Alkali silicate binders: effect of SiO2/Na2O ratio and alkali metal ion type on the structure and mechanical properties
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The influence of SiO2:Na2O molar ratio and the nature of an alkali metal (Na vs. K) in commercial aqueous alkali silicate on the microstructure, textural properties, phase composition, and hydrolytic stability of an alkali silicate binder have been investigated using scanning electron microscopy, nitrogen adsorption/desorption technique, X-ray diffractometry, thermal analysis, and dissolution tests. It has been found that microstructure and textural properties of the alkali silicate binder depend both on silica to alkali molar ratio and type of alkali metal (Na vs. K). Sodium silicate binder obtained from commercial silicate solution with lower SiO2:Na2O molar ratio (2.2) exhibits a globular microstructure of silica xerogel with high content of micropores, whereas the binder formulated with SiO2:Na2O molar ratio 3.2 is characterized by more open cluster structure with lower content of micropores. It is observed that surface specific area estimated by Brunauer, Emmett, and Teller method and mesopore volume obtained by the Barrett–Joyner–Halenda method for sodium silicate binder are substantially higher than those for potassium silicate binder. The ultimate hydrolytic stability of the sodium silicate binder increases slightly with increase in the silica to alkali molar ratio within the studied range. Decreasing in SiO2:Na2O molar ratio and replacement of sodium silicate solution by potassium silicate solution in the corresponding filled composition lead to the improvement of mechanical properties and decrease in open porosity.