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In this study, the alkaline solutions (NaOH) with concentrations from 1M to 18M, red mud (RM) and silica fume (SF) were used as reactors in geopolymer reactions. RM contains 7.40% SiO2 and 13.65% Al2 O3 and SF has 94.50% SiO2 , but only the active oxides can participate in the geopolymer reactions. The activity of the oxides was investigated by determining the compressive strength of the samples under different curing conditions. The characteristics of the geopolymer samples were determined by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermo-gravimetric analysis (TGA)/differential thermal analysis (DTA) and nuclear magnetic resonance analysis (NMR). The experimental results indicate that active silica mainly exists in SF. In the structure of geopolymers, the silicon can bond directly with each other (Si-Si) or be linked through ‘bridging’ oxygen (Si-O-Si) to form independent polymer chains, while aluminium atoms can only replace the silicon atoms in Si-O-Si polymer chains to form Si-O-Al instead. | PHYSICAL SCIENCES ENGINEERING The role of active silica and alumina in geopolymerization Van Quang Le1 Quang Minh Do2 Minh Duc Hoang3 Hoc Thang Nguyen4 Vietnam Institute for Building Materials 2Ho Chi Minh city University of Technology Vietnam Institute for Building Science and Technology 4Ho Chi Minh city University of Food Industry Received 23 January 2018 accepted 18 April 2018 Abstract In this study the alkaline solutions NaOH with concentrations from 1M to 18M red mud RM and silica fume SF were used as reactors in geopolymer reactions. RM contains 7.40 SiO2 and 13.65 Al2O3 and SF has 94.50 SiO2 but only the active oxides can participate in the geopolymer reactions. The activity of the oxides was investigated by determining the compressive strength of the samples under different curing conditions. The characteristics of the geopolymer samples were determined by using X-ray diffraction XRD Fourier transform infrared spectroscopy FTIR thermo-gravimetric analysis TGA differential thermal analysis DTA and nuclear magnetic resonance analysis NMR . The experimental results indicate that active silica mainly exists in SF. In the structure of geopolymers the silicon can bond directly with each other Si-Si or be linked through bridging oxygen Si-O-Si to form independent polymer chains while aluminium atoms can only replace the silicon atoms in Si-O-Si polymer chains to form Si-O-Al instead. Keywords active silica alumina geopolymer red mud silica fume. Classification number 2.3 Corresponding author Email quanghuce83@gmail.com Introduction Geopolymer is an inorganic polymer with structural units of SiO4 4- and AlO4 5- tetrahedrons 1 . The principle of the process is the formation of a polymer from the reaction of an alkaline solution NaOH KOH Na2SiO3 and K2SiO3 solutions with alumino-silicate resources 2 3 . The structure of the geopolymer is a bonding of amorphous or semi-crystalline metal oxides with an alkaline element 4 . Therefore raw materials for synthesising the
