Inorganic–organic hybrid geopolymers: evolution of molecular and pore structure, and its effect on mechanical and fire-retardant properties

In order to overcome the brittle behavior of conventional geopolymers, of late, a paradigm shift towards development of hybrid geopolymers has commenced. This study describes hybrids synthesized by co-milling metakaolin and solid organics (epoxy resin: diglycidyl ether of bisphenol A and hardener: dicyandiamide) followed by alkali activation. The developed hybrid geopolymers exhibit enhanced mechanical and physical properties. Physical and mechanical properties of such hybrids depend on the extent of molecular-level interactions and microstructural evolution during geopolymerisation. Evolution of molecular structure from precursor stage (co-milled samples) to hybrid geopolymers is studied using transmission electron microscopy (TEM) and ²⁷Al, ¹³C, ²⁹Si solid-state nuclear magnetic resonance (NMR) spectroscopy. NMR and TEM analyses of the hybrid geopolymers illustrate the formation of Si–O–C bonds and uniform C distribution (with no phase separation); this confirms inorganic–organic chemical interactions during geopolymerisation. Detailed assessment of pore characteristics using TEM, mercury intrusion porosimeter, and Brunauer–Emmett–Teller reveal formation of a dense gel (with reduced pore size and pore volume) in hybrid geopolymer vis-à-vis MK-based inorganic geopolymer. The implication of such microstructural features on mechanical and physical properties is discussed. Lastly, the suitability of developed hybrids as fire-retardant materials used in mass transit applications is highlighted.