Described herein are new cementitious mortars with a high degree of fluidity and high final resistance. The mortars contain water, a hydraulic binder, a pozzolanic substance (finely ground slag), a fluidifier and/or superfluidifier, a setting regulator, and aggregates having a specific granulometric distribution. The aggregates are made up of three highly monogranular fractions (A, B, C), and a fourth fraction (D) having a low monogranularity. The new mortars described herein have values of fluidity higher, by approximately 70%, than those of mortars produced with traditional aggregates. The increase in fluidity is obtained without increasing the water/cement ratio and without increasing the amount of fluidifiers/superfluidifiers present. The mortars produced according to the invention may be used to advantage in all applications in which it is required to associate a high degree of fluidity and a high final mechanical resistance, both in compression and in flexure.