Evolution in macro-micro properties of cement-treated clay with changing ratio of red mud to phosphogypsum

Abstract

To investigate the effect of combined red mud (RM) and phosphogypsum (PG) in cement-treated clay, unconfined compressive strength (UCS) tests and isotropically consolidated undrained triaxial (ICUT) tests were performed to explore the effects of RM/PG ratio on mechanical behavior of stabilized clay. Moreover, x-ray diffractometer (XRD) tests, thermogravimetric analysis/differential scanning calorimetry (TG/DSC) tests and scanning electron microscopy (SEM) tests were employed to observe the evolution in microstructure with changing RM/PG ratios. The results indicate that the combined RM and PG play a significant role in cement-based hydration reactions. The UCS at early curing stages nearly doubles with an optimal proportion of 7.5 %RM and 2.5 %PG. The stabilized clay with high contents of RM exhibits a higher maximum deviator stress and a brittle behavior in undrained shearing process. As the PG content increases, the peak strength decreases but the ductility of the specimen increases remarkably. It is believed that the RM acts as an alkali activator on clay minerals whereas the PG mainly contributes to the generation of ettringite. The combined RM and PG not only promotes sufficient pozzolanic reactions and stronger cementation bonds, but also brings out a principally denser microstructure, though a few large pores can be observed due to expansion potential of ettringite. However, the microstructure of stabilized clay becomes much looser with a high content of PG. Partial amounts of PG are found non-reactive with inadequate active aluminium ions. The mechanical behavior of stabilized clay also gets worse concerning negative effects of impurities in PG.