, binder-rich mix whose rheology every SCM change affects.