Development of membranes based on recycled geopolymer and zeolite through a cold sintering process

The cold sintering process (CSP) was used to produce membranes and improve the densification of a geopolymer-zeolite matrix, fully recycling a waste geopolymer powder. The role of CSP parameters were studied varying temperature (25, 40 and 80°C), applied pressure (56 MPa and 168 MPa) and reactive solution (H2O, KOH or NaOH 2, 4, 6, 8 M). CSP applied on the waste geopolymer powder (molar ratio Si/Al = 2.0 and K/Al = 1.0) resulted in relative densities ranging from about 50–77%, while modal pores varied from 0.006 µm to 0.017 µm in the mesopore range. Compressive strength was up to 45 MPa. The addition of 10 wt% of the waste geopolymer powder to a geopolymer- zeolite powder containing 81% of in situ synthetized NaA phase allowed a densification approaching 70% with mechanical strength up to 7.4 MPa.