Alteration in molecular structure of alkali activated slag with various water to binder ratios under accelerated carbonation

Abstract

Carbonation of alkali activated materials is one of the main deteriorations affecting their durability. However, current understanding of the structural alteration of these materials exposed to an environment inducing carbonation at the nano/micro scale remains limited. This study examined the evolution of phase assemblages of alkali activated slag mortars subjected to accelerated carbonation (1% CO2, 60% relative humidity, up to 28 day carbonation) using XRD, FTIR and 29Si, 27Al, and 23Na MAS NMR. Samples with three water to binder (w/b) ratios (0.35, 0.45, and 0.55) were investigated. The results show that the phase assemblages mainly consisted of C-A-S-H, a disordered remnant aluminosilicate binder, and a minor hydrotalcite as a secondary product. Upon carbonation, calcium carbonate is mainly formed as the vaterite polymorph, while no sodium carbonate is found after carbonation as commonly reported. Sodium acts primarily as a charge balancing ion without producing sodium carbonate as a final carbonation product in the 28-day carbonated materials. The C-A-S-H structure becomes more cross-linked due to the decalcification of this phase as evidenced by the appearance of Q4 groups, which replace the Q1 and Q2 groups as observed in the 29Si MAS NMR spectra, and the dominance of Al(IV) in 27Al MAS NMR. Especially, unlike cementitious materials, the influence of w/b ratio on the crystalline phase formation and structure of C-A-S-H in the alkali activated mortars before and after carbonation is limited.

Supercritical CO2-Induced Evolution of Alkali-Activated Slag Cements

“…This indicates the formation of highly cross-linked aluminosilicates upon carbonation. The structural changes in C-A-S-H gel to higher cross-linked aluminosilicates are previously observed [ 24 , 31 , 45 ].…”

“…The CaCO 3 thus formed acts as a diffusion barrier inhibiting the further ingress of CO 2 and carbonation [ 20 ]. Alkali-activated slag systems, unlike PC, do not have Ca(OH) 2 and hence decalcification directly happens in C-S-H [ 20 , 30 , 31 ]. The disparate carbonation mechanism involved in PC and AAS has exacted the AAS to become more vulnerable to carbonation.…”

Performance of Alkali-Activated Slag Concrete Masonry Blocks Subjected to Accelerated Carbonation Curing

“…Such a correlation is presented in Figure 10. In another study, the depth of carbonation of slag geopolymer paste, tested with phenolphthalein solution, increased with longer carbonation duration [29].…”

“…This process led to a weakening of the sample matrix. On the effect of water content on the properties of a carbon-cured, slag-based, alkali-activated material, it was found that the water-to-binder ratio had a limited impact on the microstructure before and after carbonation, as well as on the formation of carbonates [29]. Moreover, Jun et al [30] developed a carbonation-cured, slag-based geopolymer material suitable for salty and humid environments, where samples achieved most of its strength after 3 days of carbonation curing.…”