Effects of metakaolin addition on geopolymer prepared from natural kaolinitic clay

This study was undertaken to valorize naturally occurring clays in the synthesis of new geopolymers materials from the mixture of commercial metakaolins and calcined clays (550 °C). For the purpose of this work, three raw clay samples were collected from various locations in Tunisia where clayey deposits extended over large areas. A simple synthesis method was adopted by mixing calcined clays and metakaolins with an alkali silicate solution of the desired Si/K ratio. The obtained geopolymers were stored for subsequent analyses. Both metakaolins and prepared geopolymers were characterized by FTIR, XRD, TG-DTA, SEM and mechanical properties. It was demonstrated that samples based on clay and different metakaolins present amorphous consolidated geopolymer materials. Also, the obtained geopolymer materials have a smooth and amorphous appearance characterizing the polycondensation reactions. Addition of natural clay decreased the compressive strength of the geopolymer materials, because of impurities (i.e. illite, calcite, iron). Despite their low kaolinite contents, raw clays from Tunisia provided good geopolymers in the presence of metakaolins and alkaline solution.