Crystal-facet anisotropy dictates the alkali dissolution behavior of struvite

Abstract

The long-term durability of magnesium phosphate cement (MPC), particularly in repair applications, is critically limited by the chemical stability of its primary hydration product, struvite (MgNH₄PO₄·6H₂O), in alkaline environments. While struvite exhibits diverse crystal morphologies, the mechanism underlying how these morphological variations influence its dissolution behavior under strong alkali conditions remains unclear. This study systematically investigates the dissolution kinetics and microscopic mechanisms of four representative struvite morphologies – rodlike, petal-like, prismatic, and dendritic – in a highly alkaline environment (pH = 12). A combination of characterization techniques, including SEM, XRD, FT-IR, ICP-OES, and TG-DTG, coupled with theoretical calculations of surface energy and adsorption energy, was employed. The results reveal an anisotropic dissolution law governed by the preferential exposure of specific crystal facets. The rodlike morphology, possessing the highest proportion of the active [002] face, exhibits the lowest surface energy and the strongest adsorption energy for OH⁻. This leads to the preferential leaching of NH₄⁺, subsequent collapse of the crystal structure, and ultimate transformation into more stable phases like Mg₃(PO₄)₂·8H₂O, resulting in the most severe corrosion degree among all morphologies in the alkaline environment. This work elucidates the crystal-facet-dependent anisotropic corrosion mechanism of struvite, providing a crucial theoretical basis for enhancing the long-term alkali durability of MPC.