Thermal Resistance and Microstructural Response of Metakaolin- and Fly Ash–Based Geopolymers Modified with Nanosilica

In this work, the influence of nanosilica (NS) addition on the thermal and mechanical stability of metakaolin (MK)- and fly ash (FA)–based geopolymers at high temperatures (300°C, 600°C, and 900°C) is studied. A total of 16 mixes were produced by varying the binder, aggregate, and NS content. The thermal behavior was investigated by strength retention, water loss, and microstructure studies using scanning electron microscopy, energy-dispersive spectroscopy, and X-ray diffraction. Results indicate better initial strength development but higher strength decay as stress in the microstructure and gel structure breaks down and crack formation occurs, especially for MK-based systems, as the silica content increases. At 900°C, the strength drop of MK samples was more closely related to moisture loss than to FA, indicating a higher sensitivity to moisture. Additionally, NS significantly increased water loss in FA systems, while its effect was more moderate in MK matrices. Statistical analyses (univariate regression and two-way ANOVA) identified binder type as the most influential factor, with aggregate type playing a secondary role. The best performance was observed in MK–SK (sand kaolin) combinations, where NS improved matrix densification and reduced glassy phase transitions, enhancing thermal crack resistance. Overall, the study highlights the importance of optimizing NS dosage in relation to binder type, aggregate selection, and the Si/Al ratio. Proper integration of these factors is essential to maximize thermal stability and microstructural integrity in geopolymer composites.