Effect of synthesis parameters on the development of unconfined compressive strength of recycled waste concrete powder-based geopolymers

Abstract

The main object of this study is to investigate the geopolymerization potential of recycled waste concrete powder (RWCP) as well as the effects of synthesis parameters, including the molarity of NaOH solution, the mass ratio of water glass to NaOH solution (WG/NH), the mass ratio of liquid alkali activator to solid powder (L/S), and the curing conditions (curing temperature, curing time and curing method), on the development of unconfined compressive strength (UCS) of recycled waste concrete powder-based geopolymers. The recycled waste concrete powder (RWCP) (size < 0.075 mm) was utilized as source material, and water glass and sodium hydroxide (NaOH) were used as alkaline activators to synthesize geopolymer specimens. The dissolution of alumino-silicate from the RWCP under different concentrations of alkaline solutions was studied. The UCS of the resulting geopolymers synthesized under different conditions was tested. Further, the microstructure and mineral composition of the final products were characterized by X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS) technology. The results indicate that the dissolution of Si4+ and Al3+ in raw materials (RWCP) was highly affected by the alkali content and dissolution time. Increasing the alkali content, water glass content and L/S ratio was all conducive to enhancing the UCS of geopolymers. The optimum NaOH concentration corresponpded to 14 mol/L, WG/NH ratio of 1.5 and L/S mass ratio of 0.4. A maximum 7-day UCS of 14.32 MPa was achieved by wet curing at 70 ℃ for 24 hours. Microscopic analysis showed that the final products were mainly composed of amorphous hydrated calcium silicate and hydrated sodium aluminosilicate gels. However, due to the low activity and high calcium content of RWCP, most of crystalline phases and some unreacted substances still existed in the end products as inactive fillers, resulting in a relatively low UCS of the synthesized geopolymers. Baded on the research, it can be concluded that RWCP can be recycled as the precursor to synthesize geopolymer binder.