Compositional-structural Gradient Change of C(N)-A-S-H(I) Gel in Alkali-activated Slag after Magnesium Sulphate Attack

This paper presented the gradient changes of the characteristics of C(N)-A-S-H (Calcium(Sodium)-Aluminium-Silicate Hydrates) gel in alkali-activated slag (AAS) mortars subjected to sulphate attack in 10% (in mass fraction) MgSO4 solution by 29Si solid-state magic-angle Spinning nuclear magnetic resonance (SS MAS-NMR), and high-resolution transmission electron microscopy-energy dispersive X-ray spectroscopy (HR TEM-EDX). The results show that the outer layer with the thickness of 0.5-3 mm after attack is mainly composed of Si-Al gel, the middle-level layer with the thickness of 3-6 mm is M/C(N)-A-S-H gel, and the internal layer with the thickness of 6-9 mm is mainly composed of C(N)-A-S-H gel (which was almost not attacked. The stable crystal tobermorite (Ca/Si ratio=0.98) and hydrotalcite-like phase (Mg/Al ratio=2.39) could not be destroyed by sulphate attack. However, the main chain of C(N)-A-S-H gel is readily attacked by sulphate at [AlO4] bridging tetrahedra (BT). Meanwhile, the magnesium sulphate initially results in de-calcium phenomenon (Mg will replace Ca), and then generates the de-aluminium phenomenon, finally forming the Si-Al gel (main product in the out layer) when removing Ca and partial Al from the gel and charge-balancing the gel by abundant Mg ions.