New insights into the growth characteristics and crystal morphology of α-hemihydrate gypsum induced by organic acid crystallization agents during PG conversion

Abstract

The stockpiling of phosphogypsum (PG) poses a substantial risk of environmental pollution.To reduce the stockpile of PG and enhance its value, this research utilized phosphogypsum to produce α-hemihydrate gypsum (a-HH).The study elucidates the crystallization process of α-HH, which involves Ca2+ helical growth and coordinated filling by SO42- and H2O. Succinic acid, malic acid, L-aspartic acid are used as agents to transform PG into α-HH. α-HH is developed using a modified vapor pressure method. XRD, XPS, FT-IR, SEM, zeta potential, intensity testing, and molecular dynamics simulations are some methods used to study its crystal growth characteristics, morphological structure, and mechanical strength. These analyses aim to explore the impact and mechanism of succinic acid, malic acid, L-aspartic acid on the properties of α-HH. The results indicate that α-HH assumes a monoclinic crystal form, appearing as needles. Upon the addition of succinic acid, malic acid, and L-aspartic acid, the aspect ratio of α-HH crystals decreases, and the crystal morphology transitions from a rod-like needle structure to a short and stout structure.Notably, Malic acid demonstrates the most effective results.The study further demonstrates that the flexural and compressive strengths of all α-HH specimens surpass 4.5 MPa and 30 MPa, respectively, meeting the requirements for high-strength gypsum, specifically α30.Simultaneously, this study reveals that succinic acid, malic acid, L-aspartic acid form cyclic chelates with Ca2+ on the prominent surface of α-HH through oxygen atoms in the carbon base of the molecules, thereby altering the crystalline morphology of α-HH. The regulation of α-HH crystalline morphology depends primarily on the spacing and electron-donating ability between carbon-based oxygen atoms in the molecules of succinic acid, malic acid, L-aspartic acid. Introducing conjugated electron-donating groups into the adjacent positions of organic acids can more effectively enhance their ability to regulate the crystalline morphology of α-HH. This study further demonstrates that the regulation ability is directly proportional to the polarity of the organic acid molecules.