Silicate gels were prepared by mixing K 2 Oɿ3.9SiO 2 0.3 molL and AlNO 3 3 ɿ9H 2 O 1 molL solutions in various ratios, using potassium hydroxide solution 1 molL to regulate the pH. Nine gel samples were thus prepared at a range of pH conditions, filtered, washed and air-dried. About 1 molಚ of potassium was incorporated into the gels at low pH, but up to 20 molಚ 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 1000c C corresponding to mullite formation, whereas the high-potassium gels exhibited a small, gentle exotherm at about 1000-1100c C corresponding to the formation of KAlSiO 4 PAS, considered to be one of the modifications of kalsilite andor KAlSi 2 O 6 leucite, depending on the gel composition. The 29 Si and 27Al MAS-NMR spectra showed that the potassium-poor gels contain predominantly tetrahedral Si1-2Al and Q1 silicate units, and both tetrahedral and octahedral Al 3ࢪ , whereas the potassium-rich gels contain predominantly Si4Al units and solely tetrahedral Al 3ࢪ . The implications of these results for the formation of potassium geopolymers are discussed.
The synthesis of CrVI-ettringite was attempted in portlandite suspensions for the immobilization of hazardous hexavalent Cr. The reagents NaAlO 2 and CrO 3 were used by dissolving them in deionized water to obtain 0.1 molL solutions, which were mixed with portlandite suspensions and stirred for 3 h at room temperature in CO 2 -free synthetic air. CaAl atomic ratios were varied from 1.0 to 5.0 against the CrAl atomic ratios 1.0, 1.5 and 2.0. Run products were evaluated by XRD, XRF and FE-SEM. The results showed that CrVI-ettringite formed under the conditions of pH 10.9 and more always associates with calcite. The calcite was also formed due to the carbonation of the remaining portlandite CaOH 2 during drying in ambient air. The calculated yields of CrVI-ettringite generally exceeded 80 massಚ, and 85.6 massಚ was reached for the stoichiometric mixture. In the run products obtained at pH lower than 10.9, no CrVI-ettringite was identified, and the XRF analysis showed free-CaO and free-Al 2 O 3 other than free-CrO 3 potentials each corresponding to calcite, ill-crystallized gibbsite gibbsite gels and the soluble chromatite CaCrO 4 adsorbed and trapped in the jelly gibbsite gels.
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