The “Reaction” in the context of cement and concrete typically refers to the chemical and physical responses of these materials under various environmental stressors, including alkali–silica reaction (ASR), sulfate attack, and freeze–thaw cycles. These reactions can significantly influence the durability and integrity of concrete structures. Alkali–silica reaction, for instance, occurs when reactive silica in the aggregate forms a gel with alkali hydroxides in cement, absorbing water and causing the concrete to expand and crack. Sulfate attack involves chemical reactions between sulfate ions (from external sources or internal constituents) and cement components, leading to the formation of expansive compounds like ettringite and gypsum, further leading to cracking and strength loss.