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 size d mullite polycrystals. Sodium silicate solutions prepared from Na(2)O(.)Si O(2)(.)9H(2)O (A-series) and Na2O . 2SiO(2)aq (B-series) were applied as ho st solutions. Then, Al-nitrate guest solution was added drop by drop into t he host solutions to obtain precipitates, which were filtrated and washed b y deionized water. The air-dried precursor gels thus obtained were fired at elevated temperature up to 1400 degreesC and were examined by XRD. DTA and ICP techniques. The XRD results showed that the A-series gels began to cry stallize mullite at about 1000 degreesC, while the B-series at about 1100 d egreesC. According to the DTA. a sharp exothermic peak appeared at about 10 00 degreesC in the A-series gels indicating polymer character of the gels, while no such a peak in the B-series gels indicating nonpolymer character o f the gets. Chemical analysis by ICP revealed low concentration of Al2O3-co mponent in the gels, and this caused the gels consisted of mullite and sili ca potentials. Na-contaminations were extremely low. The process of mullite generation was also discussed on the basis of cell-parameters measured. (C ) 2001 Elsevier Science Ltd. All rights reserved.