The development of carbon capture, utilization and storage allows non-hydraulic low-lime calcium silicates capable of CO2 activation to serve as a sustainable alternative binder. However, the diffusion and transport mechanism of CO2 in these carbonation mediums are crucial for carbonation reaction, which heavily relies on the pore structures closely related to the water–binder ratio (w/b). This study focused on the role of w/b ratio (varying from 0.1 to 0.18) on the carbonation behavior of a carbonatable belite (β-C2S)-ternesite ( \({\text{C}}_{{5}} {\text{S}}_{{2}} {\overline{\text{S}}}\) ) binder prepared with 100% municipal solid waste incineration residues upon carbonation curing conditions of 65% RH, 20% CO2 concentration, and a temperature of 20 °C. The mechanical properties, microstructure, CO2 uptake, and phase assemblage of this binder were evaluated. The results showed that the compressive strength and CO2 uptake were increased with increasing w/b ratios from 0.1 to 0.14, which promoted the consumption of β-C2S and \({\text{C}}_{{5}} {\text{S}}_{{2}} {\overline{\text{S}}}\) , leading to an increased production of calcite. At up to 0.18, however, water overflow occurred, resulting in an approximately 78% decrease in the compressive strength. Also, increasing the w/b ratio changed the volume fraction of various pore size, particularly the predominance of large capillary pores that were beneficial to CO2 diffusion. BSE images showed that silica gel tended to distribute around the unreacted particles, while CaCO3 preferred to accumulate in the outside space.
