Mechanical properties and microstructural discrepancies of concrete with flotation-modified fly ash from circulating fluidized bed and pulverized coal furnaces

Abstract

Fly ash (FA) is a by-product of coal-fired power generation in thermal power plants. Disparities in coal types and combustion furnaces result in varying levels of unburned carbon in FA, hindering its effective utilization. In this study, FA from a circulating fluidized bed furnace (CFBFA) and pulverized coal furnace (PCFFA) was modified using froth flotation technology, and the influence of the modified FA from the froth flotation process on the mechanical properties and microstructural discrepancy of concrete was analyzed through the preparation of mortar and cement paste specimens. The results indicated that the flotation process proved effective in reducing the unburned carbon content in FA, with modified PCFFA showing higher surface area and fluidity, opposite to CFBFA. Notably, the flotation process altered CFBFA's mineral content, reducing CaSO4·2 H2O from 26.6% to 7.5% and increasing CaSO4, enhancing hydration and strength in concrete applications. The optimal FA dosage was found to be 20%, with CFBFA specimens displaying superior compressive strength across all tests. Conversely, the modified PCFFA exhibited low activity, making it challenging to reach the desired strength after 28-d. This study serves as a theoretical foundation for the application of highly unburned carbon FA in construction engineering and promotes the circular development of FA resources.