Microstructural characterisation of metakaolin-based geopolymers

Inorganic polymers (geopolymers) are formed by the alkali silicate activation of aluminosilicate materials such as metakaolin. Given correct mix-design and processing, geopolymer matrices can exhibit mechanical properties comparable or better than current cement systems, which makes them suited to a variety of construction applications. However, the variation in published mechanical and thermal properties is often a source of concern about the commercial and industrial maturity of geopolymeric materials. Experimental variations are often a result of inappropriate sample preparation, and poor quantification of system parameters. The present study details an appropriate nomenclature and standard for sample preparation to be used in geopolymeric research. These concepts are illustrated with the use of SEM to clarify the different microstructures that result from changes in the concentration of silicate and alkali type (i.e. KOH or NaOH) in the activating solution of metakaolin-based geopolymers. It is found that by increasing silicate concentration in the activating solution, the homogeneity of the bulk microstructure is improved. In all samples there is an observed increase in porosity close to unreacted metakaolin particles. It is proposed that this phenomenon is due to the nucleated growth of the geopolymer gel from the bulk to the particle surface. The macro-scale properties of these materials are correlated to each microstructure, and the results indicate that there is an optimal silicate concentration and type of alkali to use in formulation development.