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 % wollastonite—50 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 ◦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 ◦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.