Effects of CO₂ curing timing on pH, CO₂ fixation, and strength of recycled sludge soils

Expanding the use of recycled construction sludge soils requires verification of workability, mechanical performance, and environmental compatibility, particularly the mitigation of high alkalinity caused by stabilisation. Carbon dioxide curing promotes neutralisation and carbon dioxide sequestration through carbonate formation; however, the influence of curing timing within the production process remains insufficiently understood. This study investigates how the timing of carbon dioxide curing affects the physicochemical and mechanical properties of recycled soils. Ao clay adjusted to 1.25 times its liquid limit was stabilised using either 10% blast furnace slag cement type B or 25% paper sludge ash-based stabiliser. Carbon dioxide curing was applied at 0, 7, or 13 days after mixing, or not applied, under controlled conditions with a total curing period of 14 days. The pH, carbonate content, particle crushing strength, cone index, and consolidated undrained triaxial behaviour were evaluated. Carbon dioxide curing effectively reduced pH and enhanced carbon dioxide sequestration in both soils, although partial pH rebound occurred during subsequent sealed curing, particularly when curing was introduced at an earlier stage. Mechanical responses varied depending on the stabiliser type and curing timing, with notable strength reductions observed primarily in the cement-stabilised soil. These findings highlight the importance of optimising carbon dioxide curing timing to balance environmental benefits and mechanical performance in recycled soil production.