Evaluating the mechanical and diffusive leaching performances of cement-stabilized clay by mixing recycled concrete...

The use of cement to stabilize soft clay and construction sludge is common for achieving suitable soil properties in construction. To reduce costs and promote recycling, recycled concrete aggregate (RCA) from construction and demolition waste has been adopted as a cement substitute. However, the presence of hexavalent chromium (Cr(VI)) in RCA raises environmental concerns. This research investigates the mechanical properties and Cr(VI) leaching behaviors of cement-stabilized clay SC-RCA mixtures with different RCA contents (0%, 20%, 40%, and 60% by weight) as well as SC and RCA individually. Laboratory experiments including Proctor compaction, hydraulic conductivity tests, batch leaching tests (BLT), and tank leaching tests (TLT) were performed. Results show that adding RCA improves compaction and decreases hydraulic conductivity, with the lowest hydraulic conductivity at 40% RCA and minimum void ratio at 60% RCA. This is because RCAs come into contact with each other and form structures at the content of 60% RCA, causing the SC matrix to become trapped in voids between the RCA structures, resulting in ineffective compaction and increased porosity of the SC matrix. Higher RCA content leads to increased Cr(VI) leaching in BLT, but TLTs showed negligible Cr(VI) leaching. Calcium (Ca) was measured as a trace chemical to determine effective diffusion coefficients (D_eff), which were higher for specimens with 60% RCA, since the enhanced pore connectivity of well-contacted RCA particles led to an increase in K, facilitating more direct pathways and quicker diffusion of Ca through the SC-RCA matrix. The 40% RCA mixture exhibited the lowest D_eff, suggesting that it improved compaction and limited chemical leaching.