PEER-REVIEWED ARTICLE bioresources.com Suwunwong et al. (2021). “Combustion ash & roads,” BioResources 16(2), 2264-2278. 2264 Suitability and Characteristics of Combustion Residues from Renewable Power Plants for Subbase Aggregate Materials, in Thailand Thitipone Suwunwong, a,b Sutthida Boonsamran, c Kanchana Watla-iad, a,b Patcharanan Choto, a,b Nuttachat Wisittipanit, b,d Tanan Chub-uppakarn, e and Khamphe Phoungthong c,f, * Physico-chemical properties and the environmental impacts were studied relative to the leaching of rubber fly ash and bottom ash. The pozzolanic properties of fly ash and bottom ash were confirmed by the chemical composition, including silicon oxides, calcium oxides, and aluminum oxides. The geo-technical characteristics of rubber wood fly ash and bottom ash, i.e., modified compaction, plasticity, and the soaked California Bearing Ratio, were evaluated to assess the feasibility of fly ash or bottom ash mixed with lateritic soil as aggregate materials for the subbase in road construction in order to optimize the replacement of lateritic soil by fly ash or bottom ash. The leachates from rubber fly ash and bottom ash did not exceed standard thresholds. The measured characteristics of fly ash or bottom ash mixed with lateritic soil were in good alignment with the effective engineering thresholds. Recommendations were developed for safe reuse of byproducts from rubber renewable power plant in subbase road construction. Keywords: Renewable power plant; Rubber wood; Incineration residues; Construction materials; Leaching test; Road construction Contact information: a: Center of Chemical Innovation for Sustainability (CIS), Mae Fah Luang University, Chiang Rai 57100 Thailand; b: School of Science, Mae Fah Luang University, Chiang Rai 57100 Thailand; c: Environmental Assessment and Technology for Hazardous Waste Management Research Center, Faculty of Environmental Management, Prince of Songkla University, Songkhla 90112 Thailand; d: Materials for Energy and Environment Research Group (MEE), Mae Fah Luang University, Chiang Rai 57100 Thailand; e: Department of Civil Engineering, Faculty of Engineering, Prince of Songkla University, Songkhla 90112 Thailand; f: Center of Excellence on Hazardous Substance Management (HSM), Bangkok 10330 Thailand; *Corresponding author: khamphe.p@psu.ac.th INTRODUCTION Biomass power plants are an alternative pathway to reclaim a large quantity of agricultural wastes to produce electricity and to achieve the objectives of the Alternative Energy Development Plan (AEDP) in Thailand (Prasertsan and Sajjakulnukit 2006; Sutabutr 2012). However, a large expansion of biomass power plants to support the government policies has increased the wastes produced from electricity production, including fly ash (FA) and bottom ash (BA) (Sutcu et al. 2019). FA and BA are porous, solid by-products from the complete or incomplete combustion of biomass. The ash byproducts contain carbon and oxides of calcium, silicon, and aluminium matters, and their characteristics vary by temperature and conditions in the furnace (Dahl et al. 2009; Vamvuka and Kakaras 2011; Phoungthong et al. 2016b; Agrela et al. 2019). FA and BA are regarded as particularly hazardous and costly to dispose, requiring a large area for