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

Abstract

The current study proposes CO2 can increase the dissociation degree of blast furnace slag (BFS), thus demonstrate the untapped potential to maximize BFS reactivity at the early stage (3d). This low-carbon cementitious materials employs CO2 activation to increase the dissociation of [SiO4]4− and [AlO4]5− species of BFS, with a more significant effect on [AlO4]5−. This has been demonstrated by the 27Si and 29Al NMR spectral fitting statistics, a 20.12 % and 23.86 % increase in the degree of dissociation for [SiO4]4− and [AlO4]5−, respectively. The introduction of CO2 facilitated the dissolution of Ca2+ on the surface of BFS. Additionally, it accelerated the ionization of H2O, leading to the hydroxylation of [AlO4]5−, as well as, the increase in pH of the leachate. Furthermore, low carbon cementitious materials with low clinker (LT3-MSWI fly ash:BFS:PC:FGDG = 20:42:28:10) exhibit excellent compressive strength (40.57 MPa at 3d) and stabilization/solidification (S/S) properties due to a significant number of functional products. C-S-H with a lower C/S ratio, C-A-S-H, hydrocalumite, and AFt phase are the main functional products of LT3, with relative contents of 17.30 wt%, 13.60 wt%, 8.40 wt%, and 6.3 wt%, as determined by the TIMA quantitative analysis system. The role of CO2 induces the formation of longer-chain C-S-H species (MCL = 4.21) with a higher degree of polymerization (Pol = 0.25) for LT3, as contrasted with LH3, where these values stand at MCL = 3.58 and Pol = 0.14, respectively.