Investigation of the intumescence mechanism of geopolymers by solid-state Magic Angle Spinning NMR

With certain formulations, Geopolymer (GP) exhibits intumescence (expansion upon heating), forming a porous structure, providing a thermal barrier effect. However, there have been no studies demonstrating the mechanism of the intumescence and the corresponding structural changes before and after heating. In this study, GPs are made based on metakaolin and silica fume. Then, GP with two aluminum-to-silicon molar ratio (Al/Si = 0/0.55) are investigated under heating conditions. The GP (Al/Si = 0) exhibits initial softening followed by hardening of the structure (from 50 degrees C to 175 degrees C), with intumescence observed within this temperature range. In contrast, the GP (Al/Si = 0.55) does not exhibit softening or expansion. Solid-state Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR) analysis of Si-29 and H-1 were then conducted on both GPs at ambient temperature and after heating at 105/200/300 degrees C for 24 h. Results show that GP softening is due to the transition from Si(Q(2)) to Si(Q(3)) silicate units, while hardening is caused by the transition from Si(Q(3)) to Si(Q(4)) silicate units. Additionally, the proton mobility of GP is higher for lower Al/Si ratios after curing. During heating, the mobility decreases for both GPs due to the removal of free water and the breaking of hydrogen bonds. Our results extend the knowledge of GP intumescence mechanism and call attention to low Al/Si GP for fire/high temperature application.