Effect of Synthesis pH on the Preparation and Properties of K-Al-Bearing Silicate Gels from Solution

Abstract

Silicate gels were prepared by mixing K₂O•3.9SiO₂ (0.3 mol/L) and Al(NO₃)₃•9H₂O (1 mol/L) solutions in various ratios, using potassium hydroxide solution (1 mol/L) to regulate the pH. Nine gel samples were thus prepared at a range of pH conditions, filtered, washed and air-dried. About 1 mol% of potassium was incorporated into the gels at low pH, but up to 20 mol% was incorporated under high pH conditions. On heating, characteristic DTA exotherms were observed at higher temperatures, possibly providing an indication of the degree of polymerization of the gels. The low-potassium gels exhibited a sharp exothermic peak at about 1000°C corresponding to mullite formation, whereas the high-potassium gels exhibited a small, gentle exotherm at about 1000-1100°C corresponding to the formation of KAlSiO₄ (PAS, considered to be one of the modifications of kalsilite) and/or KAlSi₂O₆ (leucite), depending on the gel composition. The ²⁹Si and ²⁷Al MAS-NMR spectra showed that the potassium-poor gels contain predominantly tetrahedral Si(1-2Al) and Q1 silicate units, and both tetrahedral and octahedral Al³⁺, whereas the potassium-rich gels contain predominantly Si(4Al) units and solely tetrahedral Al³⁺. The implications of these results for the formation of potassium geopolymers are discussed.