| A geopolymer is one type of aluminosilicate inorganic polymer materials composed by tetrahedral [AlO4] and [SiO4] units with three-dimensional structures. Because of its excellent property of high strength, anticorrosion, standing fire and long life, the research of the novel geopolymer has recently become a hot spot in the field of inorganic polymer material in the world. At present, materials such as metakaolin, fly ashes and blast furnace slag are often used to produce geopolymers. However, since the chemical composition of these materials is very complex and various with their sources, their application is limited to some extent. Sol-gel process is a novel method for preparation of powders and has received much attention in recent years, due to high purity, good homogeneity and high reactivity of materials prepared in this process.As the purity and Si/Al ratio of the powders for the geopolymer are easily adjusted in the sol-gel process, the properties and composition of the powders can be controlled with relative ease. If the powders prepared by this process are able to be alkali-activated, they will be useful because the composition of powders can be designed according to our requirement, which is beneficial to control of purity and performances of geopolymer products. Hence, the application of geopolymers would be more extensive in the future.In this study, pure Al2O3-2SiO2 powders for a geopolymer were prepared by a chemical synthesis methods with tetraethoxysilane (TEOS) and aluminum nitrate (ANN) as the starting material referring to the principal composition of metakaolinite, that is, Si/Al molar ratio of 1. At the same time, alkali-activation reactivity and structure of the powders and optimization of process parameters for preparing powders were systematically investigated. The principal results are as follows.The pure Al2O3-2SiO2 precursors (powders) for a geopolymer have been prepared by the sol-gel method. The alkali-activated products derived from the powders meet the general criteria for a geopolymer. At the same time, the powders with different Si/Al molar ratios have also been prepared. As the purity and Si/Al ratio of the powders for the geopolymer are easily adjusted in the sol-gel process, the properties and composition of the powders can be controlled with relative ease. Thus it is not only easy to control purity and performances of geopolymer products, but also possible to design theoretically their composition and performances.The optimum process parameters for preparing high-reactivity powders were determined considering the compressive strength of the geopolymer products as the main criterion. The optimum process parameters for preparing high-reactivity powders are as follows:synthetic temperature is 70℃,the mole ratio of ethanol to TEOS about 6, the mole ratio of water to TEOS about 9, aging time about 12 h, drying time about 10 h, calcination temperature 700℃-800℃, and calcination time 4 h.The optimum process parameters for preparing geopolymers were determined considering the compressive strength of the geopolymers products as the main criterion. The optimum process parameters for preparing geopolymers are as follows:the mole ratio of SiO2 to Al2O3 is 3.0～3.5, the mole ratio of H2O to Al2O3 about 10, the mole ratio of Na2O to Al2O3 is about 1, curing temperature more than room temperature and less than 70℃.By NMR, XRD and FTIR analysis, some microstructure characteristics of the as-prepared powders and their alkali-activation products are similar to those of metakaolin and its alkali-activation product, respectively. However, the heat-treated temperature at which the powders begin to be alkali-activated is lower than that at which kaolin begins to transform to metakaolin.Analytical methods such as DTS, TEM, SEM, BET were employed to study the effects of process parameters, such as quantities of water and ethanol, hydrolysis temperature, aging time and drying methods, on the size, specific surface area and structure of nanometer powders. The optimum process parameters for preparing low agglomerative Al2O3-2SiO2 nanometer powders are as follows:the mole ratio of water and ethanol to TEOS are 0:1 and 16:1 respectively, synthetic temperature is 40℃, aging time is 36 h and the drying way is azeotropic distillation with microwave drying. The powders possess uniform size, laminar structure and monodispersity. The average particle diameters of the powders were in the range of 70-80 nm and the largest BET specific surface area were up to 669m2/g.The pure Al2O3-2SiO2 powders for a geopolymer have been prepared by a coprecipitation method with tetraethoxysilane (TEOS) and aluminum nitrate (ANN) as the starting material. Results show that the reactivity of the powders prepared by drop addition is higher than by bulk addition.The mechanism of the geopolymerization was studied By NMR, SEM, ICP, XRD and FTIR analysis. In dissolution process, the Si-O-Si or Al-O-Si bonds in the particles were broken and monomers and oligomers such as [Al(OH)4]-, [SiO(OH)3]-,[SiO2(OH)2]- etc. were formed. The dissolution rate of Al is faster than that of Si. In condensation process, resultants in the dissolution process condense and geopolymers are formed. The condensation process in these systems occurs in two stages:(a) quick condensation between aluminate and silicate species; followed by (b) a slow condensation stage solely involving silicate species. For the reaction system with the same nominal formula, the increase of the molar ratio of Si in alkali activator solution to total Si favors the geopolymerization in the range of low alkali activator solution modulus. The alkali-dissolvability of the powders shows that the alkali-dissolvability of the powders in NaOH solution does not represent geopolymerization activity completely.The Al2O3-2SiO2 powders with the high alkali-activation reactivity are of high contents of 5-coordinated Al. The reactivity of the powders prepared by sol-gel method is higher than that by coprecipitation method.
|
PDF Full Text Request