Abstract
Circulating Fluidized Bed (CFB) combustion is a clean technology for burning, with advantages of adapting to a large variety of fuel, high combustion efficiency, lower NOx emissions, and stable operation. The residue collected from the ash-hoppers of the electrostatic precipitator of the CFB boiler is called CFB fly ash. This paper presents the hydration development on the application of CFB fly ash to activating blast furnace slag (BFS) as a supplementary binder material (SBM) for replacement of Portland cement in making concrete. Investigation of the hydration products of cement pastes prepared with combinations of BFS and CFB fly ash were conducted by means of X-ray diffraction, thermal gravimetric analysis, and scanning electronic microscope. Test results show that the main hydration products of the CFB fly ash-BFS blended pastes were found to be hydrated calcium silicate (C-S-H), ettringite, gypsum, and some portlandite. Considering that CFB fly ash produced from the combustion of high-sulfur coke has high SO3 contents, the volume stability of mortar made from CFB fly ash-activated BFS was subjected to tests in accordance with ASTM C1012 and ASTM C1038 for evaluating the internal and external sulfate attack, respectively. The results indicate that, due to the high sulfur (SO3) content of CFB fly ash, the expansion caused by internal sulfate attack (ISA) increased with increasing proportion of CFB fly ash in the mixture. In contrast, no significant expansion was observed in the external sulfate attack (ESA) test, regardless of the proportion of CFB fly ash in the mixture. In order for the CFB fly ash to serve as a supplementary binder material and to maintain adequate volume stability, the amount of CFB fly ash used for the activation of BFS is recommended to be no more than 20% of the SBM.
Highlights
Circulating fluidized bed boiler combustion is a clean technology for coal burning, which has the advantages of adapting to a large variety of fuel, high combustion efficiency, lower SOx and NOx emissions, and stable operation over a large range of load regulation
Internal sulfate attack and volume expansion caused by the presExperimental study was conducted on mixtures of cement-blast furnace slag (BFS)-circulating fluidized bed (CFB)
The of following drawn: CFBinternal fly ashand be blended at a ratio no moreconclusions than 20% inwere order to maintain the dimensional of thefrom concrete
Summary
Circulating fluidized bed boiler combustion is a clean technology for coal burning, which has the advantages of adapting to a large variety of fuel, high combustion efficiency, lower SOx and NOx emissions, and stable operation over a large range of load regulation. The circulating fluidized bed (CFB) boiler is a means of energy-generating process by burning petroleum coke only. The high-sulfur petroleum coke is a by-product of high-sulfur oil refining industry, with a sulfur content of approximately 7%. In order to avoid blazed petroleum coke with high sulfur content from emitting overdosed sulfur dioxide (SO2 ), limestone (CaCO3 ) is introduced in the boiler for desulfuration. The ash collected from CFB boiler has a rich content of anhydrite. Poon et al [1] reported that, in the cement-fly ash binding system, calcium sulfate anhydrite can act as activating material, and a large quantity of ettringite (AFt) can be formed during the early stage of hydration in the anhydrite-activated cement-fly ash pastes
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