In-situ Wet Carbonation Activation of Red Mud Waste for Sustainable Grout Materials

The low reactivity and high alkalinity of raw red mud (RRM) limit its utilization as supplementary cementitious materials (SCMs) in Portland cement-based grouts. This work develops an in-situ wet carbonation technique to improve the chemical reactivity of RRM using flue gas CO2. Systematic investigation of the RRM before and after carbonation revealed transformed composition and microstructure. The minerals and morphology changed significantly, with nano-CaCO3 and amorphous silica/silica-alumina gels formation. This enhanced the pozzolanic reactivity of RRM. Detailed evaluation of the effects of carbonated red mud (CRM) as SCMs on the properties of Portland cement-based grouts showed accelerated hydration kinetics and refined pore structure. Replacement of 30% cement by the CRM increased the 28 d compressive strength of the grout. The underlying mechanisms were analyzed via XRF, XRD, TGA, FT-IR, 29Si NMR, SEM, and N2 adsorption–desorption testing. The nano-CaCO3 and silica/silica-alumina gels from CRM participated in additional hydration and microstructure densification. This integrated approach enables the improved performance of grouting materials through CO2 mineralization and industrial waste recycling.