Investigation of Dealumination in Phosphate-Based Geopolymer Formation Process: Factor Screening and Optimization

Abstract

In this study, phosphate-based geopolymers obtained from two aluminosilicate precursors with different mineralogical compositions were investigated. The used experimental methods were X-ray fluorescence, X-ray diffraction, atomic absorption measurements and experimental designs. Nine factors influencing the dealumination process during geopolymer formation were screened based on a Plackett–Burman design. The results show that the control factors are the P/Al molar ratio, curing temperature and curing time. These significant parameters were selected for further optimization using a central composite design. It was found that regardless of the used P/Al molar ratio, curing temperature and curing time increases generate an increase in the response. However, the P/Al molar ratio variation strongly affects Al removal only at low curing temperatures (between 22 and 50 °C) and with short curing times (between 0.2 and 3 h). The curing time parameter is the most significant factor. In addition, the same percentage of liberated Al can be achieved either by increasing the curing temperature in the earliest steps of geopolymer formation or by prolonging the curing time, even at low curing temperatures. Finally, the optimal conditions allowing maximum aluminum release are P/Al molar ratio = 2.0, curing temperature ≈ 70 °C and curing time = 4.76 h.