Effect of silica fume on geopolymer materials

What is it about?

The influence of silica fume (SF) addition on properties of geopolymer materials produced from alkaline activation of alumino-silicates metakaolin and waste concrete produced from demolition works has been studied through the measurement of compressive strength, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy (SEM) analysis. Alumino-silicate materials are coarse aggregate included waste concrete and fired kaolin (metakaolin) at 800°C for 3 h, both passing a sieve of 90 μm. Mix specimens containing silica fume were prepared at water/binder ratios in a range of 0.30 under water curing. The used activators are an equal mix of sodium hydroxide and silicate in the ratio of 3:3 wt.%. The control geopolymer mix is composed of metakaolin and waste concrete in an equal mix (50:50, wt.%). Waste concrete was partially replaced by silica fume by 1 to 10 wt.%. The results indicated that compressive strengths of geopolymer mixes incorporating SF increased up to 7% substitution and then decreased up to 10% but still higher than that of the control mix. Results indicated that compressive strengths of geopolymer mixes incorporating SF increases up to 7% substitution and then decreases up to 10% but still higher than the control mix, where 7% SF-digested calcium hydroxide (CH) crystals, decreased the orientation of CH crystals, reduced the crystal size of CH gathered at the interface, and improved the interface more effectively.

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Why is it important?

Alkaline activation of aluminosilicate wastes in the presence of silica fume and metakaolin using sodium hydroxide and sodium silicate in the ratio of 3:3 wt.% of the dry weight leads to the formation of an aluminosilicate geopolymer that possess an enhancement in both mechanical and microstructural properties. Silica fume addition is likely to result in the formation of nucleation CSH sites for the precipitation of the geopolymer product. Metakaolin inclusion greatly improves the geopolymeric structure. Silica fume addition up to 7% greatly enhances the geopolymerization process with the formation of a well-refined and compact matrix, while further increase of SF content leads to the decrease in the mechanical characteristics of the reaction product. Alkaline activation of these aluminosilicate wastes can help in an ideal utilization of demolition wastes in disposal landfills all over the world and producing valuable sustainable materials that can be better applied in building industry.