Effect of sulfate attack on geopolymer mortars at early ages of exposure

Geopolymers have generated interest due to their potential to be used as an alternative binder to Portland cement. This study investigated the effects of sodium and magnesium sulfate attack on metakaolin-based geopolymer mortars (MK) considering different concentrations of silica in the activator (Ms), Na₂O/MK ratio, and the inclusion of an air-entraining additive. Geopolymer mortars prepared with metakaolin and a combination of Na₂SiO₃ and NaOH were cured for 84 days. The visual appearance, linear dimensional change, mass variation, and microstructure were monitored during two weeks of immersion in a Na₂SO₄ and MgSO₄ solution at 40 ºC to understand the behavior of this material in early ages. The results indicated that the highest compressive strength was achieved with Ms = 1.5 and Na₂O/MK content of 22%. The addition of the air-entrainer to the geopolymeric mixtures minimized the expansion of the samples. This effect was attributed to the accommodation of the expansive phases in the air bubbles. The sulfate resistance of the geopolymeric material is significantly dependent on the dosage and different performances were found in the first weeks of exposure to sulfates.