Reactivity of Clay Minerals in Intervention Mortars

Historic lime binders were often modified by the addition of materials containing reactive silicates and aluminates. Roman builders utilized volcanic deposits from Pozzuoli near Naples, as the addition of this volcanic ash improved the performance of mortar. Pozzolans can be of natural or artificial origin; traditional artificial pozzolans were produced from natural materials, such as clay, after heat treatment. In India, pozzolan use was introduced by Mughals, and fired clay brick or ‘surkhi’ was extensively used in medieval times to improve mortar consistency and strength. However, much attention may not have been given to the mineralogical composition and firing temperature of clay, which are critical in determining the pozzolanic activity. In the present study, three different types of clays—kaolinite, montmorillonite, and a sundried brick from the field—were fired to temperatures from 600 to 900 °C to assess pozzolanic activity. The clay crystal structures are disrupted, and an amorphous phase is produced by heating at a temperature range of 700–900 °C. The crushed sundried brick obtained from the field was not pozzolanically active at any of the temperatures. The maximum pozzolanic activity was obtained at 800 °C for the kaolinite and montmorillonite clays, and the fall of pozzolanic activity after this temperature can be attributed to the formation of new minerals such as mullite and hematite. These commercial clays can be used as pozzolans in intervention mortars if they are heated between 700 and 900 °C.