Accelerated and natural carbonation of concrete with high volumes of fly ash: chemical, mineralogical and microstructural effects

Today, a rather poor carbonation resistance is being reported for high-volume fly ash (HVFA) binder systems. This conclusion is usually drawn from accelerated carbonation experiments conducted at CO(2) levels that highly exceed the natural atmospheric CO(2) concentration of 0.03–0.04%. However, such accelerated test conditions may change the chemistry of the carbonation reaction (and the resulting amount of CH and C–S–H carbonation), the nature of the mineralogical phases formed (stable calcite versus metastable vaterite, aragonite) and the resulting porosity and pore size distribution of the microstructure after carbonation. In this paper, these phenomena were studied on HVFA and fly ash + silica fume (FA + SF) pastes after exposure to 0.03–0.04%, 1% and 10% CO(2) using thermogravimetric analysis, quantitative X-ray diffraction and mercury intrusion porosimetry. It was found that none of these techniques unambiguously revealed the reason for significantly underestimating carbonation rates at 1% CO(2) from colorimetric carbonation test results obtained after exposure to 10% CO(2) that were implemented in a conversion formula that solely accounts for the differences in CO(2) concentration. Possibly, excess water production due to carbonation at too high CO(2) levels with a pore blocking effect and a diminished solubility for CO(2) plays an important role in this.