The geopolymerisation of alumino-silicate minerals

Geopolymers are similar to zeolites in chemical composition, but they reveal an amorphous microstructure. They form by the co-polymerisation of individual alumino and silicate species, which originate from the dissolution of silicon and aluminium containing source materials at a high pH in the presence of soluble alkali metal silicates. It has been shown before that geopolymerisation can transform a wide range of waste alumino-silicate materials into building and mining materials with excellent chemical and physical properties, such as fire and acid resistance. The geopolymerisation of 15 natural Al-Si minerals has been investigated in this paper with the aim to determine the effect of mineral properties on the compressive strength of the synthesised geopolymer. All these Al-Si minerals are to some degree soluble in concentrated alkaline solution, with in general a higher extent of dissolution in NaOH than in KOH medium. Statistical analysis revealed that framework silicates show a higher extent of dissolution in alkaline solution than the chain, sheet and ring structures. In general, minerals with a higher extent of dissolution demonstrate better compressive strength after geopolymerisation. The use of KOH instead of NaOH favours the geopolymerisation in the case of all 15 minerals. Stilbite, when conditioned in KOH solution, gives the geopolymer with the highest compressive strength (i.e., 18 MPa). It is proposed that the mechanism of mineral dissolution as well as the mechanism of geopolymerisation can be explained by ion-pair theory. This study shows that a wide range of natural Al-Si minerals could serve as potential source materials for the synthesis of geopolymers.