Experimental Investigation of Legacy Dumped Coal Ash as a Gravel Improvement Material for Sustainable Low-Traffic Road Rehabilitation

Coal-fired power stations have generated substantial quantities of legacy dumped coal ash, creating long-term environmental and land-use challenges. This study evaluates naturally weathered legacy dumped coal ash as an improvement material for low-traffic road sublayer rehabilitation without cement, lime, or other chemical stabilisers. Three mixtures containing 0%, 30%, and 40% ash by mass were assessed using particle size distribution, Atterberg limits, modified compaction analysis, soaked California Bearing Ratio (CBR), swell testing, X-ray fluorescence (XRF) analysis, and QEMSCAN automated mineralogical analysis. The untreated gravel exhibited a Plasticity Index (PI) of seven and a soaked CBR of 30%. At 30% ash, the PI decreased to four and the soaked CBR increased modestly to 33%, while the 40% mixture became non-plastic, exhibited the lowest swell, and recorded a CBR of 29%. The results indicate that the 30% ash mixture provided the highest bearing capacity among the investigated mixtures, whereas the 40% ash mixture provided superior moisture stability. QEMSCAN automated mineralogical analysis showed that the ash-containing mixtures were dominated by aluminosilicate-, quartz-, and glass-associated mineral phases, providing additional mineralogical evidence of modification of the gravel matrix following ash incorporation. The 30% and 40% mixtures displaced equivalent proportions of conventional gravel while beneficially reusing an existing industrial residue. The findings demonstrate measurable material-sustainability benefits through reduced conventional aggregate demand and beneficial legacy-ash utilisation while maintaining acceptable engineering functionality, supporting its potential as a secondary construction material for resource-efficient and sustainable low-volume road rehabilitation.