Reaction between Silicate and Hydroxoaluminate Ions in Aqueous Solution

The chemical status and kinetics of the reaction between silicate ions and hydroxoaluminate ions (at Si/Al molar ratio 2:1) in KOH aqueous solution has been experimentally studied. It is assumed that condensation of OH groups bonded to silicon and aluminum occurs in this reaction forming a cross-linked polymer; and that the reaction represents the essence of synthesis of so-called geopolymer from alumino-silicate minerals. Time required for the change of the solution to a gel was measured as the reaction rate criterion. Measured dependences of the gel-point time on KOH concentration in the reaction solution exhibit a linear trend. The gel-point time decreases with increasing temperature. A measurable increase in pH value of the solution appears during the reaction. Measurement of the gel-point time for different Si/Al molar ratios has confirmed that the selected ratio 2:1 has been optimal. For a quantitative description of the reaction course and for explanation of the observed phenomena, the theories of polycondensation reactions and protolytic equilibria have been applied. Theoretically calculated dependences of the gel-point time on KOH concentration are in accord with experimentally determined values, which confirms anticipated polycondensation mechanism of the reaction under study.