Water immersion induced swelling characteristics and underlying mechanisms of compacted neutralized and un-neutralized-recycled construction sludge

In this study, the swelling behaviors of recycled and neutralized soils derived from surplus construction materials, commonly referred to as construction sludge in Japan were investigated. Investigations are performed on the properties of recycled soils treated with cement and paper sludge ash for stabilization and sometimes neutralized with carbon dioxide to reduce their high alkalinity. Although the quality control during the manufacturing process confirms that both recycled soil and neutralized-recycled soil (recycled soil neutralized with CO 2 ) meet Japanese soil environmental standards and strength requirements, their use is often avoided owing to concerns regarding their expansive nature. However, the expansive nature of recycled soil and neutralized recycled soil has not been investigated in detail. Therefore, the focus of this study is on understanding how the treatments in the manufacturing of recycled soil and neutralized recycled soil affect its potential to swell when immersed in water, which is critical for ensuring the suitability of the material for construction applications. The swelling potentials of recycled and neutralized recycled soils after compaction were evaluated using a series of soaking tests. Furthermore, the underlying mechanisms were investigated through cone index tests and microstructural and crystallographic analyses. These findings indicate that recycled soils exhibit a reduced tendency to swell compared to untreated clayey soils, which is attributed to their enhanced structural integrity. In contrast, neutralized soils, especially those treated early with carbon dioxide, exhibit a higher propensity for swelling, which is linked to reduced cohesion and changes in the hydrate composition. This study underscores the importance of controlling the manufacturing and compaction processes of recycled soils, particularly the CO 2 curing time and degree of saturation during compaction, to mitigate swelling risks. Although secondary-treated soils demonstrate high strength, they also exhibit swelling under certain conditions, thus highlighting the need for the careful consideration of the hydration and compaction processes. These insights contribute to a broader understanding of recycled soil behavior, thus advocating for informed construction practices and further research to facilitate the effective and sustainable use of recycled construction materials.