Particle packing and rheology of cement pastes at different replacement levels of cement by α-Al2O3 submicron particles

•Cement pastes containing α-Al2O3 submicron particles and a PCE superplasticizer.•Microstructure of the fresh pastes is controlled by the Specific Surface Area.•Widening particle size distribution has a minor role on packing and porosity.•Progressive initial hydration retardation on increasing α-Al2O3 particle content.•Slightly enhanced microstructure of hardened pastes at replacement levels of 2 and 4wt%.