Short-term improvement of ductile geopolymer composites exposed to magnesium sulfate: Mechanical properties, sorptivity, and mechanisms

Abstract

Geopolymers are eco-friendly substitutions for cement whose manufacture causes high carbon emissions and energy consumption. Drying shrinkage and brittleness are common defects for geopolymers, and polyvinyl alcohol (PVA) fibers can be employed to suppress these defects. Thus, ductile geopolymer composites (DGC) are preferred. Magnesium sulfate attack is one of the durability concerns over the geopolymers employed for construction. However, previous studies focused on long-term exposure behaviors, and the knowledge of short-term exposure influences and mechanisms is limited. To clarify the gaps, the current research investigated the physical properties, mechanical properties, sorptivity, phases, and microstructure of DGC incorporating 0–0.6% vol.% PVA fibers, and the DGC was exposed to 5% magnesium sulfate solution for 0, 30, 60, and 90 d. According to the results, the short-term exposure increased DGC's compressive, flexural, and splitting tensile strengths by 5.25%–8.33%, 12.02%–17.17%, and 19.31%–27.93%, respectively, as well as sorptivity by 55.15%–64.48%. Short-term magnesium sulfate exposure improved mechanical properties and sorptivity, contrary to performance deterioration caused by long-term exposure. Mechanisms for the improvement are as follows: (a) sodium-calcium-magnesium sulfo-aluminosilicate (SCMSAS) crystals were formed in micropores, which reduced DGC's porosity. The decreased porosity enhanced mechanical properties and reduced sorptivity; (b) no large amounts of expansive gypsum and ettringite crystals were detected after short-term exposure, and their substantial formations were the causes of long-term exposure deterioration; (c) aluminosilicate polymer structures were improved owing to increased stability and mean chain length, which helped enhance DGC's mechanical properties.