The Influences of Soluble Phosphorus on Hydration Process and Mechanical Properties of Hemihydrate Gypsum under Deep Retarding Condition

High value-added utilization of desulfurized building gypsum as self-leveling mortar: the comprehensive effect of cement

The utilization of desulfurized building gypsum as raw material for gypsum-based self-leveling mortar (GSL) is limited by its low strength and poor water resistance. The objective of this study is to enhance comprehensive properties of GSL and prepare qualified desulfurized building gypsum-based self-leveling mortar that can be effectively applied in practical engineering projects. The influence of cement on water consumption rate of initial fluidity (W/M ratio), fluidity, setting time, mechanical strength, and water resistance of GSL were evaluated. Additionally, rheological parameter, heat of hydration, crystal morphology, and pore structure were also analyzed. Cement significantly improved the fluidity of slurry. Moreover, the compressive strength and softening coefficient of GSL reached 20.6 MPa and 0.56 at 10% cement, respectively. Furthermore, cement reduced the 30-min-fluidity loss and improved fludity by reducing the yield stress and increasing the plastic viscosity of screed. The transformation of hydration kinetics of GSL could be due to Ca2+ and OH- released by cement, thus resulting in the shortening of initial setting time and the prolongation of the interval between initial and final setting time. Scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) showed that CSH gel and AFt crystal would generate on the surface of CaSO4·2H2O crystal, making the structure more compact. Mercury intrusion porosimetry (MIP) indicated that cement greatly reduced the porosity through the water reduction effect in the early stage and continuous hydration in the later stage. The continuous hydration of cement also increased the shrinkage rate. This work was expected to provide reference for promoting the application of desulfurized building gypsum as the high value-added screed.

Effect of phosphate rock acid insoluble residue on hydration process and mechanical properties of α-hemihydrate gypsum

The application of α-hemihydrate gypsum (α-HH) is limited by several factors, such as a rapid hydration rate, short setting time, poor water resistance, and high cost. Especially because of the high production cost, although α-HH has excellent mechanical strength, it is rarely used in the field of building materials. In this study, based on the composition characteristics of new industrial solid waste (phosphate-rock acid-insoluble residue, PAIR) and to meet the needs of resource utilization, gypsum matrix composites were prepared by adding PAIR to α-HH to solve the problems of short setting time and poor water resistance of gypsum matrix composites and improve the comprehensive properties of α-HH products. The results show that when the different types of pores formed during the hydration of α-HH were filled with inert substances, such as silicon dioxide, insoluble phosphate, and calcium fluoride from PAIR, the proportion of mesopores in the composite products increased, whereas that of harmful macropores decreased. The compressive and flexural softening coefficients of the PAIR/α-HH system with 23% PAIR were the highest at 57.25 and 60.125%, respectively, and the water resistance of the system was improved; when the content of PAIR reaches 35%, the strength of the composite products decreased from 58.125 to 43.8 MPa. The HPO₄^2- in PAIR partially replaces the SO₄^2- ion in the dihydrate gypsum (DH) lattice to form a Ca(SO₄, HPO₄)•2H₂O double salt, leading to the production of eutectic phosphorus. Soluble F^-, Al³⁺, Mg²⁺, and phosphorus-containing substances in PAIR form a variety of complex ions in PAIR/α-HH aqueous solution, which are adsorbed onto the surface of the new DH phase; the crystal morphology changed from thick, long columns to clusters, thin rods, and plates, inhibiting the nucleation and growth of DH and changing its crystal growth rate and crystallization behavior. Therefore, the setting time of gypsum is prolonged; when 35% PAIR was added, the induced nucleation period of the PAIR/α-HH system was prolonged from 40 to 265 min, and the final setting time was from 12 to 360 min. By mixing solid waste PAIR, while the setting time of α-HH is prolonged, its water resistance is improved, and its mechanical strength is not significantly reduced, reducing the cost. From the perspective of economy and environmental protection, this study is a way to α-HH is widely used in the field of building materials.

Preparation of gypsum with high purity and whiteness from phosphogypsum for CO2 mineral sequestration

Phosphogypsum (PG) is a solid waste product generated during wet-process phosphoric acid production. Various impurities considerably reduce the purity, whiteness, and application range of PG. This article analyzes the physical properties of PG in detail and systematically examines the content and distribution of impurities. Based on the obtained results, a simple process for the efficient removal of almost all impurities in the PG is proposed. The purity and whiteness of the purified gypsum (CaSO₄) significantly increased to 99% and 92%, respectively. The migration of impurities and the material balance of this process were then analyzed. Most importantly, the purified gypsum showed high CO₂ sequestration efficiency for CO₂ mineral sequestration, through which a high value-added CaCO₃ product was obtained.