Hydraulic grouts, although used for a long time for injecting sufficiently large cracks, cannot penetrate into narrower spaces, such as millimetric cracks, because of clogging. This paper presents the results of research carried out by LCPC, on the possibility of changing certain properties of cement grouts by adding ultrafines (silica fumes, lime, earth of Santorin), together with superplasticizers to their formulations. A thorough study of gradings and penetrability characteristics has enabled the formation of hydraulic grouts for injection of fine cracks. Durability tests have shown their behaviour when subjected to an aggressive environment. The mechanical strength values of such grouts have been measured and the role of the association of various types of ultrafines has been examined. Finally, the bond strength of hydraulic grouts to different types of limestone has been studied by means of direct tensile test.