Effect of decarbonization of high carbon fly ash on workability, mechanical properties and durability of concrete

Abstract

The carbon content of fly ash (FA) influences the performance of concrete. In this paper, the flotation decarburization treatment was carried out on high carbon fly ash (HCFA) to obtain flotation decarburized fly ash (FDFA). The effects of flotation decarbonization treatment of fly ash on workability, mechanical properties, chloride penetration, carbonation, and water/sulfate frost damage to concrete were systemically studied. The results indicated that flotation decarbonization treatment could remarkably reduce the water demand ratio and increase the intensity activity index of fly ash. Moreover, compared with HCFA, FDFA could better enhance the workability and increase the compressive strength of concrete. When the water-cement ratio is 0.43, and the fly ash content is 35%, the slump and 56 d compressive strength of concrete with FDFA were 60% and 23% greater than that of concrete with HCFA, respectively. Furthermore, FDFA could better improve the microstructure and the resistance to chloride penetration, carbonation, and freeze-thaw of concrete. When the water-cement ratio was 0.43 and fly ash content was 35%, the carbonation depth and chloride migration coefficient of concrete with FDFA were 69.95% and 17.24% lower than those of concrete with HCFA, respectively. In comparison to water freeze-thaw cycles, sulfate freeze-thaw cycles caused more damage to concrete. The critical value of the pore diameter for pore solution freezing (i.e., 14 nm) was proposed to illustrate the damage degree of freeze-thaw to concrete. A "frost-heaving-corrosion" composite failure theory was employed to explain the failure mechanism of concrete exposed to the sulfate freeze-thaw cycles.