Preparation Of Al₂O₃-SiO₂ Powders By Chemical Synthesis And Research On Performances Of Geopolymers

Pure Al₂O₃-SiO₂ powders with different silicon-aluminum ratios were prepared by sol-gel method, with hydrated aluminum nitrate?ANN? and ethyl silicate?TEOS? as the main raw materials. The influence of different process parameters such as synthesis temperature,calcination temperature, system pH value and ratio of silicon to aluminum on the alkali activation reactivity of powders was studied.Geopolymer materials with different compositions were prepared with the powders as raw materials and water glass as alkali activator. The effect of different process parameters such as water glass modulus, solid to liquid ratio?mass ratio of powders to water glass?, curing temperature and curing time on the compressive strength of the geopolymer materials was investigated. Based on the compressive strength of the geopolymer materials as the optimization objective, the optimum process parameters for preparing high-reactivity powders are as follows:the synthesis temperature is 70?, system pH value 0.05, and calcination temperature 800?. The optimum process parameters for preparing the geopolymer materials are as follows: water glass modulus is 1.2, liquid solid ratio of water glass to powders 1.1, and curing temperature 60?.Pure Al₂O₃-2SiO₂ powder with alkali reactivity was prepared with hydrous aluminum nitrate?ANN? and tetraethoxysilane?TEOS? as the main raw materials by sol-gel method and different precipitation ways of the coprecipitation method. The results show that the alkali activation reactivity of powders prepared by sol-gel method is higher than that of the powders prepared by the coprecipitation method.For the coprecipitation method, the alkali activation reactivity of the powders prepared by the dropwise addition is higher than that of powders prepared by single-batch coprecipitation.By means of FTIR, XRD, 27 Al and 29 Si NMR, the structure of the powders and the geopolymer materials in the study were characterized. The results show that the materials fabricated with the as-prepared powders by alkali activation have the typical characteristics of the geopolymer materials. In particular, based on 27 Al and 29 Si NMR data, alkali activation reactivity of the powders was compared. 27 Al NMR spectra show that the content of 5coordinated aluminum in the powders prepared by sol-gel method is higher than that in the coprecipitation method and the content of 5 coordinated aluminum is consistent with the alkali activation reactivity of the powder. 29 Si NMR spectra show that during the activation of the powder prepared by sol-gel method, the main change of the molecular structure is that siliconwas replaced by aluminum at secondary coordination sites of the central Si atoms. However, for the powder prepared by single-batch coprecipitation, the main change is from a low degree of polycondensation to a high degree of polycondensation.