Strength Development of Alkali-Activated Fly Ash Exposed to a Carbon Dioxide-Rich Environment at an Early Age

Abstract

The development of a binder system with a lower carbon footprint as an alternative to Portland cement has been intensely researched. In the present study, alkali-activated fly ash exposed to carbon dioxide at an early age was characterized in compressive strength tests and by MIP, XRD and FT-IR analyses. The compressive strength of carbonated specimens experienced a dramatic increase in comparison to uncarbonated specimens. The microstructural densification of the carbonated specimens was evidenced by MIP. The XRD pattern showed peaks assigned to nahcolite, indicating that the pH was lower in the carbonated specimens. Under the carbon dioxide-rich environment, the aluminosilicate gel reached a more Si-rich state, which improved the mechanical properties of the alkali-activated fly ash.

An NMR Spectroscopic Investigation of Aluminosilicate Gel in Alkali-Activated Fly Ash in a CO2-Rich Environment

“…The obtained results provide new insight into the chemistry of aluminosilicate gel exposed to a CO 2 -rich environment at an early age, and explain the strength enhanced by exposure to a CO 2 -rich environment observed in a recent work conducted by the authors [12]. It should be noted that the compressive strength of AFA exposed to conditions identical to the present study can be found in [12].…”

“…It should be noted that the compressive strength of AFA exposed to conditions identical to the present study can be found in [12]. Firstly, the main gel constituents Si and Al behaved differently, while Na was in close proximity to Al.…”

“…The liquid to solid ratio was 0.5 by mass. The details regarding the sample preparation can be found in an earlier study [ 12 ]. All samples were initially cured in a chamber at 80 °C for 24 h. The following exposure condition was applied thereafter: (a) Samples were sealed to remove the effect of carbonation; or (b) samples were placed in an accelerated carbonation chamber to expose them to an atmospheric CO 2 concentration of 10% for 28 days.…”

“…This indicates that curing AFA for a sufficient period of time may lead to higher strength development under high CO 2 conditions, in contrast to that observed in the earlier work [ 1 , 11 ]. In a recent work conducted by the authors [ 12 ], AFA, which was exposed to an atmospheric CO 2 concentration of 10%, exhibited a strength greater than the unexposed one. The increased strength of the carbonated AFA may have been caused by the decrease in the porosity due to the formation of nahcolite, but the FT-IR spectra clearly showed that the gel in the carbonated AFA was in a more mature state (higher Si/Al) [ 12 ].…”

“…In a recent work conducted by the authors [ 12 ], AFA, which was exposed to an atmospheric CO 2 concentration of 10%, exhibited a strength greater than the unexposed one. The increased strength of the carbonated AFA may have been caused by the decrease in the porosity due to the formation of nahcolite, but the FT-IR spectra clearly showed that the gel in the carbonated AFA was in a more mature state (higher Si/Al) [ 12 ]. Therefore, the binding gel phase in AFA is definitely modified to some extent under carbonation and thus requires a more detailed investigation.…”