Valorization of non-beneficiated clays as supplementary cementitious materials in the production of cement-based mortar

Four kaolinitic clays and one argillite (coal mining waste) were tested for use as supplementary cementitious materials (SCMs). Their main properties were evaluated, without undergoing mineral beneficiation. Although clay impurities, such as Fe2O3, TiO2, and high levels of quartz, etc., can prevent or hinder their use in some industrial sectors, it may be less of an issue for application as SCMs as such impurities often do not change pozzolanic activity. Civil construction has used clays with different mineralogy and obtained from different locations, but the kaolinitic clay is the most suitable for SCMs in Portland cement composition. Raw clay properties do play a key role in terms of thermal behavior and pozzolanic properties, however. The current research reported herein evaluated their application to SCM; these clays were characterized in terms of chemical composition, mineral phases, particle size distribution, thermogravimetry and pozzolanic activity. After calcination, the rapid cooling of the clays proved to be favorable to reducing particle size and increasing pozzolanic activity − especially those clays rich in quartz − when they were calcined at the highest tested temperature (800 °C). Performance index (PI) of mortars with 25% of calcined clays replacement by cement showed considerable compressive strength improvement for all calcined clay samples ranging from 106% to 132%. This demonstrated that low-cost clays could replace clinker with technical, environmental, and economic benefits, especially in terms of their availability.