Influence of the degree of dehydroxylation of kaolinite on the properties of aluminosilicate glasses

The degree of dehydroxylation of kaolinite, D-TG and D-IR, respectively, is characterized by thermogravimetric analysis (TG) and Fourier transform inf rared spectroscopy (FTIR). The relation between DTG and DIR based on the in frared absorptions at 3600-3700, 915, 810, and 540 cm(-1) is established. T hree regions can clearly be distinguished: the dehydroxylation region (D-TG <0.9), the metakaolinite region (0.9 <D-TG<1) and the 'spinel' region (D-TG =1). The effect of the degree of dehydroxylation of kaolinite on the amount of reactive material is measured by the reaction enthalpy, DeltaH, of the low-temperature reaction of the dehydroxylated kaolinite with a potassium s ilicate solution using differential scanning calorimetry (DSC). |DeltaH| in creases almost linearly with DTG in the dehydroxylation region. In the meta kaolinite region, DeltaH and thus the amount of reactive material, becomes constant. |DeltaH| is sharply decreasing when metakaolinite transforms into other phases in the 'spinel' region. No significant differences in the rea ctivity of the dehydroxylates is detected with DSC. According to FTIR, the use of partially dehydroxylated kaolinite is not influencing the molecular structure of the low-temperature synthesized aluminosilicates, but residual kaolinite is retrieved as an additive.