[en] Highlights: • Ultra-high performance fibre reinforced concretes (UHPFRC) with silica fume replaced by calcined clay were characterized. • UHPFRC matrices with calcined clays gave comparable compressive strengths to that of silica fume UHPFRC matrix. • The cement hydration was hindered in the calcined clay mixes due to the consumption of water by kaolinite reaction. • Pore structure was refined by kaolinite reaction; however it did not enhance strength development. In this paper, the replacement of high cost silica fume with calcined clay was investigated in UHPFRC mixes. The impact of two grades of calcined clay on the hydration and microstructural development of UHPFRC matrices was investigated, where 54% of the cement by volume has already been replaced by a limestone with similar PSD. It was seen that comparable mixes were obtained when silica fume was replaced with calcined clay by volume. The experimental results revealed a competition for water between the cement hydration and pozzolanic reaction of kaolinite to form more AFt and AFm phases, which affected the compressive strength development of the UHPFRC matrices. This competition is more pronounced in mixes where the kaolinite content increases in the calcined clay.