AbstractThis work investigated the feasibility of replacing quartz aggregates with discarded high-density polyethylene (HDPE) particles in a cementitious matrix for secondary structural applications in civil construction. A mixture design technique was used to assess the effect of the constituents on the physical-mechanical properties of composites, such as density, water absorption, flexural strength, compressive strength, and dynamic modulus. The aggregate/cement ratio ranged from 3.76 to 5.25, and the water/cement ratio was kept constant at 0.5, whereas a minimum of 30 vol% HDPE particles was considered in the system. The optimized responses and fractions were determined by considering five different scenarios through the statistical method desirability. The gradual replacement of quartz aggregates by HDPE led to a more porous material, increasing water absorption and reducing mechanical properties. However, these composites exhibited a less rigid characteristic, revealing a less brittle failure mode. Whereas the optimized density was obtained for the minimum matrix and maximum HDPE fractions, the optimal response for the mechanical properties and water absorption presented an opposite behavior, that is, maximum cement matrix fraction and minimum HDPE fraction. Multiobjective optimization proved to be a promising technique for identifying the optimal mix of recycled microconcrete for a given application.