Reactive aggregates in the context of cement and concrete refer to those aggregates that can chemically interact with the alkaline environment of cement paste, leading to expansive reactions that may compromise the integrity of concrete structures. The most well-known type of reactive aggregate reaction is the Alkali–Silica Reaction (ASR), where reactive silica in aggregates reacts with alkalis in cement to form a gel that swells upon absorbing water, causing internal stresses and cracking in concrete. These reactions are of particular concern in severe environmental conditions, such as high humidity or temperature, which can exacerbate the effects.