Role of Clay Minerals Content and Calcite in Alkali Activation of Low-Grade Multimineral Clays

Abstract

The increased contribution of science-intensive clean materials and technologies to the total industrial production is one of the essential prerequisites for the future of sustainable industrial development. Chemical activation based on a nonfired or low-temperature approach for the production of cementitious materials from glassy aluminosilicates, including calcined clays, is an intensively developing and promising clean technology. This study investigated the potential of calcined clays as precursors of alkali-activated cements (AACs) by considering three types of low-grade multimineral clays (with a clay mineral content of <20%). The alkali activation of the calcined clays was analyzed based on the type and content of clay minerals, presence of calcite, calcination temperature, type of alkali activator, and curing conditions. The results indicated that clays containing 20% of 2∶1 layer lattice clay minerals are not suitable as precursors of AACs, while those containing 9%–12% of 1∶1 and/or 2∶1 layer lattice minerals combined with 29%–32% calcite lead to hardened pastes displaying compressive strengths of up to 12.4 MPa after 2 days and 20 MPa after 28 days and after steam curing. The reaction products of 6M NaOH-activated systems determined by XRD, TG/DSC, and FTIR spectroscopy analyses were X-ray amorphous binder gel, calcite (CaCO3), clinozoisite (Ca2Al3(SiO4)(Si2O7)(O,OH)), and gaylussite (CaNa2(CO3)(H2O)5).