YANG Ran ¹, ZHANG Ming ², HUANG Tao ³, ZHAO Shanshan ¹,

OUYANG Liangfeng ¹, WANG Xiaotong ¹, YI Guoqiang ¹

(1. Hubei University of Technology, Wuhan 430068, Hubei, China; 2. Hubei Geology and Mineral Investment Group Co., Ltd., Wuhan 430068, Hubei, China; 3. Hubei Provincial Natural Resources Investment Group Co., Ltd., Wuhan 430068, Hubei, China)

Extended abstract:

[Background and purposes] Mirabilite gypsum is an industrial by-product gypsum with a steadily increasing output. Its high-value resource utilization has become a major focus in the industry. An ideal choice for the high-value utilization conversion of mirabilite gypsum is α-hemihydrate gypsum (α-HH). It has the advantages of compact structure, high mechanical strength and wide application scenarios. Current research on α-HH preparation has been mainly concentrated on crystal modifier regulation. However, as a key factor affecting nucleation and crystal growth, systematic information of raw material particle size in the industrial pressurized aqueous solution system is still insufficient. In this work, industrial by-product mirabilite gypsum was used as raw material. α-HH was prepared by an industrially applicable pressurized aqueous solution process with a reaction time of 2 h. The effects of raw material particle size on crystal morphology, crystallinity, particle size distribution, mechanical properties and hydration properties of α-HH were systematically studied. The optimal raw material particle size for preparing high-performance α-HH was identified.

[Methods] The mirabilite gypsum raw material was mainly composed of high-purity calcium sulfate dihydrate. After drying and sieving, five groups of samples, with median particle sizes of 30 μm, 70 μm, 115 μm, 153 μm and 205 μm, were obtained. A composite crystal modifier of 0.1 wt.% maleic acid and 0.1 wt.% succinic acid was employed. α-HH was prepared under the conditions of solid-liquid mass ratio 1:2, reaction temperature of 130 ℃, stirring speed of 300 r·min⁻¹ and reaction time of 2 h. Structure and properties of the products were characterized by using SEM, XRD, laser particle size analyzer, mechanical and hydration performance tests.

[Results] Raw material particle size showed a significant regulatory effect on α-HH. With increasing raw material particle size, the overall performance rose first then fell. For the raw material with excessively small particle size of 30 μm, the large specific surface area induced an undesirably high dissolution rate. The supersaturation of the system increased sharply, resulting in excessive nucleation and competitive growth. The product was mainly composed of fine and irregular particles with low crystallinity and poor mechanical properties. When the raw material particle size was about 115 μm, the dissolution of dihydrate gypsum, the nucleation and growth of α-HH reached the optimal kinetic balance. The product exhibited regular, short and coarse columnar crystals with the highest crystallinity of 91.2%. Its 2 h flexural strength reached 8.1 MPa and oven-dried compressive strength reached 32.3 MPa. It also had the lowest standard consistency water demand and stable initial setting time, showing the best overall performance. At an excessively large particle size of 205 μm, inhomogeneous dissolution resulted in localized supersaturation fluctuations. The product contained a mixture of large crystals and fine particles, resulting in poor crystal uniformity, decreased crystallinity and significantly reduced mechanical properties. Microstructural observation of hardened paste sections showed that the product from the optimal particle size group had close crystal interlocking, low porosity and high stress transfer efficiency. In contrast, products from the raw materials with too small or too large particles showed loose structures and pore defects, which became weak points for mechanical failure.

[Conclusions] Raw material particle size exerts a regulatory effect on the dissolution rate of dihydrate gypsum, system supersaturation and ion release rate. This in turn modifies the nucleation density, growth rate and crystal morphology of α-HH. Ultimately, these variations influence crystal integrity and the packing compactness of the hydrated crystals. A median particle size of around 115 μm is optimal, in terms of maintain the thermodynamic and kinetic equilibrium of the system. It provides suitable conditions for the composite crystal modifier to exert selective regulation, finally producing α-HH with desired structure and properties. The regulation mechanism of mirabilite gypsum particle size on the production of α-HH preparation under industrial conditions was clarified and the optimal median particle size was 115 μm. This could be used as theoretical and technical support for the efficient production of α-HH to facilitate the high-value utilization of mirabilite gypsum.

Key words: particle size; mirabilite gypsum; α-hemihydrate gypsum; pressure aqueous solution method; process optimization