Surface Characterization of Cement Paste Exposed to Magnesium Sulfate

Exposure to external sulfate attack can cause significant deterioration of concrete structures. This is a particular concern for concrete structures exposed to sulfate-rich soils, seawater, or groundwater. This study aims to evaluate the impact of external sulfate attack on the surface of cement paste systems. The nanomechanical properties of the surface of the cement paste systems were evaluated using the nanoindentation test. Microstructural and chemical analyses were conducted on the cement paste systems using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX). Significantly different nanoindentation results were obtained after exposure to magnesium sulfate, coinciding with the development of degraded areas and a Mg-rich surface layer consistent with brucite formation. The microstructural and chemical analyses showed that this brucite layer forms a white layer and covers almost the entire cement paste surface. Based on the nanoindentation results, the volume fraction with an indentation modulus below 9.6 GPa increased from 3.3% before exposure to a MgSO₄ solution to 8.4% after exposure, due to the degradation of C-S-H and the formation of microcracks. The results of this study revealed that 7 days of exposure to a 2% MgSO₄ solution was enough to induce noticeable degradation of the cement paste surface. The development of a dense Mg-rich surface layer, consistent with brucite formation, may contribute to reducing ion transport through the outer cement paste region.