Reaction Mechanism And Application Study Of Phosphoric Acid-Based Geopolymers

The phosphoric acid-based geopolymer is a type of aluminosilicate inorganic polymer materials of three-dimensional network structures, consisted of tetrahedral [AlO4],[SiO4] and [SiO4] units. In comparison to alkali-based geopolymers, phosphoric acid-based geopolymers possess excellently high strength, thermal stability at high temperature, lower dielectric constant and dielectric loss. At present, the main raw material to fabricate phosphoric acid-based geopolymers is metakaolin. However, it is difficult to confirm the reaction mechanism of phosphoric acid-based geopolymers because the chemical composition of metakaolin is very complex. In order to study the reaction mechanism of these materials, it is very important to design and control the composition, properties and structures of phosphoric acid-based geopolymers. Sol-gel method is a novel method for preparation of powders and has received much attention in last two decades, due to high purity, stability properties and high activity. Meanwhile the composition of the synthetic powders can be design, which is advantageous to study the reaction mechanism of phosphoric acid-based geopolymers.In this study, chemosynthetic Al2O3-nSiO2powders (n=1,2,3) for geopolymers were synthesized by a sol-gel method with aluminum nitrate (ANN) and tetraethoxysilane (TEOS) as the starting materials, and anhydrous as the solvent. The phosphoric acid-activated properties of these synthetic powders and the optimization of process parameters of phosphoric acid-based geopolymers prepared with these powders were investigated. The reaction mechanism of phosphoric acid-based geopolymers was systematically investigated with the Al2O3-nSiO2powders and phosphoric acid as raw materials. It was exploringly studied the phosphoric acid-based geopolymer materials used in dielectric materials and porous materials. The principal results are as follows:(1) The Al2O3-nSiO2powders fabricated utilizing sol-gel method show phosphoric acid-activated properties. The effect of calcination temperture on phosphoric acid-activated properties and molecular structure of Al2O3-2SiO2powders were investigated. The results show that the Al2O3-2SiO2powders possess phosphoric acid-activated properties at200℃, where6-coordinated Al of powders was converted into5-coordinated Al. The Al2O3-2SiO2powders possess similar structure and properties with metakaolin, but the onset temperature of metakaolin converted from kaolin is480℃.(2) The properties and structure of geopolymers were investigated with Al2O3-nSiO2powders as the main precursors. The optimum process parameters were determined by the compressive strength of the geopolymer specimens as the main criterion. The effects of curing schedules, the H3PO4/Al2O3molar ratios, SiO2/Al2O3molar ratios and H2O/Al2O3molar ratios on the compressive strength of geopolymer products were studied. The optimum composition for preparing geopolymers is Al2O3-SiO2-2H2O-H3PO4. The maximum of compressive strength of phosphoric acid-based geopolymeric samples is145.3MPa after the samples were cured. The further studies were pain on the thermal stability of geopolymers and the structure of hydrothermal product. The results show that the phosphoric acid-based geopolymers present no sign of melting up to1550℃, and the structure of samples were not changed after hydrothermal procedure.(3) Similar to the alkali-based geopolymers, the reaction mechanism of phosphoric acid-based geopolymers can be divided into depolymerization and polymerization processes. The H3PO4/Al2O3molar ratios, SiO2/Al2O3molar ratios and reaction temperature have significant influences on the depolymerization and polymerization processes. The depolymerization of geopolymers is controlled by chemical reaction step for shrinking corel model. E The activation energy of this process was20.12kJ/mol. The process of polymerization reaction was monitored by the variational curves between conductivity and time. Kinetic equation for polycondensation process of phosphoric acid-based geopolymers was fitted and deduced with experimental data, and the apparent activation energy is14.13kJ/mol. From SEM-EDS, FTIR and MAS NMR anasysis, it is shown that the phosphoric acid-based geopolymers is composed of three-dimensional network geopolymeric gel and unreacted particles. In the geopolymeric gels, the three-dimensional structure is consited of [AlO4],[SiO4] and [SiO4] tetrahedras. Compared to alkali-based geopolymers, some Si-O bonds are substituted by the P-O bonds in phosphoric acid-based geopolymers, which leads to the vibrational bands shift toward higher wavenumbers from FTIR data. Therefore, the mechical properties and thermal stability of phosphoric acid-based geopolymers are superior to alkali-based geopolymers.(4) The phosphoric acid-based geopolymers were used in dielectric materials. After the geopolymer samples were cured, some adsorbed water and hydroxyl were still in the samples, which caused the dielectric constant and dielectric loss of samples increasing. The effect of water on the dielectric properties can be eliminated after the samples are calcined at350℃. The dielectric constant was4.16, and the dielectric loss was below0.005, when the testing frequency was in the range of1MHz-1GHz, which met the demand of low dielectric materials. A low dielectric constant materials with the dielectric constant of2.56was prepared by improving the porosity of samples.(5) On the basis of theoretical research, a phosphoric acid-based porous geopolymer was synthesized with metakaolin, α-Al2O3, phosphoric acid and Al powder with following system:Al2O3-SiO2-1.4H3PO4-H2O and Al powder of0.06wt%-0.2wt%. The porosity of geopolymers increased from40%to83%. The compressive strength is higher than6MPa. The porous geopolymer showed excellent thermal stability at a high temperature of1550℃. The porous geopolymers can be widely used in high temperature fields.