LC3 is one of the most promising alternatives for a high-performance blended cement with a significantly lower clinker content and with potential for worldwide adoption. For this reason, understanding its long-term performance is relevant to ensure serviceability over its lifespan. In this paper, a detailed study of phase assemblage, porosity evolution and microstructural development was conducted on LC3 samples hydrated up to 3 years and compared to 90 days specimens. It was observed that the reaction of clinker phases and metakaolin continues between 90 days and 3 years, even after depletion of portlandite. Stra center dot tlingite was observed in systems with higher metakaolin contents, in addition to hemi and monocarboaluminate. Experimental results were compared with thermodynamical modelling. New insights are obtained on the hydration mechanisms operating at later ages, when only small pores remain filled with pore solution.