Cementitious property of NaAlO2-activated Ge slag as cement supplement

Abstract

Germanium (Ge), a waste residue leaching from zinc (Zn) smelting process, has potential cementitious properties and could be recycled as a cement supplement activated by chemical reagents. In this work, a test was conducted to determine the hydration properties of Ge slag-cement-based composites with Ge slag (GS)/ordinary Portland cement (PC) contents of 0wt%, 5wt%, 10wt%, 15wt%, 20wt%, and 25wt% and water-to-binder ratio (w/b) of 0.4. The activators Ca(OH)2, AlCl3, NaAlO2, and Na2CO3 were mixed under 1wt%, 2wt%, 3wt%, and 4wt% dosages of GS weight. The composition and microstructure of the hydration products were investigated by the combined approaches of X-ray diffraction (XRD), thermogravimetry–differential scanning calorimetry (TG-DSC), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). First, the GS cementitious property is attributed to the high content of CaSO4•2H2O. Second, the activators affected the acceleration performance in the following order: NaAlO2, Na2CO3, AlCl3, and Ca(OH)2. More importantly, the 28-day unconfined compressive strength (UCS) is 45.34 MPa at the optimum formula of 0.6wt% NaAlO2, 15wt% GS, and 85wt% PC, which is 9.16% higher than the control. Thus, NaAlO2 is beneficial for the ettringite (AFt) generation, resulting in the C–S–H structure compaction. However, the Zn2+ residue inhibited the AFt formation, representing an important challenge to the strength growth with curing age. Consequently, the GS could be recycled as a supplement to the cement under the activator NaAlO2.