How do pozzolanic materials such as fly ash form cementitious compounds with lime?

Pozzolanic materials, such as fly ash, are not cementitious on their own. Instead, they consist of finely divided siliceous or aluminous substances that undergo a chemical reaction when combined with lime (calcium hydroxide) in the presence of water.12

This process occurs because the reactive silica and alumina within the pozzolan combine with the lime to form stable, cementitious compounds. Historically, this reaction was utilized by the Romans, who mixed volcanic ash with lime to create durable mortars and concretes. In modern applications, such as pozzolanic cements, the necessary lime is often provided by the hydration of the portland cement fraction within the mixture.234

By reacting with lime, these materials contribute to the strength and hardening of the final product, effectively serving as a binding agent in masonry and construction. Fly ash, a common industrial byproduct, is widely used today as an artificial pozzolan in these types of cementitious mixtures.312