The paper presents the problem of utilisation of extruded polystyrene wastes by its secondary use as a filler for cement composites. Wastes was obtained from a company carrying out thermomodernizations works in the form of shapeless, undersized construction waste. The remains of the boards were crushed and separated using the sieve method. Basic technical parameters of thus prepared waste were assessed in terms of its use as a lightweight aggregate for concrete. The technical properties of wastes were compared with those of traditional aggregates used in concrete composites. During the research work, samples of a cement composite were produced in which the waste served as a filler. In successive test series, the waste constituted 30, 50 and 100% of the aggregate substitution in the mix. Comparative samples without waste were also made. The results showed that the waste could potentially be used to produce lightweight cement composites with high strength parameters.