Mineralogical and microstructural studies of C–S–H exposed to sulphate attack

Abstract

Calcium–silicate–hydrate (C–S–H), the most important phase of hydrated portland cement, is responsible for the cementitious system’s mechanical properties performance and durability. In a severe environment, C–S–H often suffers from sulphate attack, leading to severe concrete deterioration. The present work describes a detailed mechanism of sulphate attack on C–S–H from 28 to 270 days of sulphate curing (10% Na₂SO₄ solution) using XRD, TGA, FTIR, ²⁹Si-NMR and FESEM/EDX techniques. The results show that gypsum is the only sulphate-bearing product formed by decalcification of calcium hydroxide first and then of C–S–H. Decalcification enhances polymerization in the C–S–H structure as an additional Q³ peak was observed in FTIR and ²⁹Si-NMR analysis. The microstructure of C–S–H becomes more crystalline as the sulphate curing period increases, as observed in FESEM micrographs. In addition, sulphate adsorption on C–S–H was identified through FESEM/EDX, which could play an important role in the deterioration of cementitious materials.

Keywords:

• Calcium–silicate–hydrate
• decalcification
• polymerization
• sulphate attack

Acknowledgments

The authors, Aarti Solanki and Usha Sharma are thankful to UGC, New Delhi and CSIR, New Delhi, for financial support.