Expounding the application of nano and micro silica as a complementary additive in metakaolin phosphate geopolymer for ceramic applications—micro and nanoscale structural investigation

Abstract Metakaolin phosphate geopolymers comprising poly-phospho-siloxo units are known for their structural performance, additionally advancing their microstructure with the transformation of crystalline berlinite phases at elevated temperatures. The intrinsic reaction of Al of metakaolin in the acid exploited, but the reaction of secondary silica phases is limitedly known. Metakaolin as a primary precursor (M) with the addition of 2% and 5% of nano silica (MS2 and MS5) and micro silica (MM2 and MM5) cast using 8-M phosphoric acid was cured at 80 °C. To enhance the utilization of geopolymer in any high-temperature applications, the structural transformations were studied after heating to various temperatures (200, 400, 600 and 800 °C) by XRD, Raman, TGA-DTA, SEM, XPS, FTIR and MAS-NMR. Sample M attained a strength of 46.2 MPa enhanced to 63.6 MPa in MS5 and 54.2 MPa in MM5. This can be ascribed from the transformation of Si–O–Al–O–Si into Si–O–Al–O–P from Raman bands. Comparing the chemical shift of Al (IV) to control, micro silica addition shifts the signal to a lower field (53 to 50 ppm) related to the increase of the number of Al-connected Si to give a tougher network. Nanoindentation is visualized from hardness and elasticity, and the corresponding values are 1.4 to 2.1 GPa and 0.8 to 1.4 GPa for loads ranging from 20 to 100 mN in silica-reinforced samples that are much higher than M. The micro and macro hardness is due to the reinforcement of quartz in micro silica around the gel. TGA-DTA showed that the reduction of mass loss is as high as 25.4% in control whereas 17.2% in MS5 and 15.8% in the MM5. Further, shrinkage rate in MS5 and MM5 was as low as − 1.1% and − 0.8% throughout the temperature range from 25 to 1000 °C and thus provides the way of use of nano and micro form of silica for better thermal resistance. Graphical Abstract