Abstract: The present invention relates to a novel additive for hydraulic cements, mortars and concretes, particularly for improving the mechanical strengths and the workability and for reducing sweating and segregation, which is characterized in that it is a protein hydrolysate having a pH less than or equal to 6, comprising substances selected from the group constituted by proteins, polypeptides, aminoacids and mixtures thereof, and obtained by treating 100 volumes of an aqueous composition containing at least one protein substance selected from the group constituted by proteins, polypeptides and mixtures thereof, with at least one bacterial strain producing lactic acid having a population greater than or equal to 10.sup.2 germs/cm.sup.3, in the presence of 0.5 to 30 parts by weight of a source of carbon belonging to the family of carbohydrates, for at least 2 hours, at a temperature of between 0.degree. and 65.degree. C. and at a pH less than or equal to 6.