Metakaolin-based geopolymer composites modified by epoxy resin and silane: Mechanical properties and organic-inorganic interaction mechanism

The wide application of metakaolin based geopolymer (MG) is highly limited by its brittleness. In this study, different contents of S823, S1213 and KH570 silane coupling agent (SCA) and waterborne epoxy resin (WR) were used to synthesize organic-geopolymer composites to improve the ductility of MG. The influence of the contents of these organic agents on the mechanical properties of MG was investigated with unconfined compression tests and three-point bending tests, and the organic-inorganic interaction was characterized with Scanning Electron Microscopy, Isothermal Calorimetry, solid-state Nuclear Magnetic Resonance and Nanoindentation. By adding 1% of SCA and 20% of WR, the 28-day compressive strength, flexural strength and flexural toughness of MG achieved the highest increase of 197.0%, 128.3% and 108.4%, respectively. The reaction degree of the MG-WR and MG-SCA composites were both higher than that of MG thanks to the polymerization between the organic agents and geopolymer gel, resulting in more compact microstructure. It was revealed that the organic-inorganic polymeric structure bridged by Si-O-Si and Si-O-Al bonds were formed between SCA and MG with higher content of Si O bonds, which enhanced the micro-mechanical homogeneity, and thus the strength and ductility of MG-SCA composites. In addition, the interlocked MG-WR network formed through the hydrogen bonds allowed the high-toughness WR to improve the ductility of MG. The better understanding of organic-geopolymer interaction in this study is beneficial for the synthesis of geopolymer composites with improved mechanical performance.