Turning waste into construction materials recently gets much attention from the researchers in the world due to the advantages of not only the eco-friendly environment but also the positive enhancement of material characteristics. Thus, this study investigates the feasibility of the use of a ternary mixture consisting of cement, ground granulated blast-furnace slag (GGBFS), and fly ash (FA) for producing foamed ultra-lightweight composites (FULC) with the designed dry density of approximately 700 kg/m3. The FULC specimens were prepared with various FA/GGBFS ratios (16/24, 20/20, and 24/16) and foaming agent/water ratios (1/60, 1/80, 1/100, and 1/120). The constant water-to-binder ratio of 0.2, cement content of 40 % by mass, and superplasticizer dosage of 0.2 % by mass were applied for all FULC mixtures. Properties of the FULC specimens were evaluated through laboratory tests of compressive strength, dry density, thermal conductivity, water absorption, and thermal behavior following the relevant ASTM standards. Additionally, both the microstructure observation and cost analysis of all FULC mixtures was performed. Test results show that reducing GGBFS content resulted in a reduction in the compressive strength, dry density, thermal conductivity, and cost of the FULC. A similar trend could be observed when reducing the concentration of foam in the FULC mixtures. As the results, the 28-day compressive strength, dry density, thermal conductivity, water absorption, and cost of the FULC were in the ranges of 4.41–5.33 MPa, 716–729 kg/m3, 0.163–0.182 W/mK, 41.5–48.5 %, and 15.3–20.9 USD/m3, respectively. Furthermore, the FULC exhibited excellent performance under fire conditions as the maximum temperature at the internal surface of the FULC and the normal brick walls were 122 °C and 3180°C after 120 minutes of firing, respectively. Consequently, both GGBFS and FA had enormous potential for the production of FULC.