Carbonation Induced Structural Changes in Soil-Based Alkali-Activated Binders

Alkali activated binders (AABs) are low-carbon footprint alternatives to ordinary Portland cement. Accelerated carbonation can further reduce CO ₂ emission while enhancing the binding properties of AABs. We used solid-state nuclear magnetic resonance (NMR) in combination with X-ray diffraction, Infrared spectroscopy, and density functional theory to track local structural changes as a function of material precursors, and carbonation of AABs in their physiological conditions. ²⁷ Al and ²⁹ Si solid-state NMR show a carbonation-induced conversion of Al(IV) to Al(VI) sites along with increased branching and disorder in soil containing AABs. Analysis of ¹ H- ²⁷ Al and ²⁹ Si- ²⁷ Al heteronuclear correlation NMR spectra provides details on the carbonation-prone Al-sites and their impact on the neighboring Si sites. Density functional theory calculations confirm the stability of the nuclear magnetic resonance predicted Al-sites. ²³ Na and ⁴³ Ca solid-state NMR reveal hydration layer variation and metal carbonate formation as a crucial step in carbon sequestration. This study offers insights into the carbon sequestration in AABs and its impact on the local structures critical to their binding properties.