Origin of the nano-cracking of dehydrated nesquehonite (MgCO3·3H2O)

Abstract

Hydrated magnesium carbonates (HMCs), reaction products of carbon dioxide fixation with brucite (Mg(OH)₂), exhibit unique variations in crystal structure and morphology owing to the presence of water molecules within their structures. The various crystal morphologies and dehydration reactions provide promising opportunities for the industrial use of HMCs, for example, fillers in polymer composites. This study evaluates the changes in crystal shape and structure of nesquehonite (Nq; MgCO₃·3H₂O) upon dehydration, which induces structural randomness within the Nq structure. The dehydration of Nq caused nano-cracking, including cleavage features, although the bulk crystal shape was maintained. The average cracking interval was 258 ± 155 nm. This cracking was caused by the uniaxial shrinkage of the Nq crystal structure along the a-axis, with a maximum contraction rate of 62.8% observed during dehydration. Rehydration of compounds dehydrated at 120 and 200 °C, which exhibited broadened or absent Bragg peaks in their X-ray diffraction patterns, was also examined under 90% relative humidity. Nq was regenerated in both samples after approximately six days. These results indicate that the structure of Nq was maintained during dehydration. Changes in the crystal shape and structure of Nq are fundamental to the morphological control of the dehydrated compounds (magnesium carbonates) of Nq, and the reactivity of the dehydrated compounds with water.