Abstract
This study investigates the improved reactivity of a geopolymer based on a combination of fly ash and blast furnace slag (BFS) by the addition of silica fume. The geopolymer was synthesized by activating a mixture of fly ash, BFS, and three different types of silica fume with alkali activator. X-ray diffraction (XRD) and inductively coupled plasma-optical emission spectroscopy (ICP-OES) were utilized to characterize the reaction. The silicate structure was also analyzed by nuclear magnetic resonance (NMR) spectroscopy. From these results, it was found that the replacement of fly ash with the silica fume led to a significant decrease in theQ4(1Al) and an increase in theQ4(2Al),Q4(3Al), andQ4(4Al). The Si/Al ratio of the aluminosilicate gel was relatively constant, ranging from 2.0 to 2.6, while the Si/Al ratio of the C-S-H gel increased with the addition of silica fume. Therefore, some of the Al dissolved from the slag contributed to the formation of aluminosilicate gel, and the remnant slag particles mostly participated in the formation of the C-(A)-S-H gel with a decrease in theQ2(1Al). The increase in the reactivity of slag caused by the addition of silica fume was attributed to the reaction of the Al in the slag with the silica fume.
Highlights
Geopolymers have much potential as construction materials due to their excellent mechanical properties
A diffuse band at 15–30∘ was observed in the raw silica fume, and there were only minor differences between Silica fume 1 (SF1) and Silica fume 2 (SF2) materials; a number of crystalline peaks were observed in zirconium silica fume
The Q4(2Al) and Q4(3Al) increased with a decrease in the Q4(4Al) in order to maintain a constant Si/Al ratio of the aluminosilicate gel ranging from 2.0 to 2.6. These results demonstrate that the addition of silica fume in the fly ash-slag geopolymer changed the silicate structure of C-S-H and the aluminosilicate gel, which may affect the durability of geopolymer concrete
Summary
Geopolymers have much potential as construction materials due to their excellent mechanical properties. Okoye et al [11] investigated the effects of different dosages of silica fume in fly ash-based geopolymer concrete which were activated with NaOH and sodium silicate and cured in an oven at 100∘C. The results showed that the addition of silica fume improved the compressive strength levels of the geopolymer concretes. Because geopolymers consisting of both fly ash and slag contain multiple gels (e.g., N-A-S-H, C-A-S-H) with different characteristics [17], the content, chemical compositions, and silicate structures of these gels can be changed with the addition of silica fume to the geopolymer. The role of silica fume on the characteristics of fly ash-based geopolymers needs further clarification to realize improved durability of geopolymers. This study investigates the improved reactivity of a geopolymer based on a combination of fly ash and blast furnace slag (BFS) by the addition of silica fume. The silicate structure was analyzed by nuclear magnetic resonance (NMR) spectroscopy
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