Crystallography Of Gypsum Utilization

China is the country with most abundant gypsum resource(calcium sulfate dihydrate,CaSO₄·2H₂O),a geological reserve of 57 billion tons.Meanwhile,large amounts of by-product gypsum is produced annually in industry.To develop new products and to improve the performance of products are optimal routes to the high-value-added utilization of natural gypsum and to the recycling of industrial by-product gypsum.Gypsum is one of the common scale species in industrial piping and equipment and the essence of gypsum scaling is the nucleation and growth of gypsum.Gypsum is also an important index mineral in geological evolution which is used to reflect the behavior of water molecules in planetary history.Therefore,it is necessary to deepen the understanding of the nucleation and growth process of calcium sulfate theoretically.In this work,we focus on the nucleation and initial growth of calcium sulfate crystals,reveal the formation,structure,phase transformation process and mechanism of calcium sulfate precursors,in situ observe the dynamics of nucleation and initial growth of calcium sulfate,and preliminarily reveal the relationship between classical and non-classical crystallization theories of calcium sulfate.We investigate the nucleation and growth of calcium sulfate in nanometer scale,capture amorphous calcium sulfate(ACS)as a precursor in a low calcium sulfate concentration,and investigate its structural evolution in sufficient temporal and spatial resolution.ACS nanoparticles are 20-50 nm in diameter and present an outer-layered and inner-disordered morphology.Aggregation of ACS nanoparticles gives birth to bulk ACS particle within which nanocrystalline domains sprout and develop into the crystalline gypsum phase.This work gives a clear view of the structural evolution of amorphous calcium sulfate into crystalline phase,that is,the process of ordering.We further investigate the structural characteristics of ACS nanoparticles in molecular scale,and find ACS nanoparticles have a highly disordered hydrous structure with proto-gypsum property,and Ca is octahedral coordinated with eight water molecules in the first coordination shell.During the crystallization of ACS,transport of water molecules and sulfate ions between the first and outer coordination shells promotes the formation,development,fusion and alignment of nanocrystalline domains,and realize the structural evolution from short-to medium-and long-range order.This work provides the structural information of ACS and reveals the role of water molecules in the crystallization of ACS.We investigate the growth pathway of bassanite precursor and its transformation into the stable phase of gypsum,and demonstrate a three-stage process:(1)growth of bassanite nanoparticles into bassanite nanorods or rosettes by monomer addition,(2)self-assembly of bassanite nanorods into bassanite aggregates,and(3)heterogeneous nucleation of gypsum on tips of bassanite rods in aggregates and rosettes due to an epitaxial match with[001]direction.These findings demonstrate that bassanite growth bridges classical monomer addition and non-classical self-assembly of nanoparticles,providing further proof that more than one crystallization pathway can operate simultaneously in a single system and that the precursors in multistage crystallization processes can imprint their pattern of precipitation on the final phase.Using in situ liquid cell transmission electron microscopy(TEM)to explore calcium sulfate crystallization,we demonstrate that multiple nucleation and growth pathways are simultaneously operative,including the formation of cluster aggregates as a precursor,direct nucleation of bassanite nanoparticles,and growth of bassanite nanoparticles via both monomer addition and coalescence.The behavior of cluster aggregates upon condensation suggests its liquid-like characteristic and the dissolution of the formed fused particles serve as a feedstock for bassanite growth.This study reveals a series events of calcium sulfate crystallization that advances our understanding of calcium sulfate nucleation and growth,bridging classical and nonclassical mechanisms.Altogether,this work reveals the multi-stage evolution of calcium sulfate nucleation and initial growth in nanometer scale,which has important theoretical significance and application value in the preparation of calcium sulfate products with high performance,development of new calcium sulfate products,improvement in the utilization level of gypsum resource,deepening the understanding of calcium sulfate scaling and scale control,and retrospecting the planetary hydrochemical activity and sedimentary stratigraphic records.