Multi-scale investigation of cement mortars incorporating iron ore tailings: Linking microstructure to mechanical performance and durability

The increasing demand for natural sand in concrete construction, combined with environmental and regulatory constraints, has intensified the search for sustainable alternatives both in Australia and globally. Iron ore tailings (IOT) have recently emerged as a promising substitute for natural sand. However, their widespread adoption remains limited due to uncertainties related to material variability, mix design, and the underlying structural mechanisms governing performance. This study investigates the macro- and advanced micro-scale behaviour of cement mortars incorporating IOT at 25%, 50%, and 75% replacement levels. To understand underlying governing mechanisms, detailed microstructural, micromechanical, and microchemical analyses were conducted using SEM, EDS, nanoindentation, micro-XCT, XRD, TGA, and FTIR. Results show that IOT can effectively replace natural sand without compromising performance. Flowability remained stable between 185 and 195 mm, while density increased by up to 9.3%. Compressive strength variations were within 5%, and flexural strength improved by approximately 23.7%. Microstructural analysis revealed a refined interfacial transition zone, with thickness reduced to 0.34–1.5 µm compared to 1.6–3 µm in conventional cement sand mortar. Nanoindentation indicated comparable elastic modulus and hardness across all mixes. Durability indicators and micro-XCT analyses confirmed comparable behaviour and pore networks. XRD, FTIR, and TGA confirmed stable hydration products and thermal stability. Hence, the study provides strong evidence for IOT as a sustainable fine aggregate, providing both performance reliability and valuable insight for practical implementation in concrete.