To achieve a more sustainable waste-to-energy (WTE) process, the recycling of solid waste incineration (MSWI) bottom ash (BA) has received large attention nowadays. This study investigated how WTE BA form is changed after the hydration and the impacts of WTE BA hydration on its leaching characteristics by using geometrical and leaching tests when incorporated in cement matrix for the recycling. The material composition and characteristics of anhydrous BA, hydrated BA, cement paste, Portland cement concrete (PCC), and BA-combined PCC were evaluated through scanning electron microscopy, X-ray spectroscopy and X-ray diffraction analyses. The results confirmed that the WTE BA newly formed a complex phase of hydration products in a cement matrix. Synthetic precipitation leaching procedure (SPLP) test was also conducted to investigate the leaching behaviors of alkaline components and metals of BA in the crushed BA-PCC samples. Through the leaching study, the leachability of crushed BA-combined concrete was rigorously evaluated when recycled as construction materials (e.g. base, subbase, subdrainage, etc.), which is the worst-case scenario. The results revealed that the release of highly alkaline elements increases with increasing BA content. However, the release of trace metals was reduced by 20-30% significantly when mixed properly with Portland cement concrete (PCC), which is due to both physical and chemical binding in cement hydration products. In addition, the thresholds of water regulations, set by the World Health Organization (WHO) and the Environmental Protection Agency (EPA) of the United States, were assessed as the basis for evaluating the extent of the risk of the leaching of toxic materials.Implications: This study investigated how waste to energy (WTE) bottom ash (BA) form is changed after the hydration and the impacts of WTE BA hydration on its environmental leaching characteristics by using geometrical and leaching tests when incorporated in cement matrix for the recycling. Incorporating of WTE BA in cement mixture can form new mineralogical phases of hydration products in cement matrices such as Copper Hydrogen Arsenate Hydrate and Jahnsite. A significant reduction of alkaline elements (Si, Al, and K) from crushed PCC mixed with WTE BA due to the hydration. The lowest concentrations of major alkaline elements leached from the crushed PCC containing either 10% or 20% of BA contents. The averaged leaching concentration of detected elements is substantially below the water quality guidelines (provided by U.S. EPA and WHO) except Al.