Composition and Nano-structure of C-(A)-S-H Gels in Cement Pastes after Sulphate Attack

Abstract

The nano-structural change of hydrated calcium silicate gels (C-(A)-S-H) in Portland cement (PC) and high-sulphate resistance cement (HSR) mortars under wetting-drying cycling conditions in 5% (mass fraction) MgSO4 solution was investigated by Fourier-transform infrared spectroscopy (FTIR), thermogravimetry-tifferential scanning calorimetry (TG-DSC), 29Si magic-angle spinning nuclear magnetic resonance (MAS-NMR), and transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDX). The results show that Al-tetrahedra in C-(A)-S-Hphase is plausibly the weakness during sulphate attack, the Q1 (dimer or end-member in the chain) in the main aluminosilicate tetrahedral chain can be easily attacked by MgSO4 to form M/C-(A)-S-H, and sulphate ions are easily absorbed on the surface of C-(A)-S-H through -Si-O-M2+-1/2SO42-. The chemical compositions of C-(A)-S-H in all the samples before and after sulphate attack were simulated by the tobermorite/jennite (T/J) model. The difference between the nano-structural and the average compositional formulas of C-(A)-S-H gels in two cements after sulphate attack was derived from the (T/J) model. It is necessary for the improvement of the resistance of motors against MgSO4 attack to restrict the expansive corrosion products (i.e., gypsum and AFt) and Al-tetrahedra in the aluminosilicate chain of C-(A)-S-H phase.