Sustainable synthesis of α-calcium sulfate hemihydrate from salt gypsum: Optimization of crystal morphology and mechanical properties for industrial applications Mingxia Wu a , Chang Chen a,* , Shaowu Jiu a , Yanxin Chen a,* , Yan Liu b a College of Materials Science and Engineering, Xi’an University of Architecture and Technology, Xi’an, Shaanxi 710055, China b State Key Laboratory of Green Building, Xi’an University of Architecture and Technology, Xi’an, Shaanxi 710055, China A R T I C L E INFO Keywords: Salt gypsum α-calcium sulfate hemihydrate Green production Crystal morphology Valorization and recycling ABSTRACT Salt gypsum (SG), a by-product derived from the salt manufacturing process, poses considerable challenges for efficient reuse owing to its elevated moisture and salt content. In this study, an innovative methodology was developed utilizing an oil-bath reaction unit to synthesize α-calcium sulfate hemihydrate (α-HH) from untreated SG in a sodium chloride (NaCl) solution, with the objective of achieving environmentally sustainable production. The influence of NaCl concentration, the type and content of transcrystallizing agents, reaction temperature, reaction duration, and stirring rate on the transformation process, crystal morphology, and mechanical strength of α-HH was systematically examined. The crystal morphology and chemical composition of the resultant α-HH products were meticulously characterized through XRD, SEM, EDS, FT-IR, and TG-DSC. The findings revealed that under optimized conditions (15 wt% NaCl solution, a reaction temperature of 115 ◦ C, a reaction duration of 2.0 h, a stirring rate of 200 rpm, and 0.20 wt% sodium succinate as a crystal modifier), the conversion efficiency of α-HH attained 80.90 %, accompanied by a compressive strength of approximately 31.93 MPa post-drying. NaCl induced a transformation in crystal morphology from flaky to long columnar, whereas sodium succinate further refined the morphology from long columnar to short columnar via physical adsorption mechanisms. This study proposes a high-value strategy for the valorization and recycling of industrial by-product gypsum, thereby broadening the application scope of SG and significantly enhancing its overall utilization efficiency. This approach is of paramount importance for advancing the green and sustainable development of the salt chemical industry. 1. Introduction Salt gypsum (SG), a byproduct of salt production, is categorized into three types based on its origin: marine, well, and lake SG [1,2]. Its pri- mary constituents are sodium chloride (NaCl) and calcium sulfate dihydrate (CaSO 4 ⋅2 H 2 O, DH), magnesium chloride (MgCl 2 ) and other impurities [3–7]. Due to its high water content and salinity, SG is un- suitable for direct use in the construction industry [8]. Moreover, the extensive accumulation can lead to significant environmental pollution, affecting soil, rivers, and groundwater [1]. China, with its abundant well salt resources, faces substantial challenges in managing the large quantities of SG produced, which not only exert storage pressure but also pose a considerable environmental burden [8–11]. Therefore, the resource utilization of SG waste is an urgent issue that needs to be addressed to mitigate environmental impacts and promote sustainable practices [12]. α-calcium sulfate hemihydrate (α-CaSO 4 ⋅0.5 H 2 O, α-HH), commonly referred to as high-strength gypsum, exhibits a strength range of 25–50 MPa, approximately three times that of conventional construc- tion gypsum [13]. In addition to its superior mechanical properties, α-HH is characterized by excellent workability, environmental compatibility, and biocompatibility, making it widely applicable in precision casting, high-end building materials [14], 3D printing [15,16], arts and crafts, and biomedical fields [17–20]. Traditionally, α-HH is derived from natural gypsum; however, gypsum mining has been increasingly restricted due to ecological concerns [21]. Consequently, there is a pressing need to develop alternative materials based on in- dustrial waste gypsum [13]. The conversion of industrial byproduct * Correspondence to: No. 13, Yanta Road, Xi’an, Shaanxi 710055, China. E-mail addresses: changchen420@xauat.edu.cn (C. Chen), chen_yanxin@xauat.edu.cn (Y. Chen). Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat https://doi.org/10.1016/j.conbuildmat.2025.143742 Received 14 April 2025; Received in revised form 8 August 2025; Accepted 22 September 2025 Construction and Building Materials 495 (2025) 143742 0950-0618/© 2025 Elsevier Ltd. All rights are reserved, including those for text and data mining, AI training, and similar technologies.