Using waste materials and by-products from various building sectors is gaining popularity because natural resources are quickly depleting. Geopolymer concrete is made from by-product material and is a relatively new environmental material that does not require the presence of ordinary Portland cement as a binder. This study involves producing lightweight geopolymer concretes using a slag binder and replacing the conventional fine and coarse aggregates with two types of locally available lightweight aggregate (thermostone and montmorillonite). Several sequential steps were used to process the aggregates, along with a series of tests conducted to evaluate the concrete properties in different states. The fresh state test includes the slump test, while the hardened concrete tests involve compressive strength, flexural strength, density, thermal conductivity, and water absorption. This study reveals the suitability of lightweight concrete mixtures for various constructional applications. The most reliable mixture was the GMM, which consisted of coarse montmorillonite (5–25 mm) and fine montmorillonite aggregates. This mixture exhibited the highest compressive strength of 23.3 MPa, a flexural strength of 3.25 MPa compared to other LWGC mixtures, and a low density of 1785 kg/m³. The GMM density was 23.97% lower than the reference mixture, whereas the thermal insulation significantly improved by 65.58%. Consequently, this improvement was evident in the thermal conductivity coefficient, which measured as approximately 0.349 W/m.K. In addition, the GTT mixture containing thermostone aggregate (5-25 mm) yielded the most optimal thermal insulation and lowest density of 0.267 W/m.K and 1552 kg/m³, respectively. In general, the strength and density of the LWGC mixtures in this study meet the requirements of lightweight structural applications.