Study on Preparation and Hydration Mechanism of Sand Washing Residue Mud-Based LC3 Cement by Mechanical–Thermal Activation

To enhance the application value of sand washing residue mud (SWRM) and mitigate its adverse environmental impacts, this study focuses on the resource utilization of SWRM. The research employed a combined mechanical–thermal activation method to enhance the activity of SWRM and utilized the mixed optimal design module to design and optimize the mixing ratio of SWRM-based LC3 cement. The results showed that within a ball-milling time range of 3–9 min, as the ball-milling time increased, the specific surface area increased, the median particle size D50 decreased, and the particle size was mainly concentrated within the range of 0.1–30 μm; the specific surface area of the washed sand residue after 3 min of grinding reached 1080 m2/kg, with D50 being 3.50 μm, which met the requirements for making cementitious materials. After thermal activation at temperatures ranging from 450 °C to 950 °C for the 3 min ground SWRM, the 28 d activity index showed a trend of increasing first and then decreasing with the increase in calcination temperature, and the 28 d activity index reached the maximum of 79.3% at a calcination temperature of 650 °C. The optimal mixing ratio of the AC70 group’s SWRM-based LC3 cement obtained through the mixing design was: cement clinker 66.5%, desulfurized gypsum 3.5%, activated SWRM 15%, and limestone powder (LP) 15%. The 28 d compressive strength of the AC70 group’s SWRM-based LC3 cement was 34.1 MPa, with initial setting and final setting times of 170 min and 240 min respectively, and the volume stability was qualified. The microscopic test results indicated that under the synergistic effect of alkali and salt, the silicate and aluminosilicate tetrahedral structures in the active sand-washed mud (ASWRM) decomposed, forming a C-(A)-S-H network structure, which was the main source of strength for the SWRM-based LC3 cement in the later stage. The ecological benefit calculation and analysis showed that compared with ordinary Portland cement of the same grade, the AC70 group’s SWRM-based LC3 cement had a 27.6% reduction in implicit energy consumption, a 44.1% reduction in carbon emissions, and a 25% reduction in cost. This study provides an innovative approach for the high value-added resource utilization of SWRM.