Effect of Corrosive Ions (Cl⁻, SO₄ ²⁻, and Mg²⁺) on the Nanostructure and Chloride Binding Property of C-A-S-H Gel

Abstract

The nanostructure and chloride binding capacity evolution of C-A-S-H gel exposed to aggressive solutions were investigated, utilizing ²⁹Si NMR, ²⁷Al NMR, SEM-EDS, and XRD techniques. The experimental results show that while Cl⁻ ions show a smaller effect on the microstructure of C-A-S-H sample, and SO₄ ²⁻ ions can react with C-A-S-H, resulting in decreasing Ca/Si and Al[4]/Si for the C-A-S-H gel (i e, decalcification and dealuminization). The presence of Mg²⁺ ions can aggravate the decalcifying and dealuminizing effects of SO₄ ²⁻ ions on the C-A-S-H. With decreasing Ca/Si ratio and aluminum substitution for the original C-A-S-H gel, the depolymerization degree of silicate tetrahedra increases and the calcium aluminosilicate skeleton strengthens. C-A-S-H gel with lower Ca/Si ratio and higher Al[4]/Si ratio shows gentler nanostructure evolution under chemical attack, i e, improving thermodynamic stability under chemical attack. Furthermore, the chloride binding capacity of C-A-S-H gel is decreased after the sulfate attack. Aluminum substitution can also help C-A-S-H gel resist the degraded chloride binding capacity induced by sulfate attack.

Keywords

calcium-silicate-hydrate / ions attack / Ca/Si ratio / Al[4]/Si / aluminum uptake