| In this paper, metakaolin, potassium silicate and fly ash cenosphere, etc, were used to fabricate geopolymer and cenosphere/ aluminosilicate geopolymers composite. Transformation mechanism from kaolin to metakaolin and factors which have great effect on the chemical activity of metakaolin were systematically investigatedanalyzed by XRD, SEM, NMR and HRTEM et al. The geopolymerization mechanism was studied using the synthesized metakaolin. Effects of manufacture parameters and chemical activity of metakaolin on the structure and properties of the resulted aluminosilicate geopolymer were investigated. Based on the above studies, fly ash cenosphere/ aluminosilicate geopolymers composites were successfully prepared and structure, microstructure and properties were systematically researched.The result revealed that dehydroxylation of kaolin is the precondition of the structural rearrangements, while dehydroxylation and structural rearrangements are two separate processes. After the dehydroxylation of kaolin, the structural rearrangements of Al–O structural site occured, resulting in the amorphous metakaolin. The chemical activity of metakaolin was measured by the content of four tetrahedral Al, and the more four tetrahedral Al content, the greater chemical activity of the metakaolin. With the increasing of calcined temperature (600900°C) and calcined time (18h), the chemical activity of metakaolin increased and the highest chemical activity were obtained when calcined temperature and time are 900°C and 4h, respectively.The geopolymerization mechanism of aluminosilicate geopolymers based on synthetic metakaolin are: after mixing metakaolin particles with alkali silicate solution, dissolving of metakaolin started from its surface, i.e., Q4(1Al) sites and four, five and six coordinates of Al–O sites dissolved, together with the Si–O–Si bond and Si–O–Al bound hydrolysis, resulting in the [Al(OH)4]–, [AlO(OH)3]2–, [Al(OH)4(OH2)]–, [Al(OH)5]2–, [Al(OH)4(OH2)2]–, [Al(OH)5(OH2)]2–, [SiO(OH)3]– and a small amount of [SiO2(OH)2]2– monomers. With reaction, the Si species condense with Al species to form aluminosilicate with network structure where Si is in the form of Q4(3Al) and Q4(2Al) and Al in four coordinate.With increasing the curing temperatures (2590°C) and time (428d), when curing temperature and time are 80°C and 19d, respectively, open porosity is the lowest, bulk intensity and compressive strength of aluminosilicate geopolymers reach the highest values, which are 18.5%, 1.45g·cm–3 and 124.8MPa, respectively. Properties of aluminosilicate geopolymers based on metakaolin with various chemical activities were also different. Using metakaolin caclined under 800°C and 900°C separately, compressive strengths, flexural strengths, bulk intensities and thermal conductivities of the resulted geopolymers were 73.0MPa and 111.0MPa, 8.7MPa and 32.1MPa, 1.43g·cm–3 and 2.29g·cm–3, 0.23W·m–1·K–1 and 1.10W·m–1·K–1, respectively.After the addition fly ash cenosphere, bulk density and coefficient of thermal conductivity of the composite significant decline. With 40vol.% fly ash cenosphere, the compressive strength, bulk density and thermal conductivity of the composite are 36.5MPa, 0.82g·cm–3, 0.173W·m–1·K–1, respectively, which are much better than the common cenosphere-brick material of 7.84MPa, 0.80g?cm–3 and1.59W·m–1·K–1, respectively. |
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