Microstructure and nanoporosity of as-set geopolymers

The microstructure and porosity of as-cured and crushed sodium-based geopolymers has been examined by electron microscopy (SEM, TEM, STEM), mercury intrusion porosimetry (MIP) and specific surface area measurements, respectively. These techniques indicate that that the intrinsic microstructure is nanoporous and nanoparticulate (≤5 nm in size). When made from natural metakaolin, geopolymers have an average pore radius of ∼3.4 nm, and the porosity constitutes 41% by volume of the material. When the geopolymer is made from a pure, synthetic alumino-silicate metakaolin in which the Al atom is in five-fold co-ordination, the average pore size decreases to -0.8 nm, and the pores again constitute -40% of the volume. The porosity can be varied by selection of Na⁺ or K⁺ charge balancing cation, where the latter produces a relatively homogeneous dispersion of fine, sub-micron sized pores distributed in the nanoporous body. A biomodal pore size distribution yielding pores of ∼50μm in diameter can be achieved by synthesis with hydrogen peroxide solution in a closed pressure vessel.