Which of the following is a type of supplementary cementitious material (SCM)?

Fly ash is considered a type of supplementary cementitious material (SCM) because it is a byproduct of coal combustion that can be used in concrete to enhance its properties. SCMs, such as fly ash, are typically used to replace a portion of the Portland cement in concrete mixes. They contribute to improved workability, durability, and reduced permeability of the concrete, while also reducing the overall environmental impact of concrete production since they utilize industrial byproducts.

Portland cement is the primary binding agent in concrete, but it does not fall under the category of supplementary materials. Polymer additives are often used to modify concrete properties but do not qualify as SCMs. Railway ballast, which is granular material used to support railway ties and ensure stability, is entirely different from the context of cementitious materials and has no role as a supplementary component in concrete.