Rheological behaviors and viscosity prediction model of cementitious composites with various carbon nanotubes

•Rheological behaviors and model of cement paste with various CNTs are investigated.•Adding CNTs decreases flow index and increases critical shear rate of cement paste.•Short and thin CNTs lead high yield stress and minimum viscosity of cement paste.•Surface treatment of CNTs has obvious effect on yield stress and minimum viscosity.•CNTs’ action mechanisms result from their adsorption and entanglement effects.