Research on Recycled Cement Sludge as Natural River Sand in the Production of Ultra-High-Performance Concrete

During the production and transportation of concrete, about 3–5% of cement slurry slag is attached to the concrete mixer truck and cannot be used, so it is usually treated as waste after washing. If ultra-high-performance concrete (UHPC) can be made using the slurry residue left in batching plants and concrete mixer trucks as a fine aggregate to replace natural river sand, it can not only efficiently utilize waste resources but can also reduce the exploitation of natural river sand, which is in line with the concept of green and sustainable development. In this paper, recycled cement slurry (RCS) recovered from concrete mixer trucks and batching plants was replaced by natural river sand with a mass of 0~50%. The particle gradation was optimized based on the modified Andreasen model, and the effect of RCS on the wet packing density, workability, compressive strength, and microscopic pore structure of UHPC was systematically studied. A carbon emissions assessment was conducted in combination with the whole life cycle carbon emissions assessment method (LCA). The results show that, as the RCS replacement rate increases, the wet packing density and fluidity of UHPC decrease continuously, which is related to the high water absorption rate, rough surface, and porous interior of RCS; when the RCS replacement rate is 10%, the compressive strength increases slightly, and continuing to increase the replacement rate leads to a gradual decrease in strength. The results of MIP and SEM show that an appropriate amount of RCS helps maintain a relatively dense microstructure. Higher substitution levels will lead to an increase in harmful pores, pore enlargement, and the development of microcracks; as the RCS content increases, GWP shows a small downward trend, but since UHPC carbon emissions are mainly controlled by cement, the participation of RCS has a limited emission reduction effect. Based on the comprehensive compressive strength, pore structure and carbon efficiency index, the 10% RCS replacement rate performs optimally within the experimental range of this paper and can be used as a more reasonable recommended dosage.