Effects of carbonation treatment on the properties of hydrated fly ash-MgO-Portland cement blends

• Hydrated FA-MgO-PC blends were exposed to pressurized CO2 for carbonation treatment.
• Carbonation increased the compressive strengths of hydrated FA-MgO-PC blends.
• Carbonation treatment densified the microstructure of hydrated FA-MgO-PC blends.
• Treatment with higher pressure CO2 increased compressive strength more rapidly.
• Cement blends with higher contents of FA and MgO had greater strength increases.

Alternative cement formulations with low carbon emissions have attracted increasing attentions. In this study up to 90% of the Portland cement (PC) was replaced with reactive MgO and fly ash to prepare cement blends. Cement pastes were then prepared with the cement blends and cured with CO2 after 28 d of hydration. The implication of carbonation on the compressive strengths as well as microstructures of the cement blends were investigated by using X-ray diffraction, thermogravimetric analysis, mercury intrusion porosimetry, and scanning electronic microscopy. Results showed that under moist curing condition, the replacements of PC with the reactive MgO and fly ash (FA) reduced the compressive strengths of cement blends. However, after the following treatment with pressurized CO2 (0.55 and 0.10 MPa) for different time (3 h, 15 h, 1 d or 14 d), the compressive strengths were increased significantly by up to 195%. This is attributed to the microstructure densification of cement blends in terms of pore diameter decrease, total pore volume reduction, and products conglomeration due to the formation of carbonate products, e.g. calcite, magnesium calcite, nesquehonite, etc.. Curing with higher pressure CO2 leads to faster strength development owing to more rapid penetration of CO2 and following carbonation of the cement blends.