Supplemental cementitious materials (SCMs) are powders added to concrete to replace part of the portland cement. The most common are fly ash, slag cement, silica fume and calcined clays such as metakaolin.
SCMs react with the calcium hydroxide that cement releases as it hydrates, forming more of the binding paste. The result is usually denser, less permeable concrete, and because cement manufacturing is carbon intensive, every pound replaced lowers the concrete’s embodied carbon.
The common SCMs
- Fly ash. A by-product of coal-fired power. Supply is tightening as coal plants close, and some suppliers now harvest stored ash.
- Slag cement. Ground granulated blast-furnace slag from iron production; often used at higher replacement levels.
- Silica fume. A very fine by-product of silicon alloy production; small doses make concrete much denser.
- Calcined clays. Metakaolin and similar materials, increasingly used as fly ash becomes scarcer.
Why parking cares
- Lower permeability slows chloride from deicing salts on its way to reinforcing steel. See chloride-ion contamination.
- Lower embodied carbon can support green building goals. See Parksmart and EPDs.
- Trade-offs: some SCMs slow early strength gain and need longer curing, which matters on cold-weather schedules.
The Parkonomics view
In a parking deck, the right SCM mix is a durability decision first and a carbon decision second, and it wins on both. Ask for it by performance, permeability and strength, and let the supplier tune the recipe.
Related terms: Cement, Concrete, Embodied Carbon, Chloride-Ion Contamination, Environmental Product Declaration, Sustainability