Influence of fly ash on engineering properties of cohesive soils

The need for energy constantly increases due to industrialization, urbanization, and technological developments. Thermal power plants play an important role in fulfilling such energy demand. A significant amount of coal fly ash is generated every year as lignite and anthracite coal are burnt in thermal power plants, called the coal combustion process. The fly ash is usually stored outside the power plants, causing serious environmental and economic problems. Hence, the dismissal and reuse of fly ash are of great importance. The use of fly ash in the stabilization of cohesive soils offers economical and sustainable solutions to increasing environmental problems. Fly ash does not have self-hardening and cementation properties; however, its pozzolanic effect increases significantly in the presence of compounds such as calcium oxide in an aqueous environment. In the literature, hydrated lime is often used to accelerate the pozzolanic reaction of fly ash and increase its cementation ability. In this study, instead of hydrated lime, marble powder waste was used along with Class F fly ash, obtained from the Seyitömer thermal power plant. Fly ash and marble dust waste were added to the soil (kaolin clay) at the proportions of 3%, 6%, 9%, and 12% to form soil only, soil + fly ash and soil + fly ash + marble powder mixtures. Comprehensive laboratory experiments, including consistency limit, free swelling, linear shrinkage, compaction, unconfined compression, direct shear box, falling cone, and California Bearing Ratio (CBR), were conducted on the prepared mixtures to elucidate the effects of fly ash and marble dust wastes on the physical properties and mechanical behavior of cohesive soils. Some mixtures were cured for 7 and 28 days and then subjected to unconfined compression tests to examine the effect of curing time. The detailed analysis of the experimental results showed that the liquid limit, plasticity index, swelling, and shrinkage potential of kaolin clay decreased with the increase in fly ash content. The observed reduction in these values was much higher when fly ash was used along with waste marble dust. The soil-fly ash-marble dust waste mixtures had a higher maximum dry unit volume weight and lower optimum water content values compared to the soil-fly ash mixtures. In addition, the unconfined compressive strength of the soil specimens appeared to increase significantly as marble dust waste and fly ash were added together to kaolin clay and cured for a certain period. Similarly, the undrained shear strength of kaolin clay was found to increase significantly with the increase of fly ash and fly ash-marble powder waste content. The direct shear box test results indicated that the cohesion and internal friction angle values of the soil mixtures increased with increasing fly ash and fly ash-marble powder waste content, and the mixtures had higher cohesion and internal friction angle values compared to the pure soil specimens. The CBR value of kaolin clay increased from 16.86% to 24.46% with 6% fly ash content, and this value increased to 31.20% with 6% fly ash-marble powder waste mixture. The findings presented in this study emphasize that the use of fly ash together with different industrial wastes, especially marble dust, will produce very promising results in the stabilization of problematic soils. The positive changes in the physical and mechanical properties of kaolin clay are expected to be more apparent in the case of soil-fly ash-waste marble dust mixtures as compared to soil-fly ash mixtures.