Flue gas carbonation curing of cement paste and concrete at ambient pressure

High pressure carbonation with pure gas is commonly adopted in early-age concrete carbonation curing for performance enhancement and emission reduction. However, the processes of CO2 purification and compression are energy-intensive. This paper is to investigate the feasibility of directly utilizing flue gas at ambient pressure for carbonation curing. To accommodate the low carbon dioxide concentration in flue gas, cement paste was first studied to identify desirable curing conditions and regimes. Five curing regimes including "Closed (C)", "Cyclic closed (CC)", "Continuous supply (CS)", "Continuous supply plus circulation (CSC)", and "Cyclic closed circulation (CCC)" were examined to improve the reaction efficiencies and material properties under ambient pressure flue gas carbonation. Pure gas carbonation at ambient and high pressures were studied as references. It was found that gas supply, gas circulation, curing pressure, and CO2 concentration could affect CO2 uptake. The cyclic closed circulation (CCC) process yielded the highest CO2 uptake in flue gas carbonation, reaching about 50% reaction efficiency by pure gas at the same duration. It also presented comparable strength with pure gas carbonation at 28 d. Furthermore, the microstructure of cement paste under ambient pressure flue gas carbonation appeared to be comparable to that under pure gas carbonation although its CO2 uptake was only half. X-ray diffraction (XRD) results indicated that portlandite was the first reactant during carbonation curing regardless of gas pressure or CO2 concentration. A higher quantity of well-crystalline carbonates was observed after ambient pressure flue gas carbonation under CCC regime than pure gas carbonation by thermogravimetric analysis (TGA). Additionally, the Fourier transform infrared spectroscopy (FTIR) results also suggested that ambient pressure flue gas carbonation of CCC had seemingly enhanced the intermingling effect between carbonate and C-S-H compared to pure gas carbonation at either high or ambient pressure. The net CO2 emission analysis revealed that the CO2 sequestration efficiency in cement pastes was 75% in high pressure pure gas carbonation and reached 98% in ambient pressure flue gas carbonation. Furthermore, the cost estimate showed the cost of carbonation for 1-tonne cement or sequestration of 1-tonne CO2 was lowest in ambient pressure flue gas carbonation, which resulted from the elimination of CO2 capture, purification and compression.