From Micro to Ultra-Macro Porosity in Alkali Bonded Ceramics (Geopolymers)

Alkali bonded ceramics are synthetic and amorphous alkali aluminosilicates, also known as geopolymers, with properties varying among those of ceramics, cements, zeolites or refractories. Aluminosilicate powders are chemically consolidated in alkali aqueous solutions under mild thermal treatment (<100°C). At the atomic scale, SiO4 and AlO4 tetrahedra form Si-O-Al-O rings and chains, producing a 3-D network able to endow the material with several properties. The microstructure is intrinsically mesoporous being composed of nano-particulates separated by pores, whose size can be tailored changing the Si/Al/H2O ratios, the alkali cation or the synthesis conditions. Hierarchical pore systems can be constructed combining this mesoporosity with an induced macro- and ultra-macroporosity. Indeed, direct and indirect foaming techniques can be used to vary the pores morphology, orientation and dimensional range, depending on the final uses and applications of the material. Ice-templating can be used to obtain geopolymers with unidirectional lamellar pores, while blowing agents can generate materials with isotropic rounded pores. Fillers, as zeolite or expanded vermiculite, can add porosity respectively in the micro or ultra-macro range, and further functionalize the geopolymer.