Low-field 1H NMR study on geopolymers: The effect of paramagnetic Fe(III)

Geopolymers are inorganic polymers with 3D framework structures having superior mechanical and physical properties. Metakaolin-based geopolymers synthesized using solutions of Fe(III) chloride have been studied ex-situ and in-situ using low-field 1H nuclear magnetic resonance (LF NMR). The results revealed that the introduced Fe(III) could affect the spin relaxation and the pore structure. The effect on spin relaxation was determined by quantifying the relation between surface relaxivities and Fe(III) ion content. The effect on pore size was measured by Mercury Injection Capillary Pressure (MICP), showing a two-stage evolution. The pore size increased slightly before and dramatically after the threshold of 2.0 wt% iron content. We propose herein as possible causes the reaction between hydroxide ions and Fe(III) and the formation of Si(Al)-Fe-oxyhydroxides. During the early (first three days) polymerization of geopolymers the in-situ curves revealed three stages (i.e. dissolution, condensation, and stabilization) which are affected to different degrees by Fe(III).