Effects of red mud on the properties of fresh and hardened alkali-activated slag paste and mortar

Alkali-activated slag (AAS) suffers from severe performance issues, such as uncontrolled and rapid setting, unstable rheological behaviour and considerable drying shrinkage. The main objective of this study was to ameliorate these problems by incorporating raw red mud (RM) (at contents of 10, 20, 30, and 40% by weight) and RM thermally treated at 550 and 750 °C (at 40%) into an AAS system. The fresh paste properties (measured by using the flow cone method, the mini-slump cone test and a Vicat apparatus), hardened paste properties (measured by Fourier transform infrared (FTIR) spectroscopy, thermogravimetric-differential thermal analysis (TG/DTA), XRD and scanning electron microscopy (SEM) techniques), and the mechanical strength and dimensional stability of the mortar were measured. The results indicated that incorporating RM decreased the fluidity but led to a more cohesive fresh paste with a lower consistency loss rate. With increasing RM content, both the initial and final setting times of the paste increased. The hardened paste analysis showed that the main hydration products of all the mixtures after 28 days were dominated by a C-A-S-H-type gel with a higher Al incorporation. The drying shrinkage could be explained by the cumulative pore volume in the gel pores and was larger in most samples with low RM content than that in the samples without RM.