Structural study of the effect of nano additives on the thermal properties of metakaolin phosphate geopolymer by MASNMR, XPS and SEM analysis

This investigation was performed to understand the contribution of active nano-sized ingredients to the thermal stability of nanocomposites and discuss the underlying chemical changes after thermal treatment at 800 °C. Metakaolin–phosphate geopolymer (M), nano composites were formed by the addition of 1% nanoalumina (MA) and 1% nanosilica (MS) separately at a slightly elevated temperature of 80 °C for 24 h and then air-annealed. Microstructural studies were done to understand the thermal response of samples after exposure to 800 °C (Mh, MAh and MSh). The addition of readily available nanoalumina strengthens the silicoalumina phosphate link and aluminium phosphate phases, whereas the addition of nanosilica yields silicoalumina hydrates, which was demonstrated from X-ray diffraction spectroscopy (XRD) studies. An amorphous halo observed at room temperature diminishes while exposed to elevated temperature but becomes ordered with the additional peaks of polymorphs of aluminium phosphate. The Magic angle spinning nuclear magnetic resonance spectroscopy (MAS-NMR) data of the heated samples confirms the presence of crystalline AlPO4 and non-crystalline silicate phases. The predominant presence of AlPO4 from the Q1(1Al) environment was confirmed in the 31P signal around –26 and –28 ppm as a result of the migration of phosphorous species in the vacant sites after desilication to form P(OAl)4. The 29Si chemical shift observed at 107.8 ppm in Mh and MSh is related to quartz and the cristabolite peak of MAh coincides at 111.0 ppm. This trend was confirmed from X-ray photoelectron spectroscopy (XPS) binding energy values as the higher order in Si, Al, P and O atoms in all heat-treated samples might undergo aluminium bonding with phosphate units by increasing the bond length. Amorphous aluminium phosphate and silica phases were identified in the SEM elemental analysis along with many crystalline spots in the nanocomposite. The pronounced microstructural and morphological changes in composites under thermal treatment essentially arise from the coordination change of P bond and its migration coupled with desilication of silicoalumino phosphate, which is strongly responsible for the retention of thermo mechanical strength of the acid geopolymer.