High strength metakaolin/epoxy hybrid geopolymers: Synthesis, characterization and mechanical properties

Inorganic/organic hybrid geopolymers were synthesized by using metakaolin (MK) and solid organic components comprising an epoxy resin - diglycidyl ether of bisphenol A (DGEBA), and a hardener - dicyandiamide (DICY). In this study, solid organics were chosen to obtain a homogeneous mix upon co-milling all the solid precursors viz. MK, DGEBA and DICY in a planetary ball mill prior to geopolymerisation. Inorganic/organic hybrid geopolymers synthesized were then characterized for thermal, structural and mechanical properties. Thermogravimetric analysis (TG-DTG) revealed that hybrid geopolymers restricted the degradation of organics (~72% and ~79% organics loss corresponding to 10 mass% and 20 mass% DGEBA addition, respectively) compared to their milled counterparts (~95% and ~100% organics loss corresponding to 10 mass% and 20 mass% DGEBA addition, respectively) suggesting strong inorganic/organic interactions. This was substantiated by structural characterization via X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectroscopy. Major changes in DGEBA peak positions (FTIR) and disappearance of DICY peaks (XRD, FTIR) corroborated the MK-DGEBA-DICY reactions during geopolymerisation. Microstructural studies (scanning electron microscopy and transmission electron microscopy) coupled with elemental line scan and area maps, confirmed the homogeneous distribution of organics into the inorganic geopolymer matrix with no phase separation. Incorporating 20 mass% epoxy resulted in improved compressive strength from ~20.2 MPa to ~50.6 MPa (2.5 times superior vis-à-vis only MK-based geopolymer) and higher flexural strength of 5.4 MPa. The significantly enhanced mechanical properties of hybrid geopolymers were indicative of effective inorganic/organic interactions.