| Geopolymer is a type of aluminosilicate inorganic polymer material consists of tetrahedral [AlO4] and [SiO4] units, which is of amorphous to semicrystalline. Geopolymer is of high strength, corrosion resistance, excellent thermostability and workability, due to its three-dimensional network structures, and is suitable for use in a wide range of areas including construction, refractory and immobilized nuclear waste applications. The literatures show that geopolymers contain some nanocrystallines, furthermore, the molecular structure of geopolymers is similar to that of zeolite. Therefore, it is very important to study the relationship between geopolymers and zeolites and the reaction mechanism of zeolites transforming from geopolymers.However, it is difficult to confirm the reaction mechanism of zeolites transforming from geopolymers because the chemical composition of metakaolin is very complex. High purity Al2O3-nSiO2powders (n=1,2) were synthesized by sol-gel method. The Na2O-2SiO2-Al2O3geopolymer materials were synthesized with synthetic powders. The structure of geopolymers and NaA zeolite transformed from these geopolymers were investigated by SEM、 HRTEM、XRD、FTIR and MAS NMR testing techniques. The composition and structure change rule of geopolymers and their hydrothermal products were obtained. The mechanism of NaA zeolites transformed from geopolymers was studied. The densification NaA membrane was successfully synthesized and used in desalination of seawater. The principal results are as follows:(1)Al2O3-SiO2powders and Al2O3-2SiO2powders were fabricated utilizing sol-gel method. Molecular structure and alkali-activated properties of these two powders were investigated by DTS、SEM、XRD and MAS NMR techniques. These results show that Al2O3-SiO2powders and Al2O3-2SiO2powders were amorphous and high reactivity powders. The properties and structure of these powders were similar to metakaolin. Al2O3-SiO2powders and Al2O3-2SiO2powders were ideal precursor materials used for the study of the reaction mechanism of geopolymerization and the preparation of NaA zeolites.(2)According to the theoretical proportion of NaA zeolite, the composition of raw materials of geopolymers was designed. The mechanical properties of geopolymers was studied. The optimum process parameters were determined by the compressive strength of the geopolymer products as the main criterion. The effects of curing temperature, curing time, different modulus of water glass and the molar ratios of two different Si sources on the compressive strength of geopolymers. The optimum process conditions of the preparation of geopolymers were determined as follows:the modulus of water glass was1.0; the molar ratios of the Si derived from sodium silicate(Na2O·SiO2) to the Si derived from Al2O3-SiO2powders was1:1; the curing temperature was60℃and curing time was6h.(3) The effects of the concentration of NaOH in hydrothermal mother liquid, the volume of hydrothermal liquid, hydrothermal time, hydrothermal temperature, different modulus of water glass and different molar ratios of Si source on the structure and strength of NaA zeolites converted from geopolymers. The optimum process conditions of the preparation of NaA zeolites derived from geopolymers are as flollows:the concentration of NaOH in hydrothermal mother liquid was0mol/L, the volume of hydrothermal mother liquid was50ml, the hydrothermal time was6h, the hydrothermal temperature was90℃, the modulus of water glass was1.0, the molar ratios of the Si derived from sodium silicate (Na2O·SiO2) to the Si derived from Al2O3-SiO2powders was1:1. (4) The structures of Al2O3-SiO2powders, Na2O-Al2O3-2SiO2geopolymers and NaA zeolite were analysed by HRTEM、SEM、FTIR、MAS NMR and BET techniques. Effects of the molar ratios of Si to Al and alkalinity on NaA zeolites derived from Na2O-Al2O3-2SiO2geopolymers. The relationship between Na2O-Al2O3-2SiO2geopolymers and NaA zeolites was mass transfer and genetic structure. Na2O-Al2O3-2SiO2geopolymers were synthesized by two silicon source system with Al2O3-SiO2powders and sodium silicate. Na2O-Al2O3-2SiO2geopolymers were converted into NaA zeolite after hydrothermal procedure. The reaction mechanism of NaA zeolites derived from geopolymers can be divided into depolymerization, condensation polymerization and hydrothermal crystallization processes. In depolymerization process, the Al-O and Si-O bonds derived from Al2O3-2SiO2powders were broken to form [Al(OH)4]-and [OSi(OH)3]-groups. The water glass contains a few polysilicate, except orthosilicates. In later stage of depolymerization process, pentacoordinate aluminosilicate intermidates were synthesized with [Al (OH)4]-and [OSi(OH)3]-groups, the pentacoordinate aluminosilicate intermidates formed the primary gel around Na+. In condensation polymerization process, the polycondensation of [Al(OH)4]-and [OSi(OH)3]-groups occurred to form polymer gels with the basic structure unit of-Si-O-Al-O-in the hydration layer of hydrated sodium ions which are located in initial gel, the interface between solid and liquid and pores. These gels converted into nano ordered structure. The amorphous three-dimensional network structures with tetrahedral [AlO4] and [SiO4] units generated around the nano ordered structure. In hydrothermal crystallization process, The amorphous three-dimensional network structures from Na2O-Al2O3-2SiO2geopolymers converted into NaA zeolite crystals.(5) Self-supporting NaA membrane was successfully synthesized by the method of NaA zeolite transformed from geopolymers. The microstructure and molecular structure of NaA membrane were analysed. It was investigated that the NaA membrane was used in desalination of seawater. These results show that the NaA membrane was densification uniformity, the compressive strength of NaA membrane was57.6MPa, in the NaCl solution with the mass percent of3.5%at the temperature of25℃,the water flux of NaA membrane increase from0.42to4.74kg·m-2·h-1with the film thichness of NaA membrane decreasing from9.4mm to2mm. The rejection rate of sodium ion was above99.5%, the water flux of NaA membrane was0.42kg·m-2·h-1, when the film thickness was9.4mm. The water flux of NaA membrane in NaCl solution increased with the decrease of pervaporation temperature. The water flux of NaA membrane with the film thickness of9.4mm, reached3.86kg·m-2·h-1at the pervaporation temperature of90℃. |
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