Utilization of copper tailings in the preparation of low-calcium Portland cement clinker and carbonation-hardening mechanism

This study presents a strategy for decreasing cement-production-induced CO2 emissions and achieving comprehensive utilization of copper tailings (CTs). This is realized by preparing Portland cement clinkers dominated by calcium silicate minerals with a low Ca/Si ratio through the adjustment of the siliceous (CTs) to calcareous (limestone) material ratio. Differential scanning calorimeter (DSC), quantify X-ray diffraction (QXRD), scanning electron microscopy (SEM), 29 Si nuclear magnetic resonance (NMR), and thermogravimetric (TG) analyses were employed to systematically characterize the thermodynamic behavior and mineral composition changes during the formation of the low-calcium Portland cement clinker, and to analyze their mechanical properties, carbonation products, and microstructural evolution under accelerated carbonation conditions (high pressures and CO2 concentration). CTs used as siliceous raw materials enabled the preparation of the low-calcium Portland cement clinkers with wollastonite (CS), belite (C2S), and rankinite (C3S2) as the main mineral phases at 1200 degrees C. The Ca/Si ratio and calcination temperature markedly influenced the clinker minerals. For a Ca/Si ratio of <= 1.0, CS was the main clinker mineral. When the Ca/Si ratio was between 1.0 and 1.4, the clinker comprised the C2S-C3S2 system, with the C3S2 content increasing as the calcination temperature increased. The prepared clinker achieved a compressive strength of over 100 MPa after carbonation when calcined at 1200 degrees C with a Ca/Si ratio of 1.4. Carbonation of several low-calcium-content silicate minerals produced CaCO3, mainly as spherical vaterite crystals, while silica in the system formed silica gel, which formed a reticulated structure with CaCO3.