The role of CO2 in MSWI fly ash-slag-based binder at the early stage

•The LT3 sample optimizes BFS reactivity for enhanced cement clinker substitution after carbonation.•CO2 activation stimulates the hydroxylation of Si(Al)-O bond of BFS and the dissolution rate of [SiO4]4− and [AlO4]5−.•The ion exchange between Ca2+ on the surface of BFS and H+ derived from H2O triggers the pH increase during CO2 activation.•C-S-H can be generated with a longer chain (4.21) and a higher degree of polymerization (0.25) with carbonation.