Phosphogypsum is a solid waste produced in the production of wet-process phosphoric acid. The main component is calcium sulfate dihydrate, and its constant calcium activity is very low. It is a very difficult waste to handle. The slag produced in the ironmaking processes of the Jiuquan Iron and Steel Company has high activity and can be mixed with phosphogypsum to prepare backfilling cementitious materials for the backfilling of goaf (see Introduction below) to prevent its subsidence and cracking and subsequent subsidence of the land surface above the goaf. In this paper, an orthogonal test of material strength under different ratios and finenesses was carried out, and the optimal formula of phosphogypsum-based cementitious material was obtained. On this basis, SEM (Scanning Electron Microscope) and XRD (X-ray Diffraction) were used to observe the internal structure of phosphogypsum-based cementitious materials. The results show that: when the content of quicklime is 5.5%–6.0%, the content of phosphogypsum is 30%–33%, the content of NaOH is 1.5%–2.0%, the content of Glauber’s salt is 2.5%–3%, the average particle size is 15.88 μm, and grinding aids and an early strengthening agent were added, and in the case of the strengthening agent, the 3-days and 7-days strengths of the filling reached the highest values of 1.58 MPa (mean flexural strength) and 4.1 MPa, respectively. The formation of calcium silicate hydrates gel (C-S-H) and ettringite (AFt) gradually increased with age, and the structure became denser. The ettringite plays the role of interspersed support in the filling body, thereby improving the overall strength. This research provides a theoretical basis and practical reference for the treatment of goaf.

The Sanmenxia Reservoir wetland (SRW) serves as a critical ecological buffer in the middle reaches of the Yellow River. Ongoing population growth and changes in land use have placed significant pressure on the wetland’s ecosystems. However, existing research has yet to establish a spatiotemporal analysis method to assess the impact of land use change on the ecosystem service value (ESV). This gap hinders the precise regulation and sustainable development of land resources. To gain a comprehensive understanding of the land use dynamics and ecological functions of the SRW, this study introduced a refined ESV evaluation method. This method revised the equivalent factors from spatial, temporal, and hydrological perspectives, with an emphasis on the impact of sediment discharge. Furthermore, the Patch-level Land Use Simulation (PLUS) model was employed to project the land use structure in 2030, and the corresponding ESV was analyzed. The study revealed the following: (1) Cropland constituted the primary land type in the SRW. Over the last two decades, the build-up area exhibited the most significant single land use dynamics, with the transformation of bare land to cropland spanning the widest range and largest area. (2) From 2000 to 2020, the ESV increased by 0.64 billion CNY, with the water body contributing almost all the increase. (3) The projected ESV for 2030 is 2.94 billion CNY, indicating an increase in ecological functions in the near future. Policy makers should recognizethe impact of land use change on ESV and implement measures to rebalance land use structure. While ensuring the preservation of cropland and the protection of lives and property, it is advisable to consider expanding the water body area to facilitate comprehensive sustainable development within SRW.

The Yellow River has the highest sediment concentration in the world, and the Yellow River coarse sediment mainly comes from a particular kind of argillaceous sandstone, Pisha sandstone. This paper reports an investigation of the possibility of development of low-cost engineering materials using Pisha sandstone via ion exchange modification. The effect of modifiers with different concentration on the inhibition of volume expansion and the strength enhancement of modified Pisha sandstone were studied via ion exchange solidification. The effects of the concentration of ten types of modifier solutions and curing age were considered. The hydration of the mineral components, particle surface potential and reaction products were studied, respectively, by XRD, zeta potential, TG/DTG and SEM. Expansion volume and shear strength tests were conducted to assess the volume stability and mechanical property of modified Pisha sandstone. It showed that the expansion of Pisha sandstone was controlled and that the volume stability and shear strength were improved via ion exchange modification. The results of XRD, TG/DTG and SEM showed that the spacing of the crystal layers of the Pisha sandstone clay mineral and the mass lost had decreased significantly. When the concentration of the modifier was 0.05 mol/L, the volume reduced by 54.55% maximum and the shear strength reached the peak of 138 kPa.