Accelerated carbonation of alkaline construction sludge by paper sludge ash-based stabilizer and carbon dioxide

Construction sludge frequently has high alkalinity after its generation or during the intermediate treatment process. The aim of this study is to experimentally investigate the potential of combining accelerated carbonation and a paper sludge ash-based stabilizer (PSAS) to neutralize the alkalinity of construction sludge in a short period and to improve its strength for use as a recycled material. The experimental results indicate that the addition of a PSAS significantly granulated the alkaline sludge, and once granulated, the PSAS successfully accelerated the pH neutralization of the alkaline sludge. It was also found that the decrease in dry density ρ_d and the degree of saturation S_r of the PSAS-treated sludge was able to reduce the period required for the pH neutralization, t_N. The decrease in ρ_d is thought to allow fresh CO₂ gas to penetrate the specimen more easily. However, if S_r is below a certain limit, it does not strongly facilitate the reduction of t_N. This implies that pH neutralization cannot be accelerated when the amount of water in the sludge is below a certain level. Moreover, it was found that mean particle diameter D₅₀ also affected t_N. The strength development of the PSAS-treated sludge was evaluated using a series of cone index tests. It was found that the strength of the alkaline sludge without the PSAS was significantly decreased by accelerated carbonation, but was significantly increased even after accelerated carbonation when the PSAS was present. Due to the porosities of the remaining PS ash particles, most of the contribution of the water absorption and retention performance of the PSAS to the strength development of the PSAS-treated sludge was secured after accelerated carbonation. In addition, the granulated particles of the PSAS-treated sludge retained their granular shape to some extent. Therefore, it is presumed that the friction and interlocking of the particles did not decrease significantly. It was also found that, after carbonation, the q_c of the PSAS-treated sludge increased more rapidly than that of the alkaline sludge without the PSAS. A further detailed examination of the test results showed that under air-curing conditions, the q_c of the treated sludge with accelerated carbonation increased relatively gradually compared to that of the treated sludge without accelerated carbonation.