Gypsum Crystals Research Articles

Effect of organic acid additives on sulfite oxidation in a calcium-based slurry

The forced oxidation of sulfite ions was conducted to obtain high-quality gypsum in the flue gas desulfurization system. The effects of formic, lactic, acrylic, acetic, propionic, and hexanoic acids added to calcium-based slurry on sulfite oxidation were investigated in this study. Sulfite oxidation was inhibited when the pKa value of organic acids was small or the carbon chain was long. In the identical slurry pH 6, the SO4 2- fractions of slurries with formic and lactic acids, which have the smallest pKa values, were lower than half that of the additive-free slurry. The addition of hexanoic acid with the longest carbon chain showed a similar SO4 2- fraction to that of additive-free slurry. These results indicate that the inhibitory effect on sulfite oxidation is more expressed by the acidity of organic acids. The SO4 2- fraction in the slurry affects the growth and quality of gypsum crystals. The slurry with acetic acid presented the highest SO4 2- fraction and resulted in the formation of high-quality gypsum crystals. The findings of this study can contribute to the selection of organic acid additives with high desulfurization efficiency and the production of high-quality gypsum.

Gypsum Crystals Formed by the Anhydrite–Gypsum Transformation at Low Temperatures: Implications for the Formation of the Geode of Pulpí

Determining the mechanisms of the formation of giant crystals is a challenging subject. Gypsum, calcium sulfate dihydrate (CaSO4·2H2O), is known to form crystals larger than one meter in several locations worldwide. These selenite crystals grow at different temperatures, either in sedimentary or hydrothermal systems. The famous selenite crystals of the geode of Pulpí (Almería, Spain) are known to have grown at a temperature T = 20 ± 5 °C and have been proposed to form in a subaqueous environment by a self-feeding mechanism triggered by anhydrite dissolution and the ripening of microcrystalline gypsum, enhanced by oscillations in temperature. This paper reports the monitored crystallization of gypsum crystals, from anhydrite powder dissolution, inside airtight evaporation-free reactors under oscillating low temperatures (15 °C < T < 25 °C). These crystals are clearly smaller than the ones in the Pulpí mine but exhibit similar habits (i.e., single blocky crystals and twins following the 100 twinning law). The growth rate of gypsum single crystals has been measured to be between 3.8 and 35.3 µm/day. Noteworthy, we document the occurrence of the 100 contact twinning law of gypsum, which is the most widespread twinning law in natural environments but never univocally reported in laboratory experiments. The selection of the 100 contact twinning law has been correlated to the low supersaturation values obtained in the experiment, where the concentration in these long-duration experiments can be safely assumed to be the equilibrium concentration, i.e., 0.3 (at 25 °C) ≤ SI ≤ 0.4 (at 15 °C). We discuss the relevance of our experiment for forming the gypsum crystals of Pulpí in the framework of the geological history of Pulpí mineralization. These laboratory model experiments contribute to a deeper understanding of mineral nucleation and growth processes in natural environments.

Insights into mechanism of electric field regulating calcium sulfate crystal structure and crystallization: A study by DFT calculation and experiment

Exploring the mechanism of external electric field (EF) regulating the structure and crystallization of calcium sulfate crystal is of great significance for solving the problem of inorganic salt scale in many industrial processes such as desalination and heat exchange. Here, the dispersion-corrected Density Functional Theory (DFT-D) calculations and a series of experiments were carried out to investigate the influence of EF on crystal structure, mechanical property and crystallization of gypsum crystal. The results show that under the influence of EF, the conventional cell of the gypsum crystal shortens in the direction of lattice parameter b. The water layer and the CaO8 octahedron are deformed. The stiffness and hardness of gypsum crystals decreases. The attachment energy and surface energy of (020) plane increase significantly. Both DFT-D calculations and experiment characterization results indicate that EF promotes a regular growth of gypsum crystal mainly along in the direction of (020) plane. Our study results demonstrate the possibility of electrical regulation of calcium sulfate crystal growth to solve the industrial calcium sulfate scale problem.

Integrated above and below-ground responses of the gypsum specialist Helianthemum squamatum (L.). to drought

Gypsum endemics (i.e. gypsophiles) have adapted to live in gypsum-rich soils where nutrient unbalance and drought can be extreme. Despite their relevance as plants adapted to extreme conditions, a complete analysis of the physiological responses of gypsophiles to drought is still lacking. Helianthemum squamatum (L.) Dum. Cours. is a conspicuous Iberian gypsophile that has been reported to use gypsum crystallization water during the driest period, but the mechanisms behind this process are unknown. To characterize gypsophile responses to drought and unravel the mechanisms underlying gypsum crystalline water use, H. squamatum plants were grown in pots with natural gypsum soil and gypsum soil with deuterium-labelled crystalline water. After three years, a drought experiment was carried out and whole-plant responses were investigated. Unexpectedly, the labelling treatment affected soil physicochemical characteristics and reduced microbial biomass and organic matter content, decreasing plant aerial biomass. H. squamatum plants did not use gypsum crystallization water during simulated drought neither in the labelled soil, nor in the natural one. Drought reduced plant transpiration, stomatal conductance, water use, photosynthetic rate and the foliar concentration of most elements except P and N, which were higher in the drought stressed plants. We detected increased root exudation of choline, an osmoprotector, by drought stressed plants. The drought treatment also affected the structure of microbial communities but did not reduce the relative abundance of functional microbial groups, highly adapted to the natural drought pulses. Our results highlight an integrated water-saving strategy of H. squamatum plants in the short-term, where responses at the leaf level are combined with belowground processes, like altered root exudation. Our findings also underline the remarkable resistance to drought of microbial communities present in gypsum soils.

Insights into iron-accelerated gypsum scaling mitigation strategies in nanofiltration membrane process

In this study, the impact of ferric ions on gypsum crystallization and NF membrane performance was investigated to mitigate iron-accelerated gypsum scaling. Gypsum scaling in the presence of ferric ions led to a significant decline in the water flux, but a relatively higher reversible fouling ratio was observed compared to that under individual gypsum and iron-oxide scaling. Ferric ions influenced the crystallization of gypsum on NF membranes by serving as additional nucleation sites, leading to accelerated gypsum scaling. Various mitigation methods (e.g., feed spacers and cleaning processes) were evaluated to effectively control iron-accelerated gypsum scaling. The introduction of spacers exhibited a high mitigation efficiency by enhancing the mass transfer of scalant ions due to changes in the water channel hydrodynamics, but the overall efficiency decreased over time. To enhance the mitigation efficiency, different cleaning methods (e.g., CO2 oversaturated solution, HCl, and NaCl flushing) were conducted with feed spacers. A CO2 saturated solution demonstrated high cleaning efficiency owing to the high turbulence induced by CO2 bubbles, whereas HCl flushing showed negligible efficiency. NaCl flushing exhibited the highest cleaning efficiency through mono/divalent ion-exchange effects on iron-gypsum scaling. This study provides comprehensive insights into iron-accelerated gypsum scaling mitigation strategies for nanofiltration membranes, highlighting the impact of coexisting ferric compounds on gypsum scaling.

Exploring the feasibility of using fluidized bed homogeneous crystallization technology to recover sulfate ions from briny water as calcium sulfate pellets

The increasing salinity of water sources poses significant operational and environmental challenges, highlighting the importance of developing efficient methods for recovering [SO42−] ions from briny wastewater. In this study, the potential of fluidized bed homogeneous crystallization (FBHC) technology for recovering [SO42−] ions from briny water is explored. FBHC is an innovative technique that enhances scalability and separation efficiency by promoting the growth of crystalline particles in a fluidized state. To achieve maximum [SO42−] recovery and maintain stable fluidized bed conditions, a pH of 8 is recommended. A removal ratio (RR) of 71 % was recorded at a [SO42−] concentration of 0.14 M. Various [SO42−] concentrations exhibited a molar ratio (MR) of [Ca2+]/[SO42−] = 1.5. Smaller particles, which are more soluble and exhibit less nucleation, are typically formed under conditions with lower pH and [Ca2+] levels. Intermediate-sized particles are favored at neutral to slightly alkaline pH levels and near-stoichiometric [Ca2+]/[SO42−] MR. Greater calcium content and higher pH values promote the development and agglomeration of larger particles, with 50 % of the particles exceeding an average size of 0.38 mm. In the XPS study of CaSO4 crystals (gypsum), the Ca 2p1/2 and Ca 2p3/2 peaks are observed at binding energies of 351.5 eV and 347.7 eV, respectively. Gypsum is identifiable due to the presence of clear and distinct peaks in XRD patterns. SEM analysis reveals the coexistence of amorphous patches and tiny crystalline domains on the surface. These gypsum crystals resemble stacked needle-like structures accompanied by small, powdery crystals. In an acidic environment, the primary phase is CaSO4·2H2O, which generates well-formed, elongated, or tabular crystals that are characteristic of gypsum. The FBHC process, characterized by a high crystallization ratio (CR = 68 %) and the recovery of crystal pellets (>0.3 mm), significantly reduces sludge production compared to traditional chemical precipitation methods. Its cost-effectiveness and recoverability make FBHC a promising technology for the recovery of sulfate ions from briny wastewater.

Prominent differences in the adsorption and substitution characteristics of As(III) and As(V) on gypsum during wet flue gas desulfurization

Arsenic-bearing gypsum (ABG) is a hazardous by-product of wet flue gas desulfurization (FGD) and must be strictly controlled. This study investigated the migration of arsenic species during wet FGD and proposed strategies to reduce the toxicity of FGD gypsum in operation. During gypsum crystallization, the adsorption and substitution characteristics of As(III) and As(V) exhibit significant differences. The amount of As(III) captured by CaSO4·2H2O is greater than that of As(V). The As(III) adsorbed on the growth surface of CaSO4·2H2O can be incorporated into the gypsum by substituting for SO42−, while As(V) tends to remain adsorbed on the surface. Moreover, the arsenic adsorption capacity of the incompletely oxidized product CaSO3·0.5H2O was almost three times that of CaSO4·2H2O, especially for As(III). Based on the differences in the migration characteristics of As(III) and As(V), a strategy was proposed to reduce the arsenic content and biotoxicity in gypsum during operation by enhancing the oxidation of sulfite and highly toxic arsenite, thus ensuring the safe utilization of FGD gypsum.

Bio-Inspired Fluorescent Calcium Sulfate for the Conservation of Gypsum Plasterwork.

In this work, the potential of bio-inspired strategies for the synthesis of calcium sulfate (CaSO4·nH2O) materials for heritage conservation is explored. For this, a nonclassical multi-step crystallization mechanism to understand the effect of calcein- a fluorescent chelating agent with a high affinity for divalent cations- on the nucleation and growth of calcium sulfate phases is proposed. Moving from the nano- to the macro-scale, this strategy sets the basis for the design and production of fluorescent nano-bassanite (NB-C; CaSO4·0.5H2O), with application as a fully compatible consolidant for the conservation of historic plasterwork. Once applied to gypsum (CaSO4·2H2O) plaster specimens, cementation upon hydration of nano-bassanite results in a significant increase in mechanical strength, while intracrystalline occlusion of calcein in newly-formed gypsum cement improves its weathering resistance. Furthermore, under UV irradiation, the luminescence produced by calcein molecules occluded in gypsum crystals formed upon nano-bassanite hydration allows the easy identification of the newly deposited consolidant within the treated gypsum plaster without altering the substrate's appearance.

Experimental investigation into the effect of porosity on the strains developing during anhydrite to gypsum transformation

The swelling of anhydritic claystones often leads to severe tunnel damage. Even though this phenomenon has gained significant scientific interest, particularly in the last decades, there are still open questions which introduce uncertainties in tunnel design. One question concerns the strains developing during the anhydrite to gypsum transformation (AGT). These depend, among other factors, on whether the gypsum crystals grow within the available pore space or whether they tend to push the particles apart, leading to an expansion of the matrix and, in turn, larger macroscopic strains. The experimental investigations of this paper aim to assess the influence of the initial porosity on the strains developing during AGT. Specimens consisting of highly compacted anhydrite and kaolin powders are created with varying initial porosities between 0.22 and 0.35. It is concluded that, ceteris paribus, the strains developing during AGT decreases with increasing initial porosity. The results also indicate that in the case of high initial porosity the gypsum crystals grow in the available pore space, thus decreasing the porosity, while in the case of low initial porosity, gypsum growth leads to an increase of the pore space. The results are applicable to porous media where crystallisation may occur within the pores.

Early length changes and microscopic phase analysis of phosphorus hemihydrate gypsum fireproof mortar

Abstract Coating the surface of steel structures with phosphohemihydrate gypsum fireproof mortar can improve the fire resistance of steel structures. The hydration curve of phosphorus hemihydrate gypsum (PHG) showed a monotonic increasing trend, and the hydration time was about 1 day. In order to study the hydration process of phosphorus hemihydrate gypsum fireproof mortar, its length change in the early hardening stage was experimentally investigated. The results showed that compared with the hydration time of phosphohemihydrate gypsum, the duration of early length changes in phosphohemihydrate gypsum fireproof mortar was significantly prolonged to about 4 days, and four intervals were clearly observed during this process. Hydroxypropyl methyl cellulose (HPMC) as a water retaining agent, hydroxypropyl starch ether (HPS) as a thixotropic agent, and dispersed latex powder (LP) can all make the four intervals of length changes of fireproof mortar more significant. After adding HPMC, HPS, and LP, it can promote the formation of dihydrate gypsum mainly in fireproof mortar, but dihydrate gypsum crystals were more likely to grow along the [021] direction. The addition of additives can also effectively improve the pore structure of phosphogypsum fireproof mortar.

Microbial colonization of gypsum: from the fossil record to the present day.

Microorganisms inhabiting gypsum have been observed in environments that differ greatly in water availability. Gypsum colonized by microorganisms, including cyanobacteria, eukaryotic algae, and diverse heterotrophic communities, occurs in hot, arid or even hyperarid environments, in cold environments of the Antarctic and Arctic zones, and in saline and hypersaline lakes and ponds where gypsum precipitates. Fossilized microbial remnants preserved in gypsum were also reported. Gypsum protects the endolithic microbial communities against excessive insolation and ultraviolet radiation, while allowing photosynthetically active radiation to penetrate through the mineral substrate. We here review the worldwide occurrences of microbially colonized gypsum and the specific properties of gypsum related to its function as a substrate and habitat for microbial life on Earth and possibly beyond. Methods for detecting and characterizing endolithic communities and their biomarkers in gypsum are discussed, including microscopic, spectroscopic, chemical, and molecular biological techniques. The modes of adaptation of different microorganisms to life within gypsum crystals under different environmental conditions are described. Finally, we discuss gypsum deposits as possible targets for the search for microbial life or its remnants beyond Earth, especially on Mars, where sulfate-rich deposits occur, and propose strategies to detect them during space exploration missions.

Acid rain resistance of geopolymer recycled pervious concrete featuring top-bottom interconnected pores under freeze-thaw cycles and fatigue loads

While pervious concrete with top-bottom interconnected pores enhances the performance of pervious concrete pavements, its resistance to acid rain under combined freeze-thaw cycles and fatigue loads remains unexplored. To fill this gap, this study investigated the influence of three different qualities of recycled coarse aggregates on the acid resistance, acid neutralization, and permeability of geopolymer recycled pervious concrete (GRPC) (Low-quality, middle-quality and high-quality geopolymer recycled pervious concrete (L-GRPC, M-GRPC and H-GRPC)) under the couple effects of simulated acid rain wet-dry cycle spraying, fatigue loads and freeze-thaw cycles. After experiencing the combined action of fatigue load and freeze-thaw cycle, only the compressive strength of L-GRPC increased slightly by 9.1 % after acid rain spraying, but it had a negative impact on the flexural strength. With the increase of coupling environmental factors, the neutralization depth of GRPC was less than 4 mm, and the permeability coefficient was higher than 0.5 mm/s. Furthermore, more gypsum crystals were observed in the SEM images, and new peaks corresponded to the gypsum phase (CaSO4) appeared at 2θ = 29.4° and 2θ = 21.6° in the XRD spectra. The research results provide a broader perspective for the application of GRPC in concrete pavement engineering in acid rain area.

Selective Chiral Interactions between Hydrophilic/Hydrophobic Amino Acids and Growing Gypsum Crystals.

In nature, selective interactions between chiral amino acids and crystals are important for the formation of chiral biominerals and provide insight into the mysterious origin of homochirality. Here, we show that chiral amino acids with different hydrophilicities/hydrophobicities exhibit different chiral selectivity preferences in the dynamically growing gypsum [001] steps. Hydrophilic amino acids show a chiral selectivity preference for their d-isomers, whereas hydrophobic amino acids prefer their l-isomers. These differences in chiral recognition can be attributed to the different stereochemical matching between the hydrophilic and hydrophobic amino acids on the [001] steps of growing gypsum. These different chiral selectivities resulting from the amino acid hydrophilicity/hydrophobicity are confirmed by the experimental crystallization investigations from nano regulation on dynamic steps, to microscopic modification of gypsum morphology, and to macroscopic precipitation. Furthermore, as the hydrophilicity of amino acids increases, the disparity in chiral selection rises; conversely, the increase in the hydrophobicity of amino acids results in a decline in chiral selection. These insights improve our understanding of the interaction mechanism between amino acids and crystals and provide insights into the formation process of chiral biominerals and the origin of homochirality in nature.

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.

Effect of modified amino acids on hydration behavior of β-hemihydrate phosphogypsum

In practice, it is often necessary to add retarders to β-hemihydrate phosphogypsum (β-HPG) to prolong its setting time. In this paper, the effects of modified amino acid (XK) on physical and mechanical properties, hydration characteristics, and microstructure of β-HPG were investigated using various methods including fluidity, setting time, strength, hydration heat, X-ray diffraction (XRD), scanning electron microscope (SEM), mercury intrusion porosimetry (MIP), and X-ray photoelectron spectroscopy (XPS). The findings indicate that the addition of XK enhances the fluidity of β-HPG, and as the content of XK increases, there is an occurrence of prolonged setting time but a concurrent decrease in both the compressive and flexural strength. In addition, the incorporation of XK hampers the hydration process of β-HPG, resulting in a reduction in the hydration exothermic rate. Microscopic analysis reveals that while XK does not alter the final hydration products, it does affect the growth pattern of dihydrate gypsum crystals because of surface adsorption, and specifically, the crystals become short and columnar, with a looser crystal structure. Furthermore, XK leads to larger pores and deterioration of the pore structure of β-HPG.

Influence of recycled rubber and glass aggregates on magnesium sulfate resistance of geopolymers: Physio-mechanical properties and mechanisms

Recycled rubber aggregates (RRA) and recycled glass aggregates (RGA) are sustainable alternatives to natural aggregates. However, the influences of RRA and RGA on the magnesium sulfate resistance of geopolymers remain unclear. Previous studies have focused on the influences of binder type and composition on magnesium sulfate resistance but have neglected the role of aggregates. This study bridges this gap and demonstrates the influences of RRA and RGA on the magnesium sulfate resistance of geopolymers. This study investigated the physical properties, mechanical properties and phases of FA and GBFS-based geopolymers containing 0:10 – 10:0 RRA and RGA after being exposed to 5 % MgSO4 solution for 90 days. The results showed that compared to 100 % RGA, the 90-day length and mass of 100 % RRA increased by 50.48 and 33.22 times, respectively, and the 90-day compressive, flexural and splitting tensile strengths of 100 % RRA decreased by 65 %, 43.28 % and 56.75 %, respectively. However, the changes in length and mass of 100 % RGA were very small, and the mechanical properties even slightly increased rather than decreased. RRA + RGA showed an interval performance between 100 % RRA and 100 % RGA. The different magnesium sulfate resistance of RRA and RGA were attributed to the following mechanisms. RRA caused much higher drying shrinkage of geopolymers than RGA because the soft nature of RRA increased uncoordinated deformation, leading to micro-cracks in the matrix before magnesium sulfate exposure. The pre-existing micro-cracks served as physical transportation paths for external ions to diffuse into the interior of specimens, leading to expansive gypsum crystals formed in these micro-cracks. The accumulation of expansive gypsum crystals enlarged these micro-cracks and facilitated their propagation, leading to the expansion, cracking and degraded mechanical properties. By contrast, RGA reduced drying shrinkage due to its hard nature, which limited the availability of micro-cracks as physical transportation paths for external ions and therefore reduced the formation of expansive gypsum crystals and improved the magnesium sulfate resistance of geopolymers.

Durability performance of BFRP-confined coal gangue concrete exposed to the sulfate-rich mine environment

The combination of basalt fiber reinforced polymer (BFRP) confinement and coal gangue concrete (CGC) is an economical and innovative approach for serving as a supporting structure in the underground mine. Nevertheless, the sulfate-rich mine water and mist exist in the mine environment, which may cause the performance deterioration of structures. Thus, the long-term behavior of BFRP-confined CGC in the sulfate-rich environment was systematically explored, based on the sulfate durability investigation of BFRP sheets and CGC. The tensile test of BFRP sheets and the compressive test of CGC were first carried out to evaluate their deterioration laws and mechanism in different sulfate environment. More importantly, axial compression test of FRP-confined concrete cylindrical specimens after sulfate attack were conducted, considering the effect of inner concrete, FRP layer, sulfate concentration and corrosion time on the stress-strain response and dilation properties. The results indicate that the tensile strength of BFRP sheet declined to a small extent with the extension of sulfate corrosion due to the degradation of epoxy resin and matrix-fiber interface. Additionally, the compressive strength of CGC exhibits a first increase and then decrease trend as the secondary hydration occurs and the sulfate ions react with the hydrated production to form expansive gypsum and ettringite crystals. The strength and ductility of CGC were improved by FRP confinement regardless of sulfate attack or not, whereas sulfate corrosion degraded the improvement ratio, especially the deformation capacity. Most importantly, the sulfate degradation coefficients were introduced into ultimate strength and strain models for sulfate-attacked BFRP-confined CGC, providing a basis for its design and life cycle assessment in the underground mine.

Revealing the mechanisms of soil gypsum formation in the Atacama Desert through triple oxygen and hydrogen isotopes of gypsum hydration water

The soils in the hyper-arid core of the Atacama Desert (Chile) harbor substantial quantities of soluble salts, including sulfates and nitrates. Gypsum (CaSO₄∙2H₂O) is a prevalent mineral in the Atacama region. It manifests as ∼10-cm-thick surface crusts exhibiting a spatial pattern resembling polygons, in mixtures with other minerals beneath the surface (<3 m deep) or growing in brine ponds of salars. We examine the triple oxygen and hydrogen stable isotopes (δ17O, δ18O and δD) of structurally-bonded gypsum hydration water (GHW) from Atacama gypsum deposits to investigate their formation mechanisms. We observed considerably positive Δ17O anomalies (Δ17O > 1 ‰) in GHW of samples containing substantial amounts of nitrate (generally soil horizons 0.5–2 m deep). This is interpreted as resulting from oxygen isotope exchange between soil nitrate (Δ17O > 15 ‰) and GHW (Δ17O ∼ 0 ‰), either occurring in-situ within the soils due to bacterial activity, or more likely, during cryogenic extraction of GHW at high temperature and low pH in the laboratory. However, when analyzing pedogenic gypsum in soils with negligible amounts of nitrates, the isotopic composition of its parent solution is preserved. These samples showed that gypsum formed from solutions that did not undergo significant evaporation. These waters are isotopically similar to contemporary unevaporated freshwater sources in the region. This indicates that gypsum formation likely occurred through the hydration of anhydrous or hemihydrate calcium sulfate, either in the atmosphere or in the soil. Subsequently, the gypsum was subject to recrystallization in the soil due to occasional rainfall events. This stands in contrast to subaqueous gypsum crystals growing in brine ponds of salars in the Atacama Desert, which reflect the isotopic composition of highly-evaporated parent fluids.

Deep removal of arsenic from arsenic-bearing gypsum using a seed-induced crystal control technique and its mechanisms

Arsenic-bearing gypsum (ABG), a byproduct from the lime/ferric sulfate process in the waste acid treatment of the heavy non-ferrous industry, represents a substantial hazardous arsenic (As) waste issue in China. Due to the potential arsenic leakage, the disposal of ABG poses an urgent challenge to the surrounding environment. The elevated As content and the low-value-added recycling products are the primary hurdles hindering ABG recycling. In this study, precise control over the gypsum crystal and As species in Na2SO4-H2SO4 solutions under mild conditions facilitated the deep removal of As (with a leaching efficiency of 92.45 % and a final product As concentration of 80.32 ppm) and simultaneous preparation of high-strength α-bassanite (with a compressive strength of 35.38 MPa) from the pre-treated ABG. This process occurred during the recrystallization from dihydrate gypsum (CaSO4·2H2O, DH) to α-bassanite (α-CaSO4·0.5H2O, α-HH), involving DH dissolution followed by α-HH crystallization. The encapsulated As was fully released during the dissolution process, crucial for the efficient elimination of As impurities. H2SO4 dissolved the released As, transforming it into the protonated species (H3AsO4) with a distinct structure and a less negative binding energy (ΔEB) to SO42-. This transformation prevented the chemical incorporation of the released As during the α-HH crystallization process. Additionally, the morphology and size of α-HH were controlled by seeding in the mixed solutions. The formation of large short-columnar α-HH crystals with a low specific surface area significantly reduced the surface adsorption of As, further enhancing As leaching efficiency. Importantly, the large short-columnar α-HH was identified as high-strength gypsum with high added value. This study provided innovative guidance for efficiently removing impurities from gypsum and pioneered a cost-effective approach for the clean and high-value utilization of industrial gypsum residues.

Enhancing the Performance of Hemihydrate Phosphogypsum by the Collaborative Effects of Calcium Hydroxide and Carbonation.

Normally, the acidic impurities in hemihydrate phosphogypsum (HPG) must be neutralized when HPG is utilized, and a little amount of calcium hydroxide (CH) is the best choice. In this paper, the effects of excessive CH (5 wt.%, 10 wt.%, 15 wt.% and 20 wt.% of HPG) for carbonation curing on the performance of hardened HPG paste were studied. According to the results of macro tests and microanalyses of XRD, TG, SEM-EDS, MIP and N2 physisorption, it could be verified that CaF2, Ca3(PO4)2 and a large amount of nanoscale CaCO3 crystals were produced as a result of neutralization and carbonation, and the compressive strength and the water resistance of carbonated HPG + CH paste were significantly improved due to the effects of nanoscale CaCO3 crystals on pore refinement and the coverage on the surfaces of gypsum crystals of the hardened paste. Therefore, this study suggests a feasible and green method for recycling HPG/PG, with the collaborative effects of neutralization, performance enhancement and reductions in CO2 emissions.