Phosphogypsum Research Articles

Efficiency of acid mixtures for mitigating the radioactive contaminants in phosphogypsum

This present work aims to mitigate the radioactive contaminants in Phosphogypsum (PG), a by-product generated from phosphate rock processing, by applying a new chemical additive before precipitation to repurpose it for construction materials. PG contains different concentrations of rare earth elements, trace elements, and radioactive nuclides, such as 226Ra derived from phosphate rock (PR), which could cause environmental and health risks if not properly managed. In this context, a new approach is suggested, requiring the addition of a specific combination of concentrated sulfuric acid and nitric acid or hydrochloric acid. Gamma spectroscopy has been used to determine the activity of 226Ra, which is successfully reduced by about 49 %, decreasing from 831 to 424 Bq/kg. Several radiological parameters have been calculated, including radium equivalent activity, Raeq = 437 Bq/kg, which is found to be quite close to the allowable limits for construction (370 Bq/kg). The results of the proposed approach seem to be an effective means of reducing the activity of PG, leading to enhanced environmental responsibility and adherence to regulations in the phosphate sector.

Desilication and purification of phosphogypsum waste by flotation using a photosensitive collector

Phosphogypsum (PG) is the main by-product of wet phosphoric acid production and has seriously polluted the environment. In this study, PG was purified by reverse flotation using a photosensitive azobenzene surfactant (PASF). After reverse flotation, the content of gypsum in PG increased from 89.16 % to 93.32 %, the whiteness of PG increased from 25.62 % to 60.9 %, and the content of quartz decreased from 4.87 % to 2.12 %. The Zeta potential of quartz and gypsum became positive after being treated with PASF, indicating that PASF enhanced the floatability of quartz and gypsum. The results of FTIR and XPS indicated that there were both electrostatic attraction and hydrogen bonding between PASF and quartz. Density functional theory (DFT) calculations indicated that the adsorption of PASF on the quartz surface was more stable than on the gypsum surface. Upon exposure to UV irradiation, the molecular structure of PASF was transformed from trans to cis. Meanwhile, the adsorption amount of PASF on the quartz surface was decreased and the rigidity of the corresponding adsorbed layer was increased. This work provides a method to purify PG waste, which is important for the comprehensive utilization of PG.

Reaction Characteristics of Calcination and Decomposition of Phosphogypsum in a Bubbling Fluidized Bed.

A bubbling fluidized bed reactor is designed to investigate the high-temperature calcination and decomposition characteristics of phosphogypsum (PG). The reactor employs electric heating, and a lifting discharge tube is installed in the middle of the air distributor to allow flexible switching between batch and continuous feeding modes. The batch test results show that the solid-phase bed materials with a PG particle size of 0.27-0.55 mm and a coal particle size of 0.55-0.83 mm are found to have uniformly mixed at a PG/coal mass ratio of 5:1 and calcined at a high temperature of 1100 °C for 30 min. PG completely decomposes to yield mainly CaS and a small amount of Ca2(SiO4) as the solid-phase products. For the above-mentioned optimal process parameters, a 120 min thermal-state continuous test is conducted. Feeding and discharging are performed at intervals of 30 min to maintain the stability of the static bed height inside the reactor. The results indicate that the PG decomposition rate is higher than 92% in the batch and continuous tests. However, the continuous decomposition of PG has significant process advantages, such as a higher CaS yield (71.20%) compared to that in the batch mode (64.49%). Furthermore, PG undergoes agglomeration and bonding within the particles when heated, intensifying the formation of a Ca2(SiO4) eutectic.

Enhancing compressive strength of phosphogypsum-based geopolymer cement with five alkaline activator combinations

Phosphogypsum (PG) is a bulk industrial solid waste generated during phosphoric acid manufacturing. This study investigated a novel phosphogypsum-based geopolymer (PBG) cement through L16(45) orthogonal experiments. The impact of five alkaline activators, including Portland cement clinker (PCC), sulfate alumina cement clinker (SACC), aluminate cement (AC), magnesium oxide (MgO), and calcium oxide (CaO), on the performance of PBG cement was analyzed, and the optimal mix ratio was determined. Results indicate that all four alkaline activators, excluding CaO, can enhance unconfined compressive strength (UCS) of PBG cement. Factor index analysis revealed that an optimal composition comprising 4 % PCC, 6 % SACC, 2 % AC, and 4 % MgO yielded the highest 28-day UCS of 45.63 MPa. PBG cement primarily consists of dihydrate gypsum (CaSO4·2H2O), ettringite (Ca6(Al(OH)6)2(SO4)3(H2O)25.7), and polymers. Of these constituents, the polymers accounted for 61.9 wt% of the total composition and provided most of the strength. This study provided a new approach for making better use of waste PG.

Eco-friendly solid waste-based cementitious material containing a large amount of phosphogypsum: Performance optimization, micro-mechanisms, and environmental properties

Reusing phosphogypsum (PG) waste to manufacture eco-cementitious materials is sustainable. However, PG faces the challenge of high stacking volume and low utilization rate, and the low-content PG in eco-cementitious materials currently limits its large-scale potential application. This study provides a solution to use high-content PG to fabricate eco-friendly solid waste-based cementitious material (PBC), associated with ground granulated blast furnace slag (GGBS), fly ash (FA), and hydrated lime. Respond surface methodology (RSM) was undertaken to optimize and discuss the influence of independent design factors of GGBS, FA, and hydrated lime content on the unconfined compressive strength (UCS). Besides, the micro-mechanisms and environmental properties of PBC were also investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), and the leachate test. The results indicated that the optimal raw material contents were (by mass): 50.57% PG, 28% GGBS, 20% FA, and 1.43% hydrated lime. The UCS of the optimal PBC mixture is 23.57 MPa at 28 days, which was confirmed via experimental verification (24.65 MPa). Hence, the RSM is an effective method for optimizing PBC's UCS. The micro-mechanisms analysis shows that the ettringite is the primary hydration product within the reaction system of PBC, which is attributed to the substantial amount of PG. The leachate test results indicate that the fluoride ion concentration of the leaching solution of PBC remains below the PG, and the pH value falls within the range of 6–9. The leaching test results meet the Chinese environmental standard of GB 8978-1996. The cost and CO2 emission of the optimal PBC mixture could achieve a reduction of 71.25% and 99.98%, respectively, compared to Ordinary Portland cement (OPC). The findings of this study could serve as a theoretical foundation for the practical implementation of large-scale and efficient utilization of PG waste. The developed eco-friendly PBC has excellent potential to partially substitute conventional OPC binders and reduce engineering project costs.

Assessment of the mobility of potentially toxic trace elements (PTEs) and radionuclides released in soils stabilized with mixtures of bentonite-lime-phosphogypsum.

Phosphogypsum (PG) is a solid by-product of the phosphate industry, rich in contaminants and produced in large quantities. Raw materials and stabilized specimens, consisting of bentonite-lime-PG mixtures, were characterized by mineralogical, microstructural, chemical, alpha-particle, and gamma-ray spectrometry analysis before hydration and after hardening. Compressive strength and leaching tests were performed on hardened specimens. The physicochemical parameters and chemical composition of leachates from raw materials and hardened specimens were determined. PG contains high concentrations of natural radionuclides, specially from U series. Uranium-238 activities are double in PG than the worldwide average for soil values. The mobility of PTEs from PG is Cd (2.43%), Zn (2.36%), Ni (2.07%), Cu (1.04%), Pb (0.25%), and As (0.21%). Cadmium is the cation most easily released by PG in water with a concentration 0.0316mgkg-1. When PG is added to bentonite-lime mixture, cadmium is no longer released. The radionuclide 238,234U and 210Po predominates in the leachates of PG. However, the activity of 210Po becomes negligible in the leachates of bentonite-lime-PG mixtures. The addition of PG to bentonite-lime mixtures facilitates the trapping of trace elements (PTEs) and radionuclides, providing potential applications for PG as road embankments and fill coatings.

Treating waste with waste: adsorption behavior and mechanism of phosphate in water by modified phosphogypsum biochar.

The use of green methods to treat industrial waste and waste reuse has become a key environmental issue. In order to achieve this goal, this study treated waste phosphogypsum (PG) and produced modified PG biochar to adsorb and remove phosphorus from PG leachate, so that the PG pollution problem was controlled. In this study, PG was modified with sodium carbonate (Na2CO3) to prepare a modified PG biochar that was used for the removal of phosphorus-containing wastewater. An X-ray diffraction (XRD) analysis of the modified PG revealed that the main component was calcium carbonate (CaCO3), and a suitable amount of modified PG could load calcium oxide (CaO) onto the biochar and improve its physical properties. The experimental results showed that the modified PG biochar had a maximum phosphorus adsorption capacity of 132mg/g. A further investigation of the mechanism of adsorption revealed the importance of electrostatic attraction and chemical precipitation, and it was found that the CaO in the modified PG biochar could effectively facilitate the conversion of phosphate to hydroxylapatite (Ca5(PO4)3OH) in water. The phosphorus removal rate from leachate obtained from a landfill containing PG was 99.38% for a specific dose of the modified PG biochar. In this study, a PG pollution control technology was developed to realize the goal of replacing waste with waste.

Assessment of metal(loid) and natural radionuclide pollution in surface sediments of an estuary affected by mining and phosphogypsum releases

The prolonged impact over the Tinto River estuary by both the significant pollution by acid mine drainage (AMD) affecting this river and the polluted releases from phosphogypsum (PG) piles has led to the severe environmental degradation of this ecosystem. The aim of this work was to assess the current environmental quality of the Tinto River estuary through the study of the spatial distribution of metal(loid)s and natural radionuclides in the surface sediments from the channel edge. The sediments contain mean concentrations 5–20 times higher than the background values for pollutants such as Zn, As, Cu, Pb, or U, and up to two orders of magnitude higher for P. The studied sediments are heavily polluted by toxic heavy metals and metalloids (Pb, Zn, Cu, and As) according to the US EPA guidelines. Most of the analyzed sediment samples are also strongly polluted by long-lived natural radionuclides, mainly U-isotopes and 210Pb with concentrations up to one order of magnitude higher than unpolluted sediments, mostly due to the contribution by the PG leachates. The enrichment factors (EF) were extremely high (EF > 50) for As and very severe enrichment (25 ≤ EF < 50) for P, Cd, Zn, Cu, and Pb.

Sodium doping and control during the preparation of α-hemihydrate gypsum in NaCl solution

Transformation of phosphogypsum (PG) into α-hemihydrate gypsum (α-HH) in sodium salt solution is disturbed by the doping of Na+, which causes the frost of Na+ doped α-HH. In this study, the content evolution of Na+ impurities into α-HH was clarified during the hydrothermal process at 95 ℃, the frost phenomenon of hardened gypsum containing Na+ impurities was observed and controlled. The results show that increasing the initial NaCl concentration from 5 % to 25 % can promote the phase conversion of PG into α-HH within 30 min, but this will increase the Na2O content in the product from 0.08 % to 4.37 %, which is due to the formation of the undesirable phase Na2SO4·5CaSO4·3H2O. The content of Na+ impurities entering α-HH presents a non-linear increase with the NaCl solution concentration and reaction time, which can be well described by Boltzmann function. An obvious frost phenomenon of the hardened plaster of Na+ doped α-HH can be observed after curing 3 d, and the frost is identified as Na2SO4. This work further demonstrates the feasibility that the addition of CaCl2 in NaCl solution is an effective approach to control the doping of Na+ into α-HH, the Na2O content decreases to 0.97 % in the mixed solution of 17.5 % NaCl and 10 % CaCl2, this helps to eliminate the frost phenomenon and improve the mechanical strength of α-HH.

Immobilization capacity of element arsenic in alkali-activated slag-arsenic tailing system during self-healing process

The utilization of alkali-activated slag systems for encapsulating hazardous tailings to produce polymers can effectively reduce the leaching of hazardous elements. Nevertheless, the propensity of these polymers to develop cracks during service poses a risk of secondary release of hazardous elements. For this reason, this study focused on the development of polymer materials using calcium carbide residue (CCR), phosphogypsum (PG), ground granulated blast furnace slag (GGBS), superabsorbent polymers (SAP), and arsenic-containing tailings. The self-healing behavior of the alkali-activated slag system after cracking was analyzed, specifically targeting arsenic immobilization. Additionally, the crack width, crack area, and mechanical properties of the polymers was tested to evaluate their self-healing potential. The results revealed that the main healing products consisted of calcium silicate hydrate (C–S–H), ettringite (AFt), and calcite. These products covered the inner surfaces of the cracks, resulting in a reduction by 78.1–95.9% in arsenic leaching concentration after healing for 56 days. The incorporation of PG led to increased generation of AFt and facilitated secondary hydration reactions for self-healing. This enhanced crack closure and promoted strength recovery while simultaneously improving the pore structure of the crack walls. As a result, the leaching of arsenic from the cracked polymer was effectively inhibited. The mechanism of arsenic immobilization by polymers during self-healing process was as follows: i) adsorption of AsO43− by C–S–H; ii) ion exchange of AFt with AsO43−; iii) physical encapsulation of mixed products including C–S–H, AFt, calcite, and swollen SAP.

Improving geopolymer concrete performance with hazardous solid waste phosphogypsum

Recycling the hazardous solid waste phosphogypsum (PG) in concrete is a popular trend. However, the inherent properties of PG used alone in concrete tend to limit the strength of the concrete. In this study, PG was used as a precursor for the production of phosphogypsum-based geopolymer concrete (PGC), which effectively improved the performance of geopolymer concrete by utilising the synergistic effect of different solid wastes in geopolymer concrete. The effects of PG content (0, 10, and 20 %) and alkali modulus (0.8, 0.95, 1.1, and 1.25) on the axial compressive properties and resistance to chloride penetration of PGC were investigated. The results show that 10 % PG improved the properties of geopolymer concrete the most, with a maximum increase in axial compressive strength of 17 %. Due to the addition of SO42−, the PG content also affected the optimal alkaline modulus of PGCs. Chemical composition and microstructural analyses revealed that the promotion of the geopolymer reaction by PG is crucial for PGC performance enhancement. In addition, leaching tests and carbon emission assessment, confirmed that PGCs are an environmentally friendly option. Based on the analysis of axial compression behaviour, resistance to chloride ion permeability and environmental impact, PGC with 10 % PG content and 1.1 alkali modulus is an environmentally friendly alternative to conventional concrete. This study shows that hazardous solid waste PG can improve the performance of geopolymer concrete and create a new type of solid waste concrete.

Effect of phosphorus impurities on sulfoaluminate cement-modified gypsum-based self-leveling mortar and improvement method

The preparation of gypsum-based self-leveling mortar (GSLM) from solid waste-derived sulfoaluminate cement-modified hemihydrate gypsum is an effective path to enhance its mechanical properties. However, the phosphorus impurities in phosphogypsum (PG) still pose a challenge to the hemihydrate gypsum & sulfoaluminate cement composite system. To evaluate the impact of phosphorus impurities and develop targeted removal strategies, this study first assessed the effects of different types of phosphorus impurities on the mechanical properties and hydration characteristics of the GSLM. Then, a method of pretreating PG with carbide slag (CS) was proposed, and the removal efficiency and mechanism of phosphorus were examined by ICP-OES and XPS testing. The results indicated that the soluble phosphorus impurities severely inhibit the hydration of both CŜH0.5 and C4A3Ŝ, leading to a most notable reduced mechanical performance. The sequence of inhibitory action was as follows: H3PO4 >> Ca(H2PO4)2·H2O > CaHPO4·2H2O > Ca3(PO4)2. CS could effectively immobilize the soluble phosphorus in PG. The pretreatment by CS notably enhanced the hydration degree of the GSLM by converting soluble phosphorus into Ca3(PO4)2 beforehand. In addition, the alkaline environment provided by CS promoted the ettringite formation. When the CS content approaches 3%, a PG-based GSLM with good performances meeting the G20 standard requirements of GSLM can be prepared.

Properties of Cemented Filling Materials Prepared from Phosphogypsum-Steel Slag-Blast-Furnace Slag and Its Environmental Effect.

Phosphogypsum (PG) occupies a large amount of land due to its large annual production and low utilization rate, and at the same time causes serious environmental problems due to toxic impurities. PG is used for mine backfill, and industrial solid waste is a curing agent for PG, which can save the filling cost and reduce environmental pollution. In this paper, PG was used as a raw material, combined with steel slag (SS) and ground granulated blast-furnace slag (GGBS) under the action of an alkali-activated agent (NaOH) to prepare all-solid waste phosphogypsum-based backfill material (PBM). The effect of the GGBS to SS ratio on the compressive strength and toxic leaching of PBM was investigated. The chemical composition of the raw materials was obtained by XRF analysis, and the mineral composition and morphology of PBM and its stabilization/curing mechanism against heavy metals were analyzed using XRD and SEM-EDS. The results showed that the best performance of PBM was achieved when the contents of PG, GGBS, and SS were 80%, 13%, and 7%, the liquid-to-solid ratio was 0.4, and the mass concentration of NaOH was 4%, with a strength of 2.8 MPa at 28 days. The leaching concentration of fluorine at 7 days met the standard of groundwater class IV (2 mg/L), and the leaching concentration of phosphorus was detected to be less than 0.001 mg/L, and the leaching concentration of heavy metals met the environmental standard at 14 d. The hydration concentration in PBM met the environmental standard. The hydration products in PBM are mainly ettringite and C-(A)-S-H gel, which can effectively stabilize the heavy metals in PG through chemical precipitation, physical adsorption, and encapsulation.

Rapid Synthesis of α-Hemihydrate Phosphogypsum in MgCl2 Solution by Microwave Heating.

The preparation of α-hemihydrate phosphogypsum (α-HPG) is hindered by a lengthy reaction time due to using traditional electric heating. Microwave heating was proposed for the rapid synthesis of α-HPG in MgCl2 solution. The effects of MgCl2 concentration on the heating rate of solution and conversion rate of α-HPG were investigated; the morphology of α-HPG was regulated by citric acid, and the adsorption mechanism of citric acid on the α-HPG surface was clarified. The results show that α-HPG was successfully synthesized in a MgCl2 solution by microwave heating. Increasing the MgCl2 concentration can improve both the dielectric constant and conductivity of the solution, which is advantageous for the solution to absorb more microwave energy. Consequently, the conversion rate of phosphogypsum (PG) into α-HPG was improved at higher MgCl2 concentrations. In 2.62 mol/L MgCl2 solution, the heating time is shortened to 7 min from room temperature to 95 °C, and PG was converted to α-HPG within 60 min. The morphology of α-HPG could be controlled by chemical adsorption of citric acid onto its terminal surface. In a solution containing 2.62 mol/L MgCl2 and 1.63 × 10-2 mol/L citric acid, PG was completely transformed into α-HPG at 90 min by microwave heating, whereas the reaction time was delayed to 180 min using electric heating. Accordingly, microwave heating is a highly efficient and energy-saving method for the preparation of α-HPG.

Blending as a strategy for using phosphogypsum in granular road base: physical performance and implications

The use of phosphogypsum (PG) as a construction material continues to gain considerable attention worldwide. In the United States(US), the specific use of PG as a road base material has received renewed interest on a federal level. Previous work investigated the use of road base, either PG alone or stabilized with cementitious materials. This study investigated the physical characteristics of compacted PG sources and novel blends with traditional base aggregates compared to industry and state requirements. Specimens were created by blending up to 50% PG from five gypstacks in Florida, US with limerock (LR) and recycled concrete aggregate (RCA). The LBR for PG-LR and PG-RCA blends surpassed bearing strength (LBR) requirements and ranged from 102 to 130% and 155–210% at 50:50 ratios, and increased by 2–60% with decreasing PG content from 50 to 30%. The data suggest success of PG-amended granular base reported here relies largely on the strength of the original aggregate.

Study on recycling and utilization of phosphogypsum and lithium slag in vertical barrier materials

Traditional vertical barrier materials typically have a large carbon footprint and high cost. Phosphogypsum (PG) and lithium slag (LS), possess latent cementitious activity. This study investigated the potential use of PG and LS in vertical barrier materials. The basic characteristics of the system were studied for LS:PG (mass ratio) ranging from 1:1–1:9, with a bentonite content of 10 % and a water-to-solid ratio of 0.5. The PG-LS matrix was microscopically characterized using XRD, TG, FT-IR, and SEM analysis. Additionally, an economic and sustainability assessment was performed. The results indicated that when LS:PG was in the range of 1:3–1:1, the flowability and setting time ranged from 16.15 to 21.00 cm and 51–87 hours, respectively. The bleeding rate within 12 hours was consistently below 5 %. Furthermore, the compressive strength and permeability coefficient at 28 days were within the ranges of 1.81–7.99 MPa and 2.91×10−8 to 7.98×10−9 cm/s, respectively. Excessive addition of PG was found to be unfavorable for the formation of C-(A)-S-H gel, resulting in an increase in free alkali content in the system and promoting the carbonation-induced formation of Calcite, thereby reducing the compressive strength and increasing the permeability. Compared to traditional barrier materials, the PG-LS-based barrier material exhibited a 61 % reduction in CO2 emissions and a cost reduction of 75–82 %. Additionally, the leaching concentrations of Zn, Cr, Ni, Cu, Pb, Cd, and As were below the standard limits. Therefore, the utilization of PG and LS in vertical barrier systems demonstrated excellent performance, economic feasibility, and environmental benefits, indicating significant potential for widespread application.

Investigation of mechanical, microscopic, and leaching properties of coal-based solid waste geopolymer mortar activated by soda residue and phosphogypsum

In this study, coal-based solid waste geopolymer mortar (SWCB) was prepared by using granulated ground blast-furnace slag (GGBS) and coal gasification coarse slag (CGCS) as precursors, and soda residue (SR) and phosphogypsum (PG) as activators, with gangue sand (GS) utilized as an inert filler. The corresponding compressive strength, fluidity, ion leaching, and microstructure of the developed SWCB were systematically investigated under varying solid contents, binder-to-sand ratios, and activator ratios. The findings suggest that the incorporation of activators promoted the dissolution of the silicon-aluminum phase in GGBS and CGCS into Al(OH)4−, [SiO(OH)3]−, and [SiO2(OH)2]2−, which could subsequently react with the Ca2+ and SO42− released by PG, forming AFt and C-(A)-S-H, thereby playing a crucial role in enhancing matrix strength. AFt was the predominant hydration product in the early reaction stage. The morphology of the AFt phase evolved from needle-like or filamentous to fine and coarse rods as hydration progressed. Initially, the formation of C-(A)-S-H gel increased with rising activator content before decreasing. The optimal synergy between AFt and C-(A)-S-H was observed at an activator content of 30 %. However, the growth of gypsum crystals was hindered when the activator content surpassed 30 %, resulting in a plate-like or columnar morphology. C-(A)-S-H gel exhibited remarkable adsorption capability towards P atoms attributed to intermolecular Van der Waal's forces, enabling simultaneous physical encapsulation of P atoms, while Cl element immobilization was primarily attributed to the contribution of SiOH sites to Cl adsorption.

Strength behaviour of stacked phosphogypsum incorporating dissolution–recrystallisation equilibrium

Large phosphogypsum (PG) stacks risk dam failure, with an insufficient consensus on the shear strength parameters for stability analysis. To this end, a combination of scanning electron microscopy (SEM) and triaxial tests was undertaken to investigate the underlying mechanism between crystal structure and shear strength of in situ and remoulded PG samples. The shear strength and deformation of PG were significantly affected by dissolution and recrystallisation. Dissolution weakened the cementation between particles, leading to a stabilisation of approximate 11 kPa under different confining pressures in the initial shear stage. The hardening phenomenon was related to the formation of cluster crystals under saturated conditions. An increase from 1.57 to 1.73 in the critical state stress ratio on remoulded samples occurred as the K0 consolidation time increased from 4 to 28 days. The compressive deformation of PG is accompanied by chemical consolidation, which is mainly impacted by the consolidation conditions (saturation) rather than the consolidation time. In the engineering design of the PG stacks, φ’ could be taken to a higher value at saturation and c’ could be higher when the dry density is higher than 1.2.

The Bacau (Romania) phosphogypsum stacks as a source of radioactive threat: a case study

ABSTRACT For a detailed characterization of the 5.7 106 mt phosphogypsum (PG) stack in the vicinity of Bacau city, Romania, the air dose rate (ADR) was measured in 72 points covering the stack surface, while 10 samples of stack material were collected for future analysis. Radiometric determinations showed for the ADR values varying between 364 ± 53 and 489 ± 8 nSv/h, with some extreme values of 2775 ± 734 nSv/h, significantly exceeding 90 nSv/h, the average value reported for the Romanian territory. High-resolution gamma-ray spectroscopy (HRGS), performed on 10 samples collected from the entire PG stack evidenced only the presence of 226Ra as the major radioactive element with a specific activity varied between 820 ± 150 and 5278 ± 264 Bq/kg for hot spots. Further analysis performed on a similar number of samples by X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX), evidenced, beside gypsum as the main component, traces of brushite (CaHPO4·2H2O) and ardealite (Ca2(PO3OH)(SO4)·4H2O), as well as the presence of small acicular celestine (SrSO4) agglomerates. XRF determinations of the mass fractions of major elements evidenced values such as SiO2 (2.31 ± 0.73 %), TiO2 (0.07 ± 0.01 %), Al2O3 (0.17 ± 0.04 %), Fe2O3 (0.87 ± 0.18 %), MnO (0.01 ± 0.01 %), MgO (0.17 ± 0.02 %), CaO (32.5 ± 0.82 %), Na2O (0.04 ± 0.04 %), K2O (0.05 ± 0.01 %), P2O5 (2.12 ± 0.51 %), LOI (20.2 ± 0.3 %), i.e. closer to literature reported data for PG of different provenience while the data concerning the distribution of 20 trace elements, including incompatible Sc, La, Ce, and Th were relatively closer to the upper continental crust (UCC).

Mechanical properties and environmental implications of excess-sulfate cement concrete with phosphogypsum-based cold-bonded fine aggregates

Recycling waste phosphogypsum (PG) into phosphogypsum-based cold-bonded aggregates (PCBAs) for eco-concrete production has received extensive attention due to its sustainability for construction industry development. Currently, PCBAs mainly replace the coarse aggregates in concrete while the fine aggregates remain natural sand (NS). This study substitutes NS with phosphogypsum-based cold-bonded fine aggregates (PCBFAs) for the preparation of excess-sulfate cement concrete (ESCC), and the compressive strength, tensile splitting strength, flexural strength, and stress-strain curve were studied by considering replacement rations (0%, 25%, 50%, 75%, 100%). The results indicate that when the replacement ratio is 100%, the compressive strength, splitting tensile strength, and flexural strength of ESCC were reduced by 17.2%, 26.9%, and 29.2% respectively. The addition of PCBFAs will make the ascending segment of the stress-strain curve flatter and make the descending segment steeper. Based on the tested results, the prediction formulas of the mechanical properties and the uniaxial compressive stress-strain model of the concrete under different replacement ratios are proposed. Microstructure analysis indicates that the PCBFAs gradually integrate with the cement matrix, reducing the defects in the interface transition zone. Furthermore, ESCC effectively immobilizes impurities contained in PG. The research findings demonstrate the feasibility of using PCBFAs as a substitute for NS.