Sustainable resource recovery from hydrated waste cement for calcium carbonate and silica extraction: Circular economy in construction

• A method proposed the extraction of minerals from hydrated waste cement (HWC). • HWC was treated with HCl to extract Ca 2+ and subsequently converted to CaCO 3 . • Residual after acid treatment was used for SiO 2 extraction with NaOH solution. • This approach promotes eco-friendly practices, aligning with a circular economy. The rapidly expanding infrastructure sector in Thailand has boosted the consumption of ready-mix concrete, generating significant quantities of hydrated waste cement (HWC) during the truck washing process. HWC, composed primarily of calcium silicate hydrate (CSH), calcium hydroxide (CH), and fine sand, is commonly disposed of in landfills, incurring additional costs and environmental concerns. This study explores acid and alkaline dissolutions as novel approaches to extract calcium and silicon, respectively. HWC samples were first subjected to treatment with various concentrations of hydrochloric acid (1%, 3%, and 5% HCl) to extract calcium compounds. The resulting Ca-rich filtrate was then converted to calcium carbonate (CaCO 3 ). Optimal CaCO₃ yield was achieved with 5% HCl, though higher acid concentrations also led to the leaching of Fe from HWC, resulting in an orange shade in the final CaCO 3 product and hindering the formation of pure calcite. Instead, a less stable polymorph of vaterite was primarily produced. Subsequent alkaline treatment of the acid-treated residue with 1 M sodium hydroxide (NaOH) produced a 39.8% yield of silica (SiO₂), though Fe contamination also impacted its purity. Despite these challenges, the study findings demonstrate the viability of HWC as a feedstock for the production of CaCO 3 and SiO 2 . This method presents a promising approach to construction waste management while the environmentally friendly practices are promoted in the construction sector. This innovative HWC recycling method aligns with the principles of a circular economy, emphasizing resource conservation and sustainable waste management.