Glass-ceramic foams from 'weak alkali activation' and gel-casting of waste glass/fly ash mixtures

A 'weak alkali activation' was applied to aqueous suspensions based on soda lime glass and coal fly ash. Unlike in actual geopolymers, an extensive formation of zeolite-like gels was not expected, due to the low molarity of the alkali activator (NaOH) used. In any case, the suspension underwent gelation and presented a marked pseudoplastic behavior. A significant foaming could be achieved by air incorporation, in turn resulting from intensive mechanical stirring (with the help of a surfactant), before complete hardening. Dried foams were later subjected to heat treatment at 700-900 \ub0C. The interactions between glass and fly ash, upon firing, determined the formation of new crystal phases, particularly nepheline (sodium alumino-silicate), with remarkable crushing strength (~6 MPa, with a porosity of about 70%). The fired materials, finally, demonstrated a successful stabilization of pollutants from fly ash and a low thermal conductivity that could be exploited for building applications

Bioactive glass-ceramic scaffolds from novel 'inorganic gel casting' and sinter-crystallization

Highly porous wollastonite-diopside glass-ceramics have been successfully obtained by a new gel-casting technique. The gelation of an aqueous slurry of glass powders was not achieved according to the polymerization of an organic monomer, but as the result of alkali activation. The alkali activation of a Ca-Mg silicate glass (with a composition close to 50 mol % wollastonite50 mol % diopside, with minor amounts of Na2O and P2O5) allowed for the obtainment of well-dispersed concentrated suspensions, undergoing progressive hardening by curing at low temperature (40 degrees C), owing to the formation of a C-S-H (calcium silicate hydrate) gel. An extensive direct foaming was achieved by vigorous mechanical stirring of partially gelified suspensions, comprising also a surfactant. The open-celled structure resulting from mechanical foaming could be frozen' by the subsequent sintering treatment, at 900-1000 degrees C, causing substantial crystallization. A total porosity exceeding 80%, comprising both well-interconnected macro-pores and micro-pores on cell walls, was accompanied by an excellent compressive strength, even above 5 MPa

Highly porous cordierite ceramics from engineered basic activation of metakaolin/talc aqueous suspensions

The cellular structure, in alkali activated metakaolin-based suspensions, foamed by intensive mechanical stirring, is stabilised by the viscosity increase caused by gelation, in a condition of \u2018inorganic gel casting\u2019. The approach is so flexibile that it may be applied to mixtures embedding fillers, such as reactive \u3b3-Al2O3 powders, playing a fundamental role upon ceramic conversion. The present study is dedicated to highly porous cordierite foams, obtained including talc as further component, and applying a heat treatment at 1200 121250\u202f\ub0C, in air. A key intermediate step is represented by the removal of Na+ ions from \u2018green\u2019 foams, by ion exchange in ammonium nitrate solution (24\u202fh), before ceramization. Direct ceramization is also feasible, if the gelation is achieved by reaction with tetra-methyl-ammonium hydroxide, instead of NaOH. The new gels are effective in yielding phase-pure cordierite, otherwise feasible, with NaOH activation, by means of much longer ion exchange treatment (120\u202fh)

Up-cycling of \u2018unrecyclable\u2019 glasses in glass-based foams by weak alkali-activation, gel casting and low-temperature sintering

A significant amount of glasses is landfilled because mainstream recycling solutions do not address the articles of specific chemical compositions, including opal glass and glass fibers residues. This manuscript suggests the up-cycling of these glasses through the production of highly porous glass foams. Fine glass powders were dispersed in weakly alkaline solutions, which were foamed by the combination of surfactant addition and intensive mechanical stirring. The cellular structures were stabilized first by the gelation of suspensions, upon drying, then by viscous flow sintering at 700\u2013800 \ub0C, for 10\u201360 min. The foams based on glass fibers reached an excellent strength-to-density ratio, due to the formation of closed cells and partial crystallization. The foams made from opal glass maintained a particularly uniform open porosity, and importantly, also retained the volatile and toxic fluorine due to the rapid, low-temperature sintering