Supplementary Cementitious Materials, or SCMs, are materials that are added to, or replacing a portion of portland cement and blended cements. Often, SCMs are byproducts of another industrial-producing process, such is the case with silica fume, fly ash and ground blast furnace slag, but not always. There are some SCMs, such as metakaolin, that are produced for the specific intent of being used as an SCM. The use of SCMs in a cement mixture may be needed for several different reasons and to produce specific outcomes, such as to improve strength and durability, while others may be used to lessen alkali reactivity, decrease permeability, and increase hardening properties through either hydraulic or pozzolanic activity, or both. By reducing the amount of portland cement required through replacement, SCMs can help reduce the overall carbon footprint of a project.
Common SCMs are:
- Silica Fume
- Ground Granulated Blast Furnace Slag
- Metakaolin, Calcined Clay and other natural pozzolans
- Fly Ash
- Crushed Glass Pozzolans
Depending on the chemical composition needed to achieve a specific type of concrete, a binary blend or ternary blend of cement mixture may be needed. A binary blend is when there is one SCM added to portland cement, and a ternary blend is when two SCMs are added to portland cement. These blends can significantly improve the performance of the concrete based on the intended purpose, need, or result.
Since SCMs can drastically improve performance characteristics in concrete, SCMs are required to meet particular specifications to ensure safety. Currently, slag cement must conform to ASTM C989/C989M. Metakaolin, natural pozzolans and fly ash all must meet ASTM C618, and silica fume to ASTM C1240, while Glass pozzolans must conform to ASTM 1866 M .
 ASTM C1697-2 Standard Specification for Blended Supplementary Cementitious Materials. ASTM. ORG. https://www.astm.org/c1697-21.html