Aluminium hydroxide waste based geopolymer composed of fly ash for sustainable cement materials

• Aluminium hydroxide waste (Al-waste) was firstly used as a source material for producing geopolymer.
• Fly ash (FA) and Al-waste were used in different contents for the geopolymers cured at room temperature, 60 °C and 80 °C.
• Effect of activator concentration on the geopolymer samples were investigated.
• When higher Al-waste amount was added, it was appeared to have denser structures with high strength and low porosity.

This report is the first to describe the reuse of aluminium hydroxide waste (Al-waste) for geopolymers. For cement materials, both Al-waste and fly ash (FA) were mixed at different Al-waste contents of 10–60 wt%. The mass ratio of sodium silicate (Na2SiO3) to sodium hydroxide (NaOH) solution was fixed at 2.5. Here, the NaOH concentrations of 5, 10, and 15 M were used as alkaline activators for geopolymerization. Slurry properties for the geopolymers were evaluated using viscoelasticity measurements, which demonstrated that slurry containing higher alumina has a higher elastic modulus and that the setting time is greater than that for pure FA. Then, cured at room temperature for a week and also in an oven at 60 °C and 80 °C for 24 h, the geopolymerization was increased with increasing concentration of NaOH. After curing, the mechanical properties, microstructure, bonding, and phases of the resultant geopolymers were ascertained. The contents of the Al-waste in the geopolymer influenced the geopolymer strength when NaOH concentration was changed at different curing temperatures. The Al-waste content in the resultant geopolymer at 40 wt% cured at 80 °C showed the highest compressive strength of about 40 MPa and also the highest bulk density of about 2.8 g/cm3. Furthermore, X-ray CT scanning results indicated that the geopolymer with Al-waste content at 40 wt% cured at 80 °C exhibited a dense structure, supporting the results obtained for mechanical strength.