Spatial distribution of pores in fly ash-based inorganic polymer gels visualised by Wood’s metal intrusion

Inorganic polymer cements, or ‘geopolymers’, are now finding use as a replacement for Portland cement in concrete production, and have a complex pore structure which has proven difficult to measure accurately by gas or mercury porosimetry. These materials consist of an alkali aluminosilicate-based gel binder phase, within which are embedded unreacted precursor (usually coal fly ash and/or blast furnace slag) particles. Impregnation of the inorganic polymer samples with Wood’s metal, a low-melting-point alloy which solidifies at room temperature, and examination by scanning electron microscopy, allows both the size of pores and their physical distribution within the gel to be determined. Pore sizes as small as 10 nm are directly observable in high-resolution imaging. Much of the difficulty in applying standard porosimetry techniques to inorganic polymers may be identified as being related to the presence of numerous ‘ink-bottle’ pores, as well as the very wide distribution of pore diameters (spanning several orders of magnitude). The effect of gel chemistry on pore structure, and in particular the presence of calcium in the inorganic polymer formulation, is also considered.