The Deterioration of C-S-H Gel in a Severe Sulfate Environment

Immersion tests with 5 wt.% Na₂SO₄ solution were carried out to investigate the deterioration of calcium silicate hydrate (C-S-H) gel in a sulfate environment. C-S-H gels with different Ca/(Si + Al) molar ratios were used for comparison. Particular attention was paid to the changes in element composition and Si-O-Si chain structure of the C-S-H gel. The results show that the C-S-H gels with a low Ca/(Si + Al) ratio (0.8–1.0) and appropriate Si-O-Si chain length (14.8) presented better stability in a severe sulfate environment. The C-S-H gels with a higher Ca/(Si + Al) ratio (>2.0) were more sensitive to the attack of sulfate ions. Calcium ions dissolved rapidly during the immersion process, causing the loss of cohesive strength of the C-S-H gel, and then decomposed into fine particles. The C-S-H gel with a lower Ca/(Si + Al) ratio (about 1.1) rarely leached out calcium ions and maintained good micromorphology. The ²⁹Si MAS-NMR results indicate that the Si-O-Si chains with too short or too long chain lengths will break and recombine under the attack of sulfate ions. The Si-O-Si chains with an appropriate chain length (14.8) maintained the stability of the structure of the C-S-H gel in a sulfate environment. These changes are closely related to the asymmetric layered structure of amorphous C-S-H gel. Partial calcium ions between the layers of the main chain structure of Si-O-Si are easily taken away by sulfate ions, leading to the structural instability of the C-S-H gel.