Impact of sodium sulfate solution on mechanical properties and structure of fly ash based geopolymers

In this paper, geopolymers based on two different fly ash samples were exposed to sodium sulfate (Na2SO4) solution (50 g/l) over a period of 365 days. It was found that sulfate solution attack caused a small decrease in strength of geopolymer mortars. Analysis of the Na2SO4 solutions by optical emission spectroscopy indicated that exposing of the geopolymer samples to the Na2SO4 solution had caused leaching of one of the elements of the aluminosilicate gel, silicon. Mineralogical analyses of geopolymer samples did not show formation of any new phases due to a reaction with sulfate ions. Changes in aluminosilicate geopolymer gel due to sulfate attack were investigated by electron microscopy and nuclear magnetic resonance. It was found that treatment of geopolymer samples with the sulfate solution caused breaking of -Si-O-Si- bonds in aluminosilicate gel structure. Breaking of the -Si-O-Si- bonds and leaching of Si were consequences of the increase in the pH value of sulfate solution during testing