An Experimental Study on the Thermal Behavior and Mechanical Properties of Fiber-Reinforced Geopolymer Composites Based on Fly Ash and Blast Furnace Slag

This study aims to develop environmentally friendly and sustainable geopolymer concrete (GBS) using industrial waste, and to investigate its physical, mechanical, and high-temperature performance. In the study, ground blast furnace slag (GBS) was used as the primary binder, and fly ash (FA) was substituted for the binder at varying rates (0%, 10%, 25%, 50%, and 75%). Additionally, waste marble dust was used as an aggregate, and glass fiber was used as a reinforcing element. To improve the workability of the prepared mixtures, a superplasticizer was used at a rate of 5%, and the water-to-binder ratio was maintained at 0.19 for all samples. The prepared geopolymer concrete samples were subjected to thermal curing at 70 °C for 24 hours. As a result of the experiments conducted, it was determined that as the fly ash content increased, the flow diameters of the concrete decreased and the dry density values dropped from 2136 kg/m³ (GBS 100%) to 1972 kg/m³ (FA 75%). In terms of mechanical properties, the 28-day compressive strength was measured at 50.4 MPa in samples containing 0% fly ash. In comparison, the samples containing 75% fly ash yielded a strength of 30.7 MPa, indicating that strength decreases as the fly ash content increases. In high-temperature tests, it was found that as the fly ash content increased, the loss of compressive strength decreased in samples exposed to 250, 500, and 750 °C. The compressive strength lost is 13.48% for 250 °C, 17.54% for 500 °C and 75.88% for 750 °C. At 250 and 500 °C, the compressive strength loss rate was relatively low, whereas it was significantly higher at 750 °C. The primary reason for this is the low calcium content of fly ash and the high thermal stability of amorphous aluminosilicate phases. Additionally, according to the results of the cost analysis, increasing the amount of fly ash reduces the production costs of geopolymer concrete. Although the materials used in geopolymer concrete are industrial by-products, the procurement costs of these materials are influenced by several factors, including geographical region, supply chain, usage quantity, and local industrial activities. In conclusion, it has been demonstrated that geopolymer concrete produced from industrial waste reduces environmental impacts and provides significant advantages in terms of sustainability.