With its carbon neutrality target fast approaching, China's cement industry urgently needs to develop functional cement-based materials with high carbon sequestration capacity. low-calcium carbonatable binders (LCCB) show great potential for carbon reduction because of their low calcium-to-silica ratio and low calcination temperature; but their poor carbonation performance limits their carbon-reduction applications. To address this limitation, we systematically investigated the effects of barium (Ba) ion doping and magnesium nitrate (Mg(NO₃)₂) solution on the carbonation performance of LCCB by combining first-principles calculations with experimental tests. The results indicate that Ba ion doping increases the volume of the substitution polyhedra in wollastonite (CS) and rankinite (C₃S₂), providing a thermodynamic driving force for the dissociation of calcium ions during the early stages of carbonation. When the concentration of Mg(NO₃)₂ solution is 0.1 g/mL and the Ba ion doping level is 1% (molar fraction) of the calcium ions in the raw material, the initial reaction rate and carbonation activity of LCCB after 8 h of carbonation show significant improvement. In this case, the compressive strength and carbon sequestration rate are 101.7 MPa and 18.33%, respectively, representing increases of 53.63% and 55.34%. Analyses of carbonation products and microstructure indicate that Ba ion doping acts synergistically with Mg(NO₃)₂ to promote the rapid formation of CaCO₃ and amorphous silica gel, reduce porosity, and optimize the pore structure. These findings show that a synergistic modification strategy can effectively improve the carbonation performance of LCCB.

The mechanism by which Ba ions and Mg(NO₃)₂ synergistically enhanced the carbonation performance of LCCB was elucidated by combining first-principles calculations with experiments. The energy barrier for calcium ion dissolution decreased, and the synergy between Ba ions and Mg(NO₃)₂ also accelerated the early-stage carbonation rate of LCCB, creating favorable conditions for improving the carbon sequestration efficiency and compressive strength. Moreover, the marked increase in carbonation products effectively filled the larger pores, and the presence of aragonite further promoted the carbonation process. Thus, the synergy between Ba ions and Mg(NO₃)₂ effectively improved the carbonation performance of LCCB and provided an effective strategy for developing functional cement-based materials with high carbon dioxide sequestration capacity.

Ba ion doping and Mg(NO₃)₂ act synergistically to markedly improve the carbon sequestration rate and carbonation hardening performance of LCCB. During the early stage of carbonation, Mg(NO₃)₂ and Ba ions work together to increase the carbonation reaction rate and reactivity of LCCB, thereby speeding up the carbonation process. When the Mg(NO₃)₂ content reaches 1%, its synergistic effect with Ba ions is optimal. At this dosage, compressive strength, reactivity, carbon sequestration rate, and porosity all reach their optimal values, which shows the critical importance of the Mg(NO₃)₂ level. The findings of this study expand the theoretical framework for improving LCCB's carbonation performance, offer a scientific basis for advancing the use of carbon-sequestering cement-based materials, and foster green, circular, and synergistic progress in the building materials industry.