Mimicking the cementation mechanism of ancient Roman seawater concrete using calcined clays

The negative environmental impacts of Ordinary Portland Cement (OPC)-based materials, including high carbon emission, energy-intensive manufacturing process, and freshwater consumption, can be minimized by capitalizing on the concepts of ancient Roman concrete. Accordingly, this study presents a pathway of producing durable and sustainable seawater concrete by mimicking the cementation of ancient Roman concrete. Calcined clays with blended minerals (kaolinite and montmorillonite), portlandite, and seawater were used to produce these binder mixtures. The evolutions of the reaction products of the seawater-cured paste samples were monitored for up to 56 days using Thermogravimetric analysis (TGA), X-ray diffraction (XRD), Nuclear Magnetic Resonance (NMR), and Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS). Because of the high alkalinity (pH 8.2) of seawater, it acts as an activator for the calcined clay-portlandite mixtures eliminating the need for additional alkali activators. Accordingly, in the presence of seawater, the reaction between calcined clay and portlandite happens relatively rapidly, and compressive strengths of around 17 MPa can be achieved within 7 days. The early age strengths of these samples were primarily attributed to the formation of hydrocalumite. With increasing curing duration, geopolymer gel, C-A-S-H, ettringite, and phillipsite were formed. Both C-A-S-H and phillipsite showed binding of chlorides. Only phillipsite showed sequestration of sulfates in the matrix. The calcined clay blends containing 50 to 75% kaolinite performed better than the samples containing 100% kaolinite in terms of reaction rate and compressive strengths.