Influence and Optimization of Marble Dust and Furnace Slag as Fine Aggregate Replacement on the Strength Characteristics of Pavement Quality Concrete Using Taguchi Analysis

Rapid urbanisation and expanding highway networks in India have intensified demand for Pavement Quality Concrete (PQC), driving consumption of natural river sand at environmentally unsustainable rates. Simultaneously, India's marble processing and ferrous foundry industries generate millions of tonnes of marble dust (MD) and furnace slag (FS) annually — largely disposed of as waste. This investigation systematically explores the utilisation of these industrial by-products as partial fine aggregate replacements in M40-grade PQC. Three control factors — marble dust replacement (A: 0%, 10%, 20%), furnace slag replacement (B: 0%, 20%, 30%), and water-cement ratio (C: 0.35, 0.38, 0.40) — were varied using a Taguchi L9(3�) Orthogonal Array, reducing 27 full-factorial runs to only 9 mixes. Compressive strength (7, 28, 56 days), flexural strength (MOR), and splitting tensile strength were evaluated. Taguchi S/N analysis, ANOVA, and Multiple Linear Regression were performed in Minitab 2022. The optimal combination A₃B₃C₂ (20% MD + 30% FS + W/C 0.38) yielded CS₂₈ = 50.2 MPa (+8.7% over control; +25.5% over M40 minimum) and MOR = 5.32 MPa (+18.2% over IRC:58-2015 minimum). W/C ratio is the dominant factor (ANOVA: 31.2%), followed by MD (24.8%) and FS (5.1% individually + 12.4% synergistic interaction). Microstructural analysis confirms MD micro-filling reduces ITZ width (15–25 μm → 8–12 μm) while FS pozzolanic reaction reduces total porosity from 12.4% to 8.6%. The optimised mix achieves the lowest water absorption (3.72%) and best chemical durability among all mixes, at ₹102/m³ less than conventional PQC. The findings confirm that the combined MD+FS system simultaneously enhances PQC strength, durability, and environmental performance.