Crystallization behavior and characteristics of mullites formed from alumina–silica gels prepared by the geopolymer technique in acidic conditions

Mullites were prepared by the geopolymer technique to obtain nanometer sized mullite polycrystals. Sodium silicate solutions prepared from Na2O·SiO2·9H2O (A-series) and Na2O·2SiO2·aq (B-series) were applied as host solutions. Then, Al-nitrate guest solution was added drop by drop into the host solutions to obtain precipitates, which were filtrated and washed by deionized water. The air-dried precursor gels thus obtained were fired at elevated temperature up to 1400°C and were examined by XRD, DTA and ICP techniques. The XRD results showed that the A-series gels began to crystallize mullite at about 1000°C, while the B-series at about 1100°C. According to the DTA, a sharp exothermic peak appeared at about 1000°C in the A-series gels indicating polymer character of the gels, while no such a peak in the B-series gels indicating nonpolymer character of the gels. Chemical analysis by ICP revealed low concentration of Al2O3-component in the gels, and this caused the gels consisted of mullite and silica potentials. Na-contaminations were extremely low. The process of mullite generation was also discussed on the basis of cell-parameters measured.