Mechanical, microstructural and structural investigation of phosphate-based geopolymers with respect to P/Al molar ratio

Abstract

The purpose of this work is to study the effect of P/Al molar ratio on the mechanical properties, microstructure and structure of phosphate-based geopolymers. Brazilian tests were made in order to identify the optimal P/Al molar ratio. The mechanical behavior of different samples showed a ductile behavior for P/Al molar ratio equal to 2 and a fragile one for other samples. A dense structure was observed for geopolymers that presented a fragile behavior and a bubble structure was observed for geopolymers that displayed a ductile behavior. For the structural study, different techniques were used: atomic absorption measurements, X-ray Powder Diffraction (XRD), 29Si, 27Al and 31P Magic angle spinning nuclear magnetic resonance (MAS-NMR) and Fourier Transform Infrared spectroscopy (FTIR). These techniques revealed that the geopolymer sample obtained with deficiency of acid presents an amorphous composite material composed of a geopolymeric network based on Al–O–P units and another one based on Si–O–Al–O–P units. The geopolymer sample obtained with the optimum quantity of acid also revealed an amorphous composite material. However, it does not present only these two geopolymeric networks, but also some Si–O–P units in the terminal chains of Si layer. The geopolymer sample obtained with excess of acid revealed a gel based almost on a crystalline phase of Aluminum Phosphate Silicate Hydrate compound and some amorphous geopolymeric networks.