Effect of in situ CO₂ mixing of cement paste on the leachability of hexavalent chromium (Cr(VI))

In situ CO₂ mixing technology is a potential technology for permanently sequestering CO₂ during concrete manufacturing processes. Although it has been approved as a promising carbon capture and utilisation (CCU) method, its effect on the leachability of heavy metals from cementitious compounds has not yet been studied. This study focuses on the effect of in situ CO₂ mixing of cement paste on the leaching of hexavalent chromium (Cr(VI)). The tank leaching test of the CO₂ mixing cement specimen resulted in a Cr(VI) cumulative leaching of 0.614 mg/m² in 28 d, which is ten times lower than that of the control mixing specimens. The results in thermogravimetric analysis indicated that a relatively significant amount of CrO₄²⁻ is immobilised as CaCrO₄ during the CO₂-mixing, and a higher Cr–O extension is observed in the Fourier transform infrared spectra. Furthermore, a portion of the monocarboaluminate is inferred from microstructural analyses to incorporate CrO₄²⁻ ions. These results demonstrate that in situ CO₂ mixing is beneficial not only in reducing CO₂ emissions, but also in controlling the leaching of toxic substances.