Experimental study on static and dynamic mechanical properties of phosphogypsum

Effect of phosphogypsum adding on setting kinetics and mechanical strength of geopolymers based on metakaolin or fly ash matrices

Our study aims to highlight the effects of the addition of phosphogypsum on certain fresh and hardened characteristics of geopolymer matrices based on metakaolin or fly ash. In the fresh state, workability and setting were studied by rheology and by the electrical conductivity measurement. The hardened state was characterized by XRD, DTA, SEM, and compressive strength measurement. Workability investigations reveal that the addition of phosphogypsum increases the viscosity, which limited the phosphogypsum addition rate to 15 wt% for metakaolin-based matrices and 12 wt% for fly ash-based matrices, with a setting retarding effect in both cases. Analyses of the matrices show dissolution of gypsum along with formation of sodium sulfate and calcium silicate hydrate. Moreover, the introduction of phosphogypsum to these matrices up to a mass rate of 6% has no significant effect on the mechanical strength. Beyond that rate, the compressive strength drops from a value of 55 MPa for the matrices without addition down to 35 MPa and 25 MPa when the addition rate is 12 wt% for the metakaolin-based and fly ash-based matrix, respectively. This degradation seems to be due to the increase in porosity created by addition of phosphogypsum.

Effect of soluble salts in electrolytic manganese residue on its geotechnical characteristics

Electrolytic Manganese Residue (EMR) is a solid waste containing soluble sulfate, discharged by electrolytic manganese industries. The accumulation of EMR in ponds poses a significant hazard to both safety and the environment. This study utilized innovative geotechnical test techniques to conduct a series of tests, investigating the effect of soluble salts on the geotechnical characteristics of EMR. The results revealed that soluble sulfates had a significant impact on the geotechnical characteristics of the EMR. In particular, the infiltration of water leached away the soluble salts, causing a non-uniform particle size distribution and decreasing the shear strength, stiffness, and liquefaction resistance of the EMR. Nevertheless, an increase in the stacking density of EMR could improve its mechanical characteristics and inhibited the dissolution of soluble salts. Therefore, increasing the density of stacked EMR, ensuring the effectiveness and non-obstruction of the water interception facilities, and reducing rainwater infiltration could be effective measures to enhance the safety and reduce the environmental hazard of EMR ponds.

Experimental study on dynamic behavior of polyacrylamide-reinforced tailings

Earthquakes are a significant factor that contributes to tailings dam failure. Generally, the seismic stability of a tailings dam can be increased by improving the dynamic properties of tailings. The dynamic properties of tailings can be improved effectively using polymers. In this study, the dynamic properties of polyacrylamide-reinforced tailings were investigated via a sequence of dynamic triaxial tests. The content of polyacrylamide in the test sample was 0.3%. Test results show that the cyclic liquefaction resistance, initial dynamic shear modulus, dynamic shear modulus, and dynamic shear modulus ratio of polyacrylamide-reinforced tailings were slightly greater than those of unreinforced tailings. The damping ratio of polyacrylamide-reinforced tailings was lower than that of unreinforced tailings when the dynamic shear strain exceeded 0.038%. The increase in the dynamic pore water pressure of polyacrylamide-reinforced tailings during cyclic loading decelerated significantly compared with that of unreinforced tailings. The revised Zeng model can effectively described the changes in dynamic pore-water pressure of unreinforced and polyacrylamide-reinforced tailings. The polyacrylamide can improve the structural stability of the tailings specimen and also improve the dynamic properties of the tailings, thereby enhancing the seismic stability of the tailings dam.

Effects of Freeze–Thaw Cycles on Strength and Wave Velocity of Lime-Stabilized Basalt Fiber-Reinforced Loess

Basalt fiber is a new environmentally-friendly material with excellent potential for soil reinforcement in geotechnical engineering construction. This study explores the effects of freeze–thaw cycles on the unconfined compressive strength (UCS) and P-wave velocity (V_p) of lime-stabilized basalt fiber-reinforced loess. Reinforced loess samples with different proportions of basalt fiber and lime were subjected to 0, 1, 5, and 10 freeze–thaw cycles, and their UCS and V_p were subsequently measured. The test results showed that the addition of basalt fiber and lime to loess could enhance strength and improve resistance against freeze–thaw damage, and the freeze–thaw damage of reinforced loess decreases with the increase of basalt fiber content and length. A relationship between UCS and V_p of the reinforced samples was obtained for the same number of freeze–thaw cycles, and this relationship exhibited linear characteristics. The fitting results indicate that the V_p can be used to estimate the UCS after freeze–thaw damage. The research results not only have important practical significance in the application of basalt fiber in geotechnical engineering but also provide a reference for the non-destructive testing of the strength of loess after freeze–thaw cycles.

Microbial transformations by sulfur bacteria can recover value from phosphogypsum: A global problem and a possible solution

Rising global population and affluence are increasing demands for food production and the phosphorus (P) fertilizers needed to grow that food. Essential are new approaches for managing the growing amount of phosphogypsum (PG) that is a by-product of phosphoric-acid production from phosphate rock. Today, only ~15% of the worldwide production of PG is recycled, mainly for agriculture and road construction. This review addresses microbial valorization of PG through strategies that apply sulfur-transforming bacteria: sulfate-reducing bacteria (SRB) and sulfur-oxidizing bacteria (SOB). The focus is on recovering elemental sulfur (S⁰), which can be used to make the sulfuric acid needed to produce phosphoric acid from rock phosphate. Our review provides in-depth understanding of the microbiological, chemical, and technological bases for microbial reclamation of S⁰ from PG. The review presents the principles and practices for sulfate leaching from PG, reduction of sulfate to sulfide by SRB, and oxidation of sulfide to S⁰ by SOB. The choice of electron donor for SRB, control of oxygen delivery to SOB, and nutrient requirements are emphasized. Although microorganism-based technologies for PG reclamation are far from mature, the efficiency of such SRB- and SOB-based processes has been documented at laboratory and industrial scales. This review should spur biotechnological advances toward recovering value from PG.