Alkali-activation reactivity of chemosynthetic Al2O3–2SiO2 powders and their 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectra

• Powder prepared by sol–gel method had higher alkali-activation reactivity.
• The powders were investigated by 27Al and 29Si MAS NMR.
• The Al(V) contents of the powders were in agreement with their alkali-activation reactivities.
• Si was replaced by Al at secondary coordination sites during calcination of the powders.

Pure Al2O3–2SiO2 powders were prepared by sol–gel and coprecipitation methods, and their alkali-activation reactivities were compared. The alkali-activation reactivity of the powder prepared by the sol–gel method was higher than that of the powder prepared by the coprecipitation method. The powders were investigated by 27Al and 29Si magic-angle spinning nuclear magnetic resonance spectroscopy (MAS NMR) to understand the relationship between their structure and alkali-activation reactivity. The 27Al MAS NMR data showed that the five-coordinate Al content of the powder prepared by the sol–gel method was higher than that of the powder prepared by coprecipitation. The higher content of five-coordinate Al corresponded to higher alkali-activation reactivity. The 29Si MAS NMR data showed that for the powder prepared by the sol–gel method, silicon was replaced by aluminum at secondary coordination sites of the central Si atoms during calcination. However, for the powder prepared by single-batch coprecipitation, the main change was from a low degree of polycondensation to a high degree of polycondensation.

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