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The article presents the results of physical and chemical properties of lightweight aggregates (LWA) obtained by the thermal treatments of raw composition based on fly ash, supplied by electric plants from Serbia. The production process of LWA consists of raw material preparation, plastic shaping-extrusion, granulation, and thermal treatment at three temperatures: 1100, 1150, and 1200 °C. The final firing temperature ( T = 1150 °C) is chosen based on the mechanical and physical properties of the designed aggregates. The particle-size distribution of the LWAs is unimodal ( d ≈ 16 mm) while the density value varies from 0.98 to 1.99 g/cm. The water absorption values are determined by use of two methods: 24 h of soaking in cold water and 5 h of boiling. The thermal conductivity of unbound, fired LWA particles is determined by measuring the amount of axially transferred heat in the stationary state. The obtained value of the LWA thermal conductivity ( λ = 0.0872 W/mK, T = 1150 °C) is suitable for the production of structural concrete blocks with improved thermal insulating properties. Because of their high-porosity and -compressive strength values, the designed LWA could be used instead of the conventional aggregates in the production of concrete blocks. Consequently, a real valorization of the waste material such as fly ash in Serbia was established. [ABSTRACT FROM AUTHOR]