Sulfate attack on geopolymer: effect of the proportion of binder and air-entraining additive

Abstract

This research aimed to evaluate the resistance to sodium and magnesium sulfates of geopolymeric mortars, prepared with different proportions of alkaline activators of silicate and sodium hydroxide (Ma), Na2O content in relation to metakaolin (MK) and silica modulus (Ms). Additionally, an air-entraining additive was also used in an attempt to mitigate the expansive effects of sulfate attack for 20 weeks. The sulfate resistance properties were evaluated by the change in longitudinal length, mass variation, compressive strength, flexural tensile strength, and microstructure. SEM/EDS analyses suggest the formation of caminite and a change in the structure of the aluminosilicate gel to M-A-S-(H) in mortars exposed to MgSO4, while thenardite was indicated as a harmful product in mortars subjected to Na2SO4. The experimental results also showed that the greater amount of Na2SiO3 in the geopolymer formulation contributes to a denser structure and lower water absorption rate, but resulted in greater physical deterioration and expansion after immersion in sodium magnesium sulfate. The air-entraining additive did not minimize the tensions generated by the sulfate attack. In mortars synthesized with Na2O/MK = 15% and Ms = 1.5 (GEO-15-1.5) and Na2O/MK = 22% and Ms = 1.0 and 1.5 (GEO-22-1.0 and GEO-22-1.5), the compressive strengths were similar to standard samples when immersed in MgSO4, while in the Na2SO4 solution the increase in strength was attributed to the temporary refinement of the pores by sulfate crystals.