Which of the following is a common type of supplementary cementitious material?

Fly ash is a common type of supplementary cementitious material (SCM) used in concrete production. It is a byproduct of coal combustion in power plants and is utilized in concrete to enhance its properties, including workability, strength, and durability. When used as a partial replacement for Portland cement, fly ash can improve the long-term strength development and reduce the permeability of concrete, making it more resistant to chemical attacks and mitigating issues like alkali-silica reaction.

In contrast, the other options do not fit the definition of supplementary cementitious materials. Concrete block is a building material used primarily as a structural element rather than an additive; granite aggregate refers to coarse material used in concrete mixes and does not serve as a cement replacement; cement mortar is a mixture of cement and sand used for binding masonry units but is not an SCM itself. Hence, fly ash stands out as the appropriate choice in this context.