The effects of flaky particles in coal gangue manufactured sand on the performance and sulfate resistance of cement mortar

The utilization of coal gangue manufactured sand (CG-MS) presents a sustainable alternative to natural aggregate, yet the influence of its inherent flaky particle morphology on cement mortar performance remains systematically unexplored. This study systematically investigated the effects of flaky particle size (1.18–4.75 mm) and content (0%–30%) on the workability, mechanical properties, and sulfate attack resistance of mortars, prepared using both pure cement and cement-fly ash (20% replacement) systems. The results indicate that increasing the content of flaky particles significantly increases sand voidage, leading to a substantial decrease in mortar fluidity, while the water retention rate remains stable. The compressive and flexural strengths of the mortar achieve an observed optimum when the flaky particle content is limited to 15%–20%. Microstructural analysis confirms that this optimal range enhances the mechanical interlock at the interfacial transition zone and refines the pore structure by reducing the total volume of harmful pores. Conversely, excessive flaky particles increase total porosity and promote particle agglomeration. Sulfate attack tests further reveal that a suitable content of flaky particles can delay the penetration of erosive agents and product expansion, maintaining better durability, though this protective effect is limited under high-concentration sulfate conditions. The geometric characteristics of flaky particles are the primary control factor governing the rheology and durability of CG-MS mortar. This research provides essential guidance for optimizing manufactured sand gradation and promoting the sustainable, high-value utilization of coal gangue resources.