Neutralization and CO₂ fixation behavior of alkaline recycled soil using column tests with CO₂ ventilation

Column tests were conducted to elucidate the pH neutralization and CO₂ fixation behavior of alkaline recycled soils permeated with CO₂. The ventilation period required for complete CO₂ fixation (t_EOF) was estimated from the test results of each column. The maximum amount of CO₂ captured per gram of dry soil ((m_CO2)^max) was determined based on the inflow and outflow of CO₂ for each specimen. The investigation into the effects of soil density, specimen height, CO₂ gas concentration, and flow rate on t_EOF and (m_CO2)^max revealed that t_EOF increased with higher dry density (ρ_d) or specimen height (H), as a denser and larger specimen consumed more CO₂ gas. Conversely, t_EOF decreased with a higher CO₂ gas concentration (C) or flow volume (Q) because a low C or Q resulted in an insufficient CO₂ gas supply to match the consumption induced by the reaction. Lower C and Q tended to yield higher (m_CO2)^max, whereas higher ρ_d and H resulted in higher (m_CO2)^max. Furthermore, (m_CO2)^max increased with t_EOF, with a slight reduction in pH after neutralization with increased t_EOF. This phenomenon was attributed to the longer ventilation period enabling more Ca to react with the CO₂ gas and maintain the equilibrium state of the reaction. Finally, the evaluation of the CO₂ consumption rate (CSR) for each column specimen revealed that a higher C and Q resulted in a lower CSR, whereas a higher H resulted in a higher CSR. Additionally, a longer t_EOF was associated with a higher CSR, although some variation was observed.