Microstructure properties of poly(phospho-siloxo) geopolymeric network with metakaolin as sole binder reinforced with n-SiO2 and n-Al2O3

Abstract

This study discusses the influence of the addition of small amounts of nano silica (n-SiO2, NS) and nano alumina (n-Al2O3, NA) to the metakaolin binder hardened by phosphoric acid and cured at an elevated temperature (80 ​°C) for 24 ​h. The condensed phosphor polymeric unit made up of Si–O–Al–O–P is studied by regulating the Si/P ratio at 1.7 in the control sample (MK0) and at 1.42 by using a powdered form of 1% n-SiO2 (MS) and 1% n-Al2O3 (MA). The addition of NS improved the average compressive strength of the specimens (MS) to 58.3 ​MPa, which is 25% higher than the control specimen, and the addition of NA led to a sharp increase of compressive strength by 45% (65.2 ​MPa) for the MA specimens. SEM micrographs revealed a higher compactness of the microstructure. The characteristic changes occurring in the polyphospho-silioxo network were analysed using X-ray diffraction (XRD), Fourier transforms infrared (FT-IR) and magic angle spinning nuclear magnetic resonance (MAS-NMR) (29Si, 27Al, 31P). The formation of P–O–Al during the reaction has been ascertained from the presence of the Q1(2Al), Q2(2Al) and Q1 (0,1Al) environments in the 31P NMR spectrum. The nano additive does not alter the tetra-coordinated alumina, whereas it enhances Hexa coordination of Al as confirmed from the 27Al NMR spectra. The coordination of Si with Al and P is confirmed from the 29Si spectral analysis in all the specimens and the addition of NA to metakaolin binder yields more crystalline phases of AlPO4 embedded in an amorphous matrix with a higher Al/P ratio as evident from EDAX analysis.