Investigation on the Production of a Mineral Additive from Concrete Waste and Design Method for Mix Proportion

ABSTRACT Recycled powder recycling is a popular global research hotspot, with its optimal replacement ratio being a key research focus. Traditional studies limit the recycled powder content in cement to below 30 % to guarantee stable cement performance. Recycled cement was defined as the composite material prepared by mixing hardened cement powder (HCP) with ordinary cement as a mineral admixture. This study explored the mechanical performance of recycled cement paste with different HCP dosages. The effects of citric acid and polyhydroxy acid water reducer dosages on the setting behavior and fluidity of recycled cement paste were also studied. The mineral phases, crystal structure, and micro-morphology of recycled cement hydration products were further analyzed, and a corresponding mix proportion design method for HCP-based recycled cement was established. Test results indicated that recycled cement paste with 70 % HCP replacement achieved a compressive strength above 40 MPa, and the strength increased to over 45 MPa at a 60 % HCP dosage. For HCP treated at 1,000°C, 5 %, 10 %, and 15 % citric acid additions led to initial setting times of 45, 100 and 180 min, and final setting times of 180, 450, and 540 min, respectively. With the cement/HCP ratio rising from 3:7 to 7:3 and water reducer dosage increasing from 2 % to 6 %, the paste fluidity was significantly improved. Its spreading area varied from 40.7 to 176.3 cm2, and the spreading diameter increased from 7.1 to 14.9 cm.