Effects of several factors in accelerated carbonation on neutralization and carbon dioxide capture of alkaline sludges treated with paper sludge ash-based stabilizer

Recently, a technology was developed in Japan to accelerate the carbonation of alkaline construction sludge and reduce its alkalinity. Alkaline construction sludge is treated in advance using a paper sludge ash-based stabilizer (PSAS) so that it becomes granular and well aerated. Subsequently, CO2 gas is injected from the bottom to the top of the treated sludge layer in the underground pit. This technology can reduce the pH of alkaline sludge within short periods; however, the completion period of pH neutralization has often been predicted empirically. This may be due to the combined effects of density, water content, particle size, PSAS amount in the sludge, and CO2 concentration in the accelerated carbonation on the pH neutralization process. However, there have been few studies on how these factors affect the pH neutralization and CO2 capture of PSAS-treated alkaline sludges subjected to accelerated carbonation. Therefore, in this study, the effects of several accelerated carbonation factors on neutralization and CO2 capture from alkaline sludge were experimentally investigated. PSAS-treated alkaline sludges with different densities, water contents, particle sizes, and amounts of PSAS were cured in incubators with controlled CO2 concentrations. Based on the pH measurements, the pH neutralization completion period (tN) was evaluated for each alkaline sludge sample. The test results revealed that tN increased with increasing dry density and particle size and decreasing CO2 concentration. The results also suggest that tN increases with increasing amounts of PSAS. However, the water content had a more significant effect on tN than the PSAS amount. There may be an optimum water content for pH neutralization if conditions other than the water content are the same. From the results of the carbonate content evaluation, the amounts of CO2 captured in each specimen (mCO2) and mCO2 at tN were determined for each alkaline sludge sample. The results revealed that the density