The use of polymeric microparticles having a cavity in hydraulically setting building material mixtures is described, from 1 to 100% by volume of the cavity of these microparticles being filled with water. In this way, remarkable concrete resistance to the freezing and thawing cycle is achieved, these microparticles providing improved protection for the concrete from the effects of the freezing and thawing cycle, even at a diameter of from 0.1 to 1 .mu.m and in doses which are 1 to 2 orders of magnitude lower than those described in the prior art. Moreover, the compressive strength of the correspondingly hardened concrete is substantially improved, which was likewise not foreseeable.