NH₄⁺-mediated crystallization mechanism and structural evolution of Core-Shell porous Vaterite-rich CaCO₃ from concrete wastewater

The concrete industry faces critical challenges in managing highly alkaline, calcium-rich wastewater and reducing its substantial carbon footprint. This study presents a sustainable valorization strategy to transform this wastewater into high-value, low-carbon materials via CO₂ mineralization. An NH₄⁺-mediated crystallization approach was developed, governed by a coupled “crystal face selectivity-oriented assembly-Ostwald ripening” mechanism. Under optimized conditions, we successfully synthesized a unique core-shell porous vaterite. When utilized as a 10% cement replacement, this functional material not only enhances compressive strength by 6.38% but also reduces thermal conductivity by 13.1%, effectively breaking the strength-insulation trade-off through matrix pore refinement. We elucidated a five-stage formation model encompassing amorphous CaCO₃ (ACC) precipitation, ACC agglomeration, NH₄⁺-directed oriented attachment, shell fusion, and final internal core evacuation via Ostwald ripening. This “waste-to-resource” paradigm achieves a 15.5% reduction in total carbon emissions through synergistic CO₂ sequestration and cement substitution. Our findings provide a fundamental chemical framework for the rational design of low-carbon building materials from industrial waste streams.