Effects of different activators on workability, mechanical properties and microstructure of alkali-activated slag-copper slag

Recycling and utilizing copper slag (CS) as a precursor to prepare alkali-activated blast furnace slag-copper slag (AAS-CS) is significant both economically and environmentally. However, the impact of activator type on the performance of AAS-CS has not yet been systematically evaluated. In this study, AAS-CS mortars were prepared with different activators including Na2SiO3, NaOH, Na2CO3, and Na2SO4. The effects of activator types on the setting time, fluidity, hydration heat, and mechanical properties of AAS-CS were determined. Meanwhile, the micro-structure and pore structure are characterized using X-ray diffractometer (XRD), Fourier transform infrared spectrum (FT-IR), thermogravimetric analysis (TGA), scanning electron microscope (SEM) and nuclear magnetic resonance (NMR), respectively. The results showed that the compressive strength and the flexural strength of the samples at 28 days were ranked as Na2SiO3 > NaOH > Na2SO4 > Na2CO3, suggesting that Na2SiO3 was the optimal activator for AAS(-)CS. The 28-day compressive strength and the 28-day flexural strength of samples activated with Na2SiO3 were 64.3 MPa and 7.9 MPa, respectively. Compared to the samples activated with other activators, more hydration products were observed in the sample activated with Na2SiO3, resulting in a more compact microstructure, which is consistent with the results of pore structure test. In addition, ettringite (AFt) is found in the sample activated with Na2SO4, and the sample activated with Na2CO3 exhibited a relatively loose microstructure with several microcracks. These findings can provide valuable insights for the future application of AAS-CS.