Characterization of hydrated magnesium carbonate materials with synchrotron radiation-based scanning transmission X-ray spectromicroscopy

Formation of hydrated magnesium carbonate (HMC) cement phases from the carbonation of brucite has been investigated using synchrotron radiation (SR) based scanning transmission X-ray microscopy (STXM) at Mg and O K-edges. HMC is formed from the reaction of brucite and sodium bicarbonate in water. The introduction of magnesium acetate as a ligand influences the characteristics of the resulting precipitates in an aqueous setting. Particularly, significant morphological development of HMC phases and the complete carbonation of brucite with the longest reaction time are observed. The analysis successfully distinguishes various HMC phases, including nesquehonite, dypingite, giorgiosite, and an undefined phase, shedding light on the structural and chemical differences of formed HMCs. Notably, there is a transformation in HMC formation when magnesium acetate is introduced, leading to the creation of agglomerates that weave together into an interconnected network with a dense microstructure, a key factor in enhancing cohesive performance for HMC cement. We demonstrate here that, owing to simultaneous microscopic and X-ray absorption near edge structure (XANES) spectroscopic observations with STXM-XANES, different HMC phases can be resolved and a comprehensive investigation of their interconnections studied, proving STXM-XANES to be a powerful tool for studies of cementitious materials. We observed a strong orientation effect, where the relative angle between the crystal axis and polarization vector of X-rays affects the observed XANES spectrum. The report also discusses the challenges and future prospects of synchrotron radiation-based STXM-XANES in studying HMC materials and their pivotal role in the carbonation process of Mg-based cements, paving the way for innovative and sustainable construction materials.