Study on the mechanical properties and microstructure of chitosan reinforced metakaolin-based geopolymer

Abstract

Due to the high brittleness and poor toughness, the applications of geopolymer as building materials are highly limited. In this paper, the chitosan (CS), a type of organic polymer, was used to improve the mechanical properties of metakaolin-based geopolymer (MKG), especially its flexural toughness. The effects of chitosan content, modulus ratio of alkali activator (molar ratio of SiO2 to Na2O), and the concentration of alkali activator on the mechanical properties were investigated. Fourier Transform Infrared Spectroscopy (FTIR), Nuclear Magnetic Resonance (NMR), and Scanning Electron Microscopy (SEM) were employed to study the microstructure and the toughening mechanisms. The results showed that the strength and flexural toughness of MKG were significantly improved in the presence of chitosan. As the dosage of chitosan increased from 0 to 2.0 wt%, the flexural and compressive strength of MKG increased first and then decreased. When the chitosan content was 1.0 wt%, the compressive strength and flexural strength of MKG at 28 d increased by 15.5% and 33.3%, respectively, compared with those of the blank group. Furthermore, the flexural toughness coefficient at 28 d was 83.63% higher than that of the blank group. It was noteworthy that with the addition of 1.0 wt% chitosan, the optimal modulus ratio and the concentration of alkali activator were 1.6 wt% and 10.0 wt%, respectively. Microscopic measurements indicated that –CONH2 in chitosan and –OH in MKG could bond together to form C-O-Si and lead to the decrease of the polymerization degree of [SiO4] tetrahedron. Moreover, the long chain molecules of chitosan bonded with MKG could form a three-dimensional interpenetrating network, thus resulting in the reinforcement of MKG at both molecular and submicroscopic levels. This study reveals the toughening mechanism of chitosan on MKG and can provid a technical support for the composition design of chitosan-reinforced geopolymer materials.