Strength and microstructure of water treatment residue-based geopolymers containing heavy metals

This research investigated the use of water treatment residue (WTR) as an alumino-siliceous material from which to synthesize geopolymers. WTR with an initial SiO2:Al2O3 ratio of 1.78 was pretreated by calcining at temperatures of 600, 800 and 900 °C for 1 h. NaOH was reacted with WTR at an Na2O:SiO2 ratio of 0.25. Without thermal treatment of the WTR, XRD showed that dehydroxylation of the halloysite did not occur, and thus, no strength was gained during the early stages. Samples prepared from WTR calcined at 800 °C exhibited the highest strength at all curing times. FTIR spectra revealed that the major asymmetric vibration band shifted at 1000 cm−1, corresponding to the hydration products. When plating sludge was added at 50 wt%, the strength gained by the sample after 28 days was only 34% of the control sample prepared from WTR calcined at 800 °C.