Unravelling the role of iron oxidation states in alkali-activated slags: A multinuclear solid-state NMR study on polymerization and structural evolution

Abstract

Understanding the structure and polymerization behavior of Fe-bearing alkali-activated slags (AASs) is crucial for optimizing their macroscopic properties and expanding their applications in sustainable construction materials, radioactive waste storage, carbon sequestration, and other environmental technologies. This paper investigates the effect of iron oxidation state in precursor slags on the polymerization and microstructural evolution of synthesized AASs using advanced solid-state NMR with 1D MAS and 2D 3QMAS experiments. ⁵⁷Fe Mössbauer spectroscopy and Raman spectroscopy further provide insights into Fe coordination and phase composition. The synthesized slags were designed with controlled Fe content (<10 wt%), ensuring sufficient NMR spectral resolution. The results show that after 7 days of curing, AASs synthesized from Fe²⁺-rich slag exhibits lower structure polymerization compared to those derived from the Fe³⁺-rich slags. After 1 year of storage, Fe²⁺-rich AAS undergoes further polymerization, leading to the formation of highly connected silicate structures. Based on the NMR analysis, we propose that Fe³⁺ is incorporated into the Al^VI site in hydrotalcite or as a charge balancing cation near the Al^IV site in the bridging positions, with only a minor fraction potentially in tetrahedral coordination. This study highlights the critical role of iron oxidation state in tuning the polymerization and structural evolution of AASs, providing a fundamental understanding that can guide the design of next-generation Fe-bearing alkali-activated materials.