The term “replacement level” in cement and concrete refers to the proportion of cement that is substituted by supplementary cementitious materials (SCMs) such as fly ash, slag, or silica fume. These materials are used to replace a part of the ordinary Portland cement (OPC) in a concrete mix to improve various properties of the concrete and to reduce the environmental impact of cement production. The mechanism behind this involves the pozzolanic or hydraulic reactions of SCMs, which contribute to the strength and durability of the concrete by refining the microstructure and reducing the permeability of the cement paste.