Optimization of geopolymers based on natural zeolite clinoptilolite by calcination and use of aluminate activators

Natural zeolite clinoptilolite was calcined at different temperatures (700–1000 °C) in order to increase its reactivity as geopolymer precursor. The clinoptilolite structure was completely destructed at 900 °C and the material was called “metazeolite”. Geopolymer pastes were prepared with different SiO₂/Al₂O₃ ratio by using silicate and aluminate activators, including alkaline waste solution from a Dutch aluminium anodizing industry. The prepared geopolymers were cured at 80 °C and were characterized by XRD, FTIR and SEM. The metazeolite based geopolymer activated with potassium silicate solution possessed the highest compressive strength (43 MPa ± 4 MPa). The silicate and hydroxide activated geopolymers showed relatively high shrinkage. On the other hand, by using aluminate solutions the shrinkage of geopolymers was significantly decreased. The aluminate activated geopolymer contained newly formed phillipsite and zeolite Na-P. However, there was no significant difference in geopolymer product when using chemical grade aluminate solutions and industrial waste aluminate solutions. The use of alkaline waste solutions is recommended because of economic and environmental benefits.