Introduction. The main requirement of formability in the 3D concrete printing (3DCP), in contrast to the classical technology of concrete, is the dimensional stability of freshly laid layers. The dimensional stability is expressed in maintaining the geometric dimensions of the molded element, solidity, uniformity, and position in space under the influence of technological and operational factors, and determines the quality of construction products. The influence of the unmodified cement-sand mortar mobility on the quality of 3D printed hardened composites based on them has been studied. Materials and methods. The mortar mobility was determined following Russian State Standard GOST 5802-86. The samples were molded by layer-by-layer extrusion on a construction 3D printer "AMT S-6044" ("SPETSAVIA LLC", Yaroslavl, Russia). Determination of defects of cement-sand mortar mixture and hardened composites was carried out by visual instrumental method. Structure kinetics of the mortar mix was determined to ASTM C403 using a pocket penetrometer C194. Results. It was revealed that 3D printing on the investigated 3D printer is possible on cement-sand mortar mixtures of mobility classes from Pk 2 to Pk 4; however, the products have defects and damage, their appearance and nature are very different. The 3D printed mortar mobility has a significant effect on the structure kinetics and the deviation of geometric parameters. The expediency of using the penetrometric method in the study of the formation of the plastic strength of the printed mixture both in the coagulation and in the initial crystallization period is shown, which is effective in predicting the ability of the mixture to hold the formed layers without deforming the previous ones. It has been established that there are no optimal compositions of a cement-sand mortar mixture with a certain mobility class, which simultaneously meets all the requirements of the considered quality indicators of hardened composites. Conclusions. It was shown the need to apply such an approach to modifying the compositions of raw mixtures for 3D concrete printing, in which, first of all, their optimal rheological and technological properties will be achieved, ensuring high quality of molded composites increases.