Effect of hydration assemblage on the autogenous shrinkage of alkali-activated slag mortars

The purpose of this work is to characterise the autogenous shrinkage performance of alkali-activated slag (AAS) mortars prepared with four activator types (i.e. sodium hydroxide, waterglass, calcium hydroxide/sodium carbonate and calcium oxide/sodium carbonate), with ordinary Portland cement (OPC) mortar as the reference sample. The hydration assemblage and nanostructure of calcium aluminosilicate hydrate (C-A-S-H) were investigated using a series of microscopic characterisations, including X-ray diffraction (XRD), thermogravimetric analysis (TGA) and silicon-29/aluminium-27 nuclear magnetic resonance (²⁹Si/²⁷Al MAS NMR) analysis combined with thermodynamic modelling. The results show that the autogenous shrinkage of mortar samples is highly associated with C-A-S-H type and mass content in the cementitious systems. Compared with the OPC system, the larger C-A-S-H mass fractions in AAS systems affect the microstructure rearrangement, contributing to higher autogenous shrinkage. Furthermore, the autogenous shrinkage in AAS systems is negatively correlated with the medium chain length and aluminium (Al) incorporation degree of C-A-S-H.