Multiscale Characterization of Fly Ash–Based Geopolymer and Type V Portland Cement Exposed to MgSO4

Abstract

Fly ash–based geopolymer is an attractive supplemental cementitious material that has been receiving great attention from the community; however, its durability characteristics such as resistance to sulfate-related damage have not yet been fully examined. This study investigates the properties of fly ash–based geopolymer and its counterpart, Type V portland cement paste/mortar, when they are exposed to MgSO4 solution. Toward that end, a multiscale characterization was conducted. Changes in nanomechanical properties due to MgSO4 exposure were tracked and quantified by nanoindentation. Scanning electron microscopy and energy-dispersive X-ray spectroscopy were coupled to characterize microstructural and chemical changes at different MgSO4 exposure levels. Moreover, the effect of MgSO4 solution on macroscale properties, including changes in mass and compressive strength, were examined. Results indicated that exposure to MgSO4 solution affected the two cementitious materials very differently. No significant sign of deterioration was observed in fly ash–based geopolymer, although MgSO4 changed the chemical compositions by increasing Mg content and decreasing Na in the original N-A-S-H gel. On the contrary, Type V portland cement presented the degradation of the main hydration product due to the decalcification process with increasing MgSO4 exposure. This resulted in a significant drop of Ca/Si ratio and compressive strength after 6 months of MgSO4 immersion. Test-analysis results in different length scales in this study imply that fly ash–based geopolymer can be a durable material under MgSO4 environments.