Impact of accelerated carbonation on geotechnical properties of recycled soil from construction sludge stabilized with paper sludge ash-based stabilizer and blast furnace cement

• Paper sludge ash-based stabilizer (PSAS) and blast furnace cement type B (BFCB) were used in this study. • An accelerated carbonation method was applied to neutralize the alkalinity of recycled construction sludge, producing carbonated recycled soil. • Carbonation reduced the pH to approximately 8.6, satisfying Japanese environmental standards and improving compaction behavior. • Carbonation reduced the cone index to 0.60 – 0.95 times its original value, depending on the water content. • Carbonation modified the cone index and permeability coefficients because of ettringite loss. In Japan, it is imperative to develop sustainable methods for treating and recycling construction sludge. Use of conventional highly alkaline binder additives such as cement and lime risks environmental contamination. As a sustainable alternative, an accelerated carbonation method using carbon dioxide (CO 2 ) was developed, and its effectiveness in treating recycled sludge stabilized with a paper sludge ash-based stabilizer and blast furnace cement type B was demonstrated. However, a significant research gap remains in understanding how carbonation influences geotechnical properties because most prior studies primarily focused on pH reduction via additive selection. This study addresses this gap through a comparative experimental analysis of carbonated and non-carbonated soil samples. The innovation of this work lies in evaluating carbonation as both an environmental neutralization process and a treatment that alters compaction, strength, and permeability. The results showed that carbonation reduced the soil pH to approximately 8.6, meeting environmental standards, while improving the maximum dry density ( ρ dmax ) and compaction behavior. However, the cone index ( q c ) results revealed a reduction in the recycled soil strength owing to hydrate decomposition and weakened particle bonds. This can be minimized by maintaining optimal water content before treatment. Furthermore, analysis based on the k 15 coefficient (permeability at 15 °C) suggests that carbonation affects hydraulic conductivity. In some cases, hydraulic conductivity increases because of ettringite decomposition, leading to the loss of crystalline structure and creation of continuous voids. These findings highlight that, although carbonation offers environmental benefits, its application must be carefully managed to ensure compliance with geotechnical performance requirements.