Focuses on the chemical and structural long-term performance of aged MSWI bottom ash used as a reinforcement layer in a 60metre-long stretch of a test road. The bottom ash was stored in open air for some time before use. Sampling was performed eight years after construction. Epoxy-impregnated polished slabs from undisturbed samples representing cross-sections through the sub-base layer were used for a microtextural and chemical characterization by SEM/EDS and XRPD. No cracks that implied movement or rotation of particles in the road construction or disturbance due to the sampling process were found. This made it possible to study the chemical and structural processes on a micro-scale. The SEM/EDS analysis showed that most particles had reacted to some extent but were not disturbed by movements in the road construction or sampling. It is concluded that the microstructural stability of the studied MSWI bottom ash is high.