The formation of crystalline phases in metakaolin-based geopolymers in the presence of sodium nitrate

Geopolymers generated by alkali-activation of amorphous aluminosilicate sources are considered as an alternative immobilizing matrix for hazardous and nuclear wastes. Low and intermediate level nuclear waste streams are often highly alkaline saline solutions containing various concentrations of nitrate salts. The aim of the research project presented here was to study the effect of nitrate ions on the formation and evolution of metakaolin-based geopolymeric systems at moderate temperatures, i.e., at 40 °C. Metakaolin was alkali-activated using NaOH solutions of varying concentrations, yielding H₂O:OH^- ratios of 5.50, 9.15, 13.75, and 27.50. Sodium nitrate was added to the activation solutions at a constant [NO₃ ⁻]: [OH⁻] ratio of 0.25. Most geopolymeric mixtures were designed to obtain a Na₂O:Al₂O₃ ratio of 1.00 for nitrate-free mixtures, or 1.25 for those including sodium nitrate. In addition, the effect of deviation from these values (Na₂O:Al₂O₃ ratios of 0.84 and 1.20 for nitrate-free samples, 1.04 and 1.50 for nitrate-bearing samples) was also studied. The samples were cured in sealed containers at 40 °C for periods ranging from 1 day to 3 months. The products were characterized by X-ray diffractometry, Fourier transform mid-infrared spectroscopy, and scanning electron microscopy as well as by compressive strength measurements. The results demonstrate the influence of composition, alkalinity, SiO₂:Al₂O₃ ratio, Na₂O:Al₂O₃ ratio, nitrate concentration, and curing times on the mineralogy of the geopolymeric matrix. Various crystalline phases such as zeolite A, zeolite X, and nitrate-bearing phases, namely nitrate sodalite and nitrate cancrinite, were identified among the reaction products. The sequence of phase evolution in these geopolymeric systems was elucidated.