Wastewater-Derived Mineral Recovery for Carbon-Neutral Construction Materials

Abstract

Chinenye Elizabeth Onumadu and Adeel Patrick

Cement production accounts for approximately 8% of global anthropogenic CO2 emissions, driven largely by limestone calcination and high-temperature clinker sintering. The increasing scarcity of high-quality supplementary cementitious materials (SCMs) further exacerbates the sustainability challenge facing the construction sector. Municipal and industrial wastewater streams constitute a continuously available, yet largely untapped, source of dissolved and particulate calcium (Ca), magnesium (Mg), and silicates (Si), presenting a promising circular economy pathway for simultaneous resource recovery and carbon footprint reduction in construction materials.

The objective of this study was to recover a multi-mineral blend from wastewater through an integrated chemical process and to evaluate its efficacy as a sustainable cementitious additive in ordinary Portland cement (OPC) systems, targeting both performance parity and significant embodied carbon savings.

A pH-swing precipitation sequence combined with controlled CO2 carbonation was developed to selectively recover calcium carbonate (CaCO3), magnesium-bearing phases (primarily nesquehonite and brucite), and amorphous silicates from both synthetic and real municipal/industrial wastewater matrices. Process parameters including pH, CO2 flow rate, and membrane pre-concentration were optimized for multi-mineral yield and product reactivity. The recovered blend was characterized by X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and laser diffraction particle sizing. Blended OPC pastes and mortars incorporating 5–15% replacement levels were prepared. Compressive strength development was assessed at 1, 7, 28, and 90 days. Microstructural evolution and phase assemblages were examined using XRD, TGA, and mercury intrusion porosimetry (MIP). Hydration kinetics were monitored via isothermal calorimetry.

High recovery efficiencies were achieved: 88% Ca, 72% Mg, and 65% Si under optimized conditions. At 10% replacement, the multi-mineral blend yielded a 28-day compressive strength of 51.8 MPa, closely approaching the 53.2 MPa control mortar. Mercury intrusion porosimetry revealed an 18% reduction in total porosity, attributed to the nucleation effect of fine CaCO3 particles (d50 ≈ 3 μm) and the pore-filling/pozzolanic action of recovered silicates, which increased secondary C-S-H and C-A-S-H formation as confirmed by TGA. The 15% blend exhibited modest strength reduction due to dilution. Life-cycle assessment demonstrated a 24% reduction in embodied CO2 emissions compared with reference OPC formulations, resulting from both direct CO2 mineralization and clinker displacement. Wastewater-derived minerals thus constitute a viable low-carbon cementitious additive. The integrated recovery process is ready for pilot demonstration and offers a scalable route toward carbon-neutral construction materials.

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