Nano-Structural Change of C(N)-A-S-H Gel in Alkali-Activated Slag Pastes Subjected to Wetting-Drying Cyclic Sulphate Attack

Abstract

Nano-structural change of calcium(sodium) aluminium silicate hydrates ( C(N)-A-S-H) gel in alkali-activated slag (AAS) mortars after wetting-drying cyclic attack in 5 % (mass fraction,the same below) and 10% sulphate solutions was investigated. The structural and compositional changes of aluminosilicate chains in the main binding phases (C(N)-A-S-H) in AAS mortars were analyzed by Fourier-transform infrared spectroscopy, thermogravimetry-differential scanning calorimetry, 29Si magic angle spinning nuclear magnetic resonance, backscattered scanning electron microscopy and transmission electron microscopy-energy dispersive X-ray spectroscopy. The results show that the sulphate attack increases the degree of reaction of ground granulated blast-furnace slag regardless of sulphate salt type. The main corrosion products in MgSO4 solutions are gypsum, which is not detected in the samples subjected to Na2SO4 solutions. The C(N)-A-S-H with longer chains prefers to be attacked in MgSO4 solutions, whilst the end-members of the chains or short chains of C(N)-A-S-H are broken in Na2SO4 solutions. The C(N)-A-S-H after cyclic water attack is a mixture of tobermorite/jennite dimer to undecamer (T/J2-T/J11), while the Na2SO4 solutions attack results in the destroy of shorter chains and the increase of protonation (or Na-balanced ratio), thus forming a new C(N)-A-S-H gel.