The global cement industry is a major source of CO₂ emissions, making its green transition an urgent priority. Utilizing industrial solid waste to prepare low calcium carbonatable binder (LCCB) is considered as a promising approach that can simultaneously achieve solid waste valorization and CO₂ sequestration. However, some impurity ions commonly appeared in solid waste, such as Mn ions from manganese slag, can dissolve into the mineral phases of LCCB, affecting its structure and carbonation behavior. The mechanisms underlying these effects remain unclear, hindering the optimization of material performance. This study was to clarify the solid solution behavior of Mn ions in LCCB and their mechanism of action during the carbonation process. The doping behavior of Mn ions in the main mineral phases was analyzed and their influence on the carbonation reactivity and microstructural evolution of the material was discussed via combining density functional theory calculations with experimental validation. This work could provide a theoretical basis for the design of high-performance LCCB materials based on manganese-containing solid waste.

LCCB clinker was synthesized with analytical-grade chemicals. To investigate the doping effect of Mn ions, MnO₂ was incorporated at varying levels (i.e., 0%(in mass), 0.15%, 0.25%, and 0.50%). The raw mixtures were calcined at 1280 ℃ for 120 min, followed by rapid cooling. The resulting clinker was ground and subsequently compacted at 4 MPa to form the specimens, which were then subjected to carbonation curing in 0.3 MPa CO₂ (99.9% purity) for 8 h. The microstructural morphology and elemental distribution were determined by scanning electron microscopy (SEM) equipped with energy-dispersive X-ray spectrometry (EDS) and backscattered electron (BSE) detection analysis. The phase composition was characterized by X-ray diffraction (XRD) coupled with Rietveld refinement. The CO₂ uptake was determined by thermogravimetric analysis (TGA), while the carbonation reactivity was assessed by isothermal calorimetry. The pore structure was characterized by low-field nuclear magnetic resonance (LF-NMR). The compressive strength was measured on a WDW-200 testing machine at a constant loading rate of 0.5 mm/min. Furthermore, the crystal models of CS, C₃S₂, and C₂AS were constructed based on the first-principles calculations to investigate the doping behavior of Mn ions.

This study reveals that Mn ion doping significantly enhances the carbonation hardening performance of LCCB via reducing the total bond order density of Ca—O bonds. Mn ions preferentially occupy Ca sites in LCCB, primarily because this substitution pathway has the lowest defect formation energy, and the resulting Mn-O bonds exhibit a chemical environment that is highly similar to the original Ca—O bonds, while maintaining analogous orbital hybridization characteristics. At an MnO₂ doping level of 0.5%, the material properties are improved. After 8-h carbonation, the compressive strength is increased by 80.70%, compared to the blank group, and the porosity is decreased to 20.29%. Mn doping promotes the formation of more CaCO₃, and its content is increased by 47.42% relative to the blank group, significantly increasing the proportion and thermal stability of calcite. The carbonation reaction calorimetry analysis shows that the exothermic peak and cumulative heat release in the doped system both increase, indicating a higher degree of reaction. The aforementioned enhancement effects are mainly attributed to the reduction in the total bond order density of Ca—O bonds due to Mn doping, which weakens the lattice's restraint on Ca²⁺, thereby promoting its dissolution and accelerating the carbonation reaction process. The microstructural analysis demonstrates that Mn doping leads to carbonation products dominated by layered calcite, forming a dense network-like skeleton. Meanwhile, unreacted mineral phases are encapsulated by amorphous phases, and CaCO₃ effectively fills pores, collectively contributing to the improvement in mechanical properties. This research provides a theoretical basis for the high-value utilization of manganese-containing solid waste in low-carbon cementitious materials.

The results indicated that Mn ion doping and its promoting effect on carbonation performance could be highly significant, especially at the optimal doping level (i.e., 0.5%). Under this condition, the carbonated samples exhibited a compressive strength increase of 80.70%, a porosity reduction of 20.29%, and a CO₂ uptake enhancement of 47.42%, compared to the blank group. Mn ions could substitute Ca sites and dissolve into various mineral phases of LCCB, inducing a lattice contraction and a rightward shift in diffraction peaks. This could weaken the total bond order density of Ca—O bonds, facilitating Ca²⁺ dissolution and accelerating the carbonation reaction process. Furthermore, doping could promote the formation of calcite with a higher thermal stability, whose layered stacking structure significantly improved the material densification and mechanical properties. This study could provide a theoretical basis and technical pathway for the resource utilization of manganese-containing solid waste in high-performance, low-carbon cementitious materials, having an important value for promoting the green transition of the cement industry.