Alkali–silica reaction (ASR) expansion in concrete is a chemical reaction process where reactive forms of silica in the aggregates react with alkali hydroxides in concrete, leading to the formation of a gel that swells as it absorbs water from the surrounding cement paste. This phenomenon typically occurs in environments where there is sufficient moisture, and it can lead to significant structural damage over time. The reaction begins when the alkaline environment of the concrete, which can be exacerbated by the presence of external sources of alkalis, facilitates the dissolution of amorphous or poorly crystalline silica found in certain aggregates. As this gel absorbs moisture, it expands, exerting expansive pressure within the concrete, which can lead to cracking, spalling, and, ultimately, structural failure if not properly managed or mitigated.