Influence of Expanded Polystyrene on the Mechanical and Tribological Performance of Alkali-Activated Lignocellulosic Ash Composites

Lignocellulosic waste materials (LWM), such as wood ash (WA) and sugarcane bagasse ash (SCBA), have emerged as promising alkali-activated precursors. In this study, WA- and SCBA-based matrices were modified with metakaolin (MK) and volcanic ash (VA) as complementary reactive precursors, while recycled expanded polystyrene (EPS) was incorporated as a lightweight modifier. Composites were prepared using a 15 M NaOH activator and characterized by compressive strength, Vickers hardness, and ball-on-disc tribological tests under dry sliding conditions to establish the relationship among precursor composition, mechanical behavior, and surface performance. SCBA-based composites exhibited higher compressive strength than WA-based systems, reaching 4.47 MPa in SCBA–VA formulations, whereas the hybrid WA–SCBA–VA formulation achieved the highest compressive strength (7.55 ± 0.25 MPa, n = 5). Although EPS reduced compressive strength, it increased surface hardness, with the highest value recorded for formulation 9BPS (25.4 HV). The WA–SCBA–VA formulation also exhibited the lowest wear rate (0.09 mm3/N·m), indicating superior tribological performance. Overall, VA was more effective than MK in enhancing the combined mechanical and tribological behavior of WA–SCBA-based composites. This study provides a systematic comparative assessment of the tribological performance of lignocellulosic ash-based alkali-activated composites, offering new insights into the relationship between precursor composition and surface performance in lightweight sustainable materials.