Effect of the microstructure in the optimization of geopolymeric membrane supports for microfiltration of wastewaters

Alkali activated materials and geopolymers have attracted a lot of research interests in the last 20 years thanks to their excellent performances in terms of mechanical, thermal and durability properties. In addition, these materials show promising properties in terms of sustainability when compared to traditional binders commonly used in the building sector (i.e., ordinary Portland cement) and to traditional and advanced ceramics (i.e., membranes, adsorbents, and catalysts). Sustainability in geopolymers can be reached by using a broad variety of industrial by-products as raw materials and by ambient or low temperatures (< 100 °C) consolidation. The aim of the present study was the optimization of microstructural properties of one-part geopolymers, based on flash-sintered metakaolin and anhydrous sodium silicate, for microfiltration membrane support in wastewater treatments. Investigated geopolymers were shaped by uniaxial pressing and cured at 70 °C for 24 h. Several parameters in terms of mix design optimization (i.e., alkali concentration and water content) and pressing pressure were investigated to obtain a microstructure suitable for the preparation of the support of flat asymmetric membranes. Assessment of the total open porosity and the pore size distribution was carried out by mercury intrusion porosimetry, while microstructural observations were performed by field emission gun-scanning electron microscopy (FEG-SEM) coupled with energy dispersive spectroscopy (EDS). It was found that pressing is a particularly suitable shaping method for one-part geopolymers thanks to the use of low water amount for the activation of the mix. In addition, increasing the water content (up to 15 wt%) and pressing pressure (up to 20 MPa) allowed the formation of a dense geopolymeric gel, while alkalis concentration, ranging between Na/Al = 0.7 and 1.1, did not strongly influence the microstructure. Finally, the optimized parameters for producing flat support for microfiltration membranes are a Na/Al molar ratio equal to 1.0, 12 wt% water content and 2 MPa of pressing pressure. These conditions allowed to obtain the same range of open porosity and pore size distribution characteristic of ceramic membrane support used for the same purpose.