Degree of reaction and phase content of silica-based one-part geopolymers investigated using chemical and NMR spectroscopic methods

One-part geopolymers were synthesized from two different silica materials (a silica-rich residue from chlorosilane production and a commercial microsilica) and sodium aluminate at three different SiO₂/Al₂O₃ ratios and a nominal water/solids ratio of 0.5. The degree of reaction of the silica in the cured geopolymers (i.e. the fraction of silica dissolved to form aluminosilicates and minor products) was determined using two different methods: chemical attack with HCl to dissolve the reaction products and evaluation of peak areas of ²⁹Si MAS NMR spectra. It was found that the degree of reaction of the silica decreases with increasing the silica content of the starting mix, and that it is almost constant after 1 day of curing and almost independent from the kind of starting silica. From the results of the NMR-based method, the mean SiO₂/Al₂O₃ ratio of the reaction products (aluminosilicates and minor products) can be estimated to be ca. 2.0, nearly independent of the starting composition of the geopolymers. The dissolution method is biased, but of sufficient precision to be useful for following changes of the degree of reaction. Major crystalline phases in the cured geopolymers are zeolite A and/or hydrosodalite. Depending on the starting composition, the relative amounts of these zeolites vary; additionally, sodalite (only for the residue from chlorosilane production with >1 wt% Cl⁻), faujasite, and zeolite EMT can appear in the geopolymers. The ²⁹Si and ²⁷Al MAS NMR results indicate mainly Si(4Al) and Al(4Si) sites, in line with the presence of zeolite A, hydrosodalite, sodalite, and geopolymeric gel of comparatively low SiO₂/Al₂O₃ ratio.