Carbonation Resistance of Alkali-Activated Slag Under Natural and Accelerated Conditions

In this paper, carbonation resistance of alkali-activated slag (AAS) pastes exposed to natural and accelerated conditions up to 1 year was investigated. Two aspects of carbonation mechanism were evaluated. The first was the potential carbonation of the main binding phases in finely powdered AAS pastes. The second was the reactivity and diffusivity of CO2 within the bulk AAS paste. From Fourier transform infrared spectroscopy and thermogravimetric analysis coupled with mass spectroscopy time-series measurements, it was found that powdered AAS was largely carbonated within 28 days with a CO2 uptake of 14 wt%. The main carbonation products were calcium carbonates. Nevertheless, the bulk paste samples were highly resistant to carbonation, regardless of the exposure conditions. The findings showed that the pH value (initial pH > 12) and strength of the samples did not decrease under accelerated carbonation compared to those of the samples exposed under natural conditions. The mineralogy of the samples in these two carbonation exposures did not alter either, except for outdoor conditions. The gel pores were dominant in the pastes (pore size in range of 2–15 nm). The dense microstructure was the main barrier for CO2 to diffuse and further react with binding phases.