Nanoscale insights on structure and properties of C-(A)-S-H gel in cementitious materials under sulfate attack

Sulfate attack poses a significant durability challenge to cementitious materials by degrading microporous hydration products. This study investigates the impact of sulfate attack on the structure and properties of C-(A)-S-H gels with varying Ca/Si and Al/Si ratios. The findings reveal that the susceptibility of C-A-S-H(Calcium Alumino-Silicate Hydrate) gels to sulfate attack is highly dependent on their chemical composition. Key observations include the formation of gypsum and ettringite crystals, with their quantities increasing with higher Ca/Si and Al/Si ratios. Our research demonstrates that sulfate attack induces decalcification and dealumination in C-A-S-H structures, leading to substantial changes in aluminosilicate chain morphology. Importantly, we identified that C-A-S-H crystals exhibit optimal sulfate resistance at an Al/Si ratio of 0.1. This study also elucidates the transformation of aluminum coordination states within the C-A-S-H gel under sulfate attack and highlights the role of Al/Si ratios in promoting specific structural changes that enhance sulfate resistance. These insights can provide guidance for developing more durable concrete formulations, particularly for applications in sulfate-rich environments.