Natural pozzolans are volcanic or sedimentary materials that can enhance the properties of cement and concrete. They contain silica and alumina, which react with calcium hydroxide in the presence of water to form compounds that improve strength and durability. Common sources include volcanic ash, pumice, and calcined clay. These materials have been used for centuries in construction, dating back to ancient Roman structures. Today, natural pozzolans are valued for their sustainability, as they can reduce the carbon footprint of concrete production. They also help improve resistance to chemical attacks and reduce permeability, making structures more resilient.