Strength and resistance to sulfate and sulfuric acid of ground fluidized bed combustion fly ash–silica fume alkali-activated composite

• Silica fume was used to improve the properties of FBC fly ash geopolymer composite.
• Intergrinding of FBC fly ash and silica fume was used.
• Silica fume improved the strength and resistance to sulfate and acid of geopolymer.
• Silica fume converted CH to C–S–H which was more stable at high temperature up to 800 °C.

Fluidized bed combustion (FBC) is an environmentally friendly process for burning of coal and is used in many small factories located in urban area. The FBC fly ash is an environmental problem and needs good disposal or utilization. This research studied the strength and resistance to sulfate and acid of alkali-activated FBC fly ash–silica fume composite. The FBC fly ash was interground with silica fume (at the dosage levels of 1.5%, 3.75% and 5.0%) to make the source material homogenous with increased reactivity. Addition of silica fume enabled the adjustment of SiO2/Al2O3 ratios (6.55-7.54) of composite and improved the strength and resistance to sulfate and acid of composite. The composite with 3.75% silica fume showed the optimum strength with 28-day compressive strength of 17.0 MPa. The compressive strengths of composite with 3.75% silica fume immersed in 5% magnesium sulfate solution and 3% sulfuric acid solutions were substantially higher than the control. The strength loss was from the high calcium content of FBC fly ash and incorporation of silica fume thus increased the durability of the composite.

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