Changes in chemical phases and microscopic characteristics of fly ash blended cement pastes in different CO2 concentrations

The effects of CO2 concentration on changes in chemical phases and microscopic characteristics for fly ash (FA) blended cement pastes were investigated in this study. Several microscopic test methods, including X-ray diffraction (XRD), thermo-gravimetric analysis (TGA), 29Si nuclear magnetic resonance (29Si NMR) and scanning electron microscope (SEM), were used to characterize the chemical compositions and microscopic features. The XRD results showed that the precipitation of allotropic calcium carbonate (CC̅) includes calcite (c), aragonite (a) and vaterite (v). The ratio of c/(a + v) was around 0.6 under 3% and 20% CO2, while more percentage of calcite was generated under 100% CO2 (c/(a + v) = 0.79). The precipitation of more calcite than vaterite and aragonite happened with the CO2 concentration elevated to 100%. TGA analysis indicated that the total content of CC̅ was similar under all accelerated conditions and higher than that under natural carbonation. Additionally, in the 29Si NMR spectra, more C-S-H (about 70%) was decalcified after accelerated carbonation compared with natural carbonation (54.1%). The decalcification degree was the same for 3% and 20% CO2 and showed the highest value under 100% CO2. The microstructure changes characterized by SEM observation exhibited denser microstructure after carbonation with the formation of CC̅ but no apparent difference was observed with different CO2 concentrations based on the SEM pictures. Compared with the carbonation of ordinary Portland cement (OPC) paste, the carbonation of FA blended cement paste was more inclined to precipitate as calcite than vaterite and aragonite and caused a lower decalcification degree of C-S-H. Overall, similar to OPC paste, the carbonation results obtained in natural and accelerated conditions for FA blended cement pastes were different and the conditions between 3% and 20% CO2 were similar while 100% CO2 showed different results