Performance Optimization of Green Mortar Using Graded Ferrochrome Slag and Dune Sand as Aggregates

The massive expansion of global construction projects has caused a shortage of river sand (RS) as a construction raw material, necessitating the development of alternative materials to alleviate this pressure. In this study, ferrochrome slag (FS) and dune sand (DS) were utilized as composite aggregates to completely replace RS in building materials. Systematic tests were conducted to evaluate the effect of gradation on the flowability and mechanical properties of mortars with composite aggregates, clarifying the influence mechanism through microscopic physical phase tests. The test results show that the grading optimization improves flowability by 12.8–15.9% and enhances the 28-day compressive strength of mortars by 20.5–23.2%. The optimized gradation with a DS proportion of 0.3 has the highest performance, with 28-day compressive and flexural strengths of 59.81 MPa and 8.30 MPa, which are 29.4 and 11.9% higher than those of RS aggregate mortar, respectively. Microstructural analysis reveals that optimized gradation reduces porosity by 7.4–10%, leading to denser structures with fewer cracks and pores. The optimal use of DS and FS as alternative aggregates significantly reduces costs and potential carbon emissions, as the cost efficiency (CP) and ECO2 efficiency (CI) values of the optimized mixture decreased by 47.3 and 27.7% respectively, compared to the control group. The materials developed in this study exhibit excellent engineering application potential, and the performance-based material optimization method provides a theoretical basis and practical reference for the design of alternative building materials made with solid waste.