•Disk or rotor–stator dissolver is not suitable to disperse ultrafine milled cement.•Ultrasound is much more effective than disc or rotor–stator dissolvers.•Dispersion with ultrasound makes it possible to seal fracture aperture down to 54 µm. Ever higher demands on the sealing of rock around underground structures drive the development of cement-based grouts. The results of the previous study demonstrated that it is possible to mill cement to approximately 20 to 25 µm and use it to seal fracture aperture down to approximately 70 µm. Further milling deteriorates the penetrability of the grouts due to the flocculation of the fine particles. This study investigates ultrasound as a technique to improve dispersion of grouts based on ultrafine milled cement. The grout tested is based on cement where 95% of the cement mass has particles smaller than 12 µm and is very hard to disperse. Dispersion with ultrasound is compared with a conventional laboratory mixer equipped with a disk and rotor–stator system. The dispersion efficiency of this mixing equipment was tested by a filter pump. The results confirmed that a conventional laboratory mixer equipped with a disk is not at all suitable for dispersion of this grout. The laboratory mixer equipped with a rotor–stator system is much better than a disc but still cannot disperse this fine milled grout properly. Ultrasound was found to be the most effective method. The dispersion achieved, measured with a filter pump, was around 54 µm which indicates that it is possible to seal fracture aperture now down to 54 µm, which is a significant improvement.