What chemical compound is primarily responsible for expansion and cracking due to alkali-silica reaction?

The chemical compound primarily responsible for expansion and cracking due to alkali-silica reaction (ASR) is alkali-silica gel. ASR is a chemical reaction that occurs between the alkaline hydroxides in cement paste (primarily sodium hydroxide, NaOH, and potassium hydroxide, KOH) and certain reactive forms of silica found in some aggregates. These reactive forms of silica include opaline silica, chert, and strained quartz. The alkaline hydroxides react with the reactive silica to form alkali-silica gel. This gel is hygroscopic, meaning it absorbs water from the surrounding cement paste. As the gel absorbs water, it expands significantly, creating internal pressure within the concrete. This pressure can exceed the tensile strength of the concrete, leading to cracking. The cracking is typically characterized by a map-cracking pattern on the surface of the concrete. The alkali-silica gel continues to absorb water and expand over time, exacerbating the cracking and leading to further deterioration of the concrete. To mitigate ASR, strategies include using non-reactive aggregates, limiting the alkali content of the cement, incorporating supplementary cementitious materials (SCMs) like fly ash or slag, which reduce the alkalinity of the pore solution, and using lithium-based admixtures, which can reduce the expansion of the alkali-silica gel. For example, concrete structures built with reactive aggregates and high-alkali cement are prone to ASR, while those built with non-reactive aggregates or low-alkali cement are less susceptible.