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 degrees 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 mu m to 0.017 mu m 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.