The study, published in Case Studies in Construction Materials, systematically evaluated how water reducers and retarders govern the behavior of phosphogypsum-based slurries across two fundamentally different extrusion printing strategies. In the rapid-setting route, mixing, stirring, and extrusion are compressed into a window of just three to five seconds using a rotary planetary printhead whose outer cylinder and inner blades counter-rotate to homogenize the paste on the fly. In the premixed route, by contrast, the slurry must survive premixing, storage, and delivery before it ever reaches a 10-millimeter nozzle. Each route places contradictory demands on the fresh material, and the researchers showed that admixture selection must be tailored accordingly rather than borrowed from conventional casting practice.

The raw phosphogypsum itself is dominated by calcium sulfate dihydrate, with rhombic plate-like crystals carrying residual silica and trace phosphate, fluoride, and metal impurities. These soluble phosphorus species and impurity ions interfere with crystal growth and setting behavior, which is why untreated phosphogypsum historically has been confined to low-value blocks and panels. The team tested three water reducers for the rapid route, a naphthalene-based system, a melamine-based system, and a polycarboxylate ether, each representing a distinct dispersion mechanism. The naphthalene and melamine chemistries disperse particles primarily through electrostatic repulsion, while the comb-shaped polycarboxylate molecule adds steric hindrance, physically preventing particles from approaching one another.

Setting measurements revealed striking differences. The polycarboxylate system produced an initial setting time of nine minutes and a final set at nineteen minutes, the broadest working interval among the water reducers, while the melamine system set fastest at four and nine minutes. But the researchers caution that setting time alone is a misleading proxy for printability. In extrusion printing, the material must flow continuously under shear inside the delivery system and nozzle, then recover structural resistance almost immediately after deposition to hold its filament geometry and bear the weight of subsequent layers. A slurry that is too fluid slumps; one that is too stiff clogs the pump.

Rheometry using a rotational rheometer with a stepwise shear protocol, ramping from rest to 140 reciprocal seconds and back down, quantified this trade-off. The polycarboxylate-modified slurry showed the highest torque, indicating strong structural resistance that favored filament integrity after extrusion, yet it still extruded continuously and produced a smooth filament surface with only isolated pores. The melamine system, though less resistant, achieved the smallest single-filament lateral path deviation, roughly 0.5 to 0.7 millimeters, compared with about 2.1 to 2.6 millimeters for the polycarboxylate. The naphthalene system fared worst, with porous surfaces and boundary fluctuations near two millimeters. The lesson is that extrusion continuity, surface quality, and path accuracy are related but non-equivalent measures of printability, and the highest rheological resistance does not automatically deliver the most accurate geometry.

The premixed route told an even clearer story about the limits of retardation. Three retarders were compared at identical dosage: a bio-based commercial retarder, citric acid, and sodium gluconate. Sodium gluconate extended setting the longest, to twenty-six minutes initial and thirty-six minutes final, yet its printed filaments fractured, spalled, and contracted, with path deviations of 3.8 to 5.3 millimeters. Citric acid flowed easily but left periodic surface ridges and lateral wandering of nearly four to 5.6 millimeters because the deposited material remained too deformable. Only the bio-based retarder, occupying an intermediate setting and rheological regime, balanced open time against post-deposition structural build-up, holding deviations to 0.6 to 0.8 millimeters. In premixed printing, more retardation is emphatically not better; the optimum is a middle ground.

Multilayer tests confirmed the fresh-state findings at structural scale. Seven-layer specimens printed with the polycarboxylate system formed a complete square column 114.9 millimeters tall, though layer heights drifted from 18.5 millimeters at the base to 13.8 at the top, revealing cumulative compression. The bio-retarder system produced a circular structure of 121.4 millimeters with remarkably uniform layers between 17.0 and 17.3 millimeters, indicating excellent layer-height control during vertical accumulation. Neither system collapsed or delaminated, demonstrating that properly regulated phosphogypsum can sustain genuine multilayer construction rather than single decorative filaments.

Hardened-state characterization explained the strength differences without invoking new chemistry. X-ray diffraction and thermogravimetric analysis showed that every admixture system retained gypsum dihydrate as the dominant phase with essentially unchanged dehydration behavior, so no admixture fundamentally altered the reaction pathway. Nitrogen adsorption and electron microscopy, however, revealed dramatic differences in pore structure and crystal packing. The polycarboxylate and bio-retarder specimens developed dense, interlocking networks of short columnar and acicular crystals with few large voids, reaching compressive strengths of 15.5 and 17.1 megapascals respectively. The naphthalene, citric acid, and sodium gluconate systems left looser, more heterogeneous skeletons with visible pores and discontinuous crystal contacts, consistent with their lower strengths.

The broader significance extends beyond one waste stream. The authors frame their results through two stage-dependent pathways: a fresh-state pathway in which admixtures govern setting and rheology to control extrusion and buildability, and a hardened-state pathway in which they shape crystal growth and pore development to control strength. Crucially, the microstructure is not treated as the direct cause of printing behavior, avoiding a common conflation in the additive manufacturing literature. For a field racing to print with alternative binders, earth, geopolymers, gypsum, and low-carbon concretes, the message is that admixture effectiveness is route-dependent, and material design must target a process-specific window rather than maximize flowability, setting delay, or strength in isolation.

For the billions of tons of phosphogypsum languishing in stockpiles, the demonstration that pretreated material can be printed into sound multilayer structures with compressive strength above 17 megapascals opens a credible path to high-value utilization compatible with automated construction. Gypsum’s inherent advantages, rapid setting, early strength, and good shape retention, make it unusually well suited to extrusion printing, provided the chemistry is tuned to the machine. Turning a disposal problem into printable walls would be a satisfying circular-economy outcome; the deeper contribution may be the design principle that makes it possible.

Subject of Research: Admixture regulation of phosphogypsum-based materials for extrusion-based 3D printing in construction

Article Title: Evolution of extrusion, buildability, and mechanical properties of phosphogypsum-based 3D printing materials under admixture regulation

Article References: Wang, Y., Li, G., Wen, Y., Tamon, U., Zhang, T., Chang, J., & Wang, L. (2026). Evolution of extrusion, buildability, and mechanical properties of phosphogypsum-based 3D printing materials under admixture regulation. Case Studies in Construction Materials, 25, Article e06540. https://doi.org/10.1016/j.cscm.2026.e06540

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06540

Keywords: phosphogypsum, 3D printing, additive manufacturing, water reducers, retarders, rheology, setting time, buildability, compressive strength, gypsum crystals, solid waste recycling, construction materials

The research, led by Lei Meng and Bin Lian of Nanjing Normal University, together with colleagues at Guizhou Institute of Technology and the Chinese Academy of Sciences, set out to test whether the mineral locked inside phosphogypsum could be biologically transformed into something genuinely useful for agriculture. The team’s strategy rested on a well-known geochemical pathway called the oxalate-carbonate pathway, in which living organisms convert calcium sulfate into calcium oxalate minerals and then, through bacterial degradation, into calcium carbonate. If the sequence could be demonstrated in the laboratory and then in living soil, phosphogypsum might find a second life as a soil amendment rather than rotting indefinitely in storage stacks.

The first step was a controlled chemistry experiment. The researchers exposed phosphogypsum to three different organic acids under identical conditions: oxalic acid, citric acid, and another common low-molecular-weight organic acid. The result was strikingly selective. Only oxalic acid managed to attack the gypsum crystal structure, dissolving the calcium sulfate dihydrate and reprecipitating it as whewellite, the monohydrate form of calcium oxalate. Citric acid, despite its reputation as an effective chelator, left the gypsum mineral phase essentially untouched. This selectivity matters because it identifies oxalate as the specific chemical lever needed to pull calcium out of phosphogypsum, and it explains why the team then turned to organisms that either produce or consume that exact compound.

Enter Aspergillus niger, a ubiquitous black mold famous among microbiologists for its industrial productivity and notorious among mycologists for its aggressive acid secretion. When the researchers cultivated A. niger in the presence of phosphogypsum, the fungus did precisely what the pure oxalic acid had done, but through its own metabolism. Over eight days of growth, the pH of the culture medium plummeted from 5.12 to 2.82, a dramatic acidification driven by the fungal excretion of oxalic acid and other metabolites. As the acidity climbed, the team tracked a progressive disappearance of the original gypsum mineral phase, accompanied by the appearance of two calcium oxalate minerals: whewellite, the monohydrate, and weddellite, the dihydrate. The fungus, in effect, was mining the waste pile with its own chemistry, converting an industrial byproduct into minerals that are otherwise associated with plant tissues, forest soils, and even human kidney stones.

The second microbial player then took the stage. Azospirillum brasilense strain OX-1 is an oxalotrophic bacterium, meaning it can metabolize oxalate as a source of energy and carbon. In the process, it raises the local pH and releases calcium that combines with carbonate to form calcite, the stable polymorph of calcium carbonate. When the researchers fed OX-1 the calcium oxalate generated either by pure oxalic acid treatment or by the A. niger transformation, the bacterium completed the circuit: the oxalate minerals were degraded and calcite formed in their place. This two-step handoff, from an acid-producing fungus to an oxalate-consuming bacterium, mirrors the natural oxalate-carbonate pathway that operates in soils around the world, where fungi and bacteria jointly convert organic oxalate into long-lived mineral carbon. Here, the same pathway was harnessed to transform a waste product into a benign, soil-friendly carbonate.

Demonstrating the chemistry in a flask is one thing; proving it matters in living soil is another. For that, the team ran a 45-day pot experiment using Suaeda salsa, a halophytic plant well adapted to saline and alkaline conditions, which are precisely the kinds of degraded soils where a gypsum-based amendment might be most valuable. The experimental design compared untreated controls against phosphogypsum alone and phosphogypsum combined with the OX-1 bacterium. The differences were unambiguous. Plants receiving the combined phosphogypsum and OX-1 treatment reached a mean height of 11.03 centimeters, with a standard deviation of 0.67 centimeters, and a mean stem diameter of 0.81 millimeters, plus or minus 0.07 millimeters. The untreated controls, by contrast, managed only 4.67 centimeters of height and 0.32 millimeters of stem diameter. In other words, the combined treatment more than doubled plant height relative to the control.

The benefits were not limited to what could be measured with a ruler. The combined application of phosphogypsum and OX-1 significantly altered several soil physicochemical properties, including soil pH and available phosphorus, the form of the nutrient that plants can actually absorb. This makes mechanistic sense. The gypsum component supplies calcium and sulfur, while the microbial transformation pathway releases and mobilizes residual phosphorus trapped in the waste, converting it into plant-available forms. At the same time, the bacterial degradation of oxalate and the precipitation of calcite can moderate soil acidity and contribute stable inorganic carbon to the soil. For saline-alkali soils, which cover vast areas in northern China and elsewhere and are notoriously difficult to remediate, an amendment that simultaneously supplies calcium, improves phosphorus availability, and buffers pH would be an attractive proposition.

The broader context gives the findings their urgency. Phosphogypsum is produced on a staggering scale, and only a fraction of it is currently recycled into construction materials, ammonium sulfate, or other products. The remainder sits in impoundments that pose risks of leachate contamination of groundwater and, in some regions, concerns about radionuclide content inherited from the phosphate ore. Reviews of phosphogypsum management have repeatedly called for circular-economy solutions that move beyond storage, and recent work has explored everything from high-purity calcium carbonate production to the recovery of phosphate and fluoride from leachates. What distinguishes the new study is its explicitly biological approach: rather than using energy-intensive chemical processing, it enlists soil microorganisms to do the mineral transformation at ambient temperatures, in principle directly in the field.

The authors are careful to frame the work as preliminary, and several important questions remain open. The most significant is whether the two-step transformation, fungal oxalate production followed by bacterial oxalate degradation, can actually proceed sequentially in the rhizosphere, the narrow zone of soil surrounding plant roots where chemistry is governed by root exudates and dense microbial communities. Laboratory cultures are far simpler environments than living soil, and factors such as competing microbes, variable moisture, and the buffering capacity of real soils could slow or redirect the pathway. The long-term environmental effects also need evaluation, including the fate of trace impurities in phosphogypsum as the gypsum matrix dissolves and whether repeated applications could accumulate unwanted elements in soil or crops. A companion study by some of the same authors on high-dosage phosphogypsum in potted amaranth underscores that dosage and soil type matter for safety.

Even with those caveats, the study offers a compelling proof of concept with an almost poetic symmetry. A waste product of industrial agriculture, stockpiled by the billions of tonnes, is transformed by a mold and a bacterium into calcite and plant-available nutrients, and the plants grown in the amended soil respond with more than double the height of untreated controls. The oxalate-carbonate pathway, long studied as a curiosity of soil geochemistry and carbon sequestration, here becomes a practical tool for waste valorization. If follow-up work confirms that the fungal-bacterial relay can operate in real rhizospheres at field scale, phosphogypsum could shift from being one of the fertilizer industry’s most stubborn liabilities to a raw material for restoring the very soils that agriculture depends on. The research was funded by the Science and Technology Support Plan of Guizhou Province and the Major Scientific and Technological Achievement Transformation Project of Guizhou Province, reflecting regional interest in turning a local waste burden into an agricultural asset.

Subject of Research: Microbial biotransformation of phosphogypsum into calcium oxalate and calcite for use as a soil amendment

Article Title: Biotransformation of phosphogypsum and its potential for soil-based applications: a preliminary study

Article References: Meng, L., Xiao, D., Liu, X., Cao, Y., & Lian, B. (2026). Biotransformation of phosphogypsum and its potential for soil-based applications: a preliminary study. 3 Biotech, 16(10), Article 426. https://doi.org/10.1007/s13205-026-05064-8

Image Credits: AI Generated

DOI: 10.1007/s13205-026-05064-8

Keywords: phosphogypsum, Aspergillus niger, Azospirillum brasilense, calcium oxalate, whewellite, weddellite, calcite, oxalate-carbonate pathway, soil amendment, soil microbiology, bioremediation, saline-alkali soil

Phosphogypsum is the bulky, problematic by-product of phosphoric acid production for fertilizer manufacturing. Global stockpiles have now exceeded six billion tonnes, with another 200 to 250 million tonnes added every year, while utilization rates remain stubbornly low. Chemically, the material is dominated by gypsum, calcium sulfate dihydrate, with calcium oxide content commonly around 30 percent by weight. That makes it an tantalizingly abundant calcium reservoir for mineral carbonation, the process in which CO2 reacts with calcium- or magnesium-bearing phases to form stable carbonate minerals such as calcite. Unlike steel slag or carbide slag, however, phosphogypsum holds its calcium in sparingly soluble gypsum rather than in readily reactive lime or calcium silicates, so its carbonation depends entirely on whether that bound calcium can first be coaxed into solution.

Researchers led by Ying Shi and colleagues at Central South University in China tackled this problem with a comparative experiment of unusual rigor. They tested four activators, ammonium chloride, sodium acetate, ammonium acetate, and sodium gluconate, against plain deionized water, to dissolve calcium out of washed phosphogypsum from Guizhou Province. The resulting calcium-rich leachates were then carbonated under an identical protocol: pH was raised above 12 with sodium hydroxide, and each leachate was exposed to a CO2 atmosphere in sealed gas-tight bags for fourteen days, with the gas renewed daily. By measuring dissolved calcium at every stage and characterizing every solid that emerged, the team could link three successive steps that most studies treat in isolation: calcium release, calcium consumption, and carbonate product formation.

The leaching results were dramatic. Water alone extracted only modest amounts of calcium, leaving dissolved concentrations between roughly 726 and 833 milligrams per liter. Ammonium chloride nearly tripled that, sodium acetate and ammonium acetate pushed higher still, and sodium gluconate was in a class of its own, sustaining dissolved calcium concentrations around 15,300 milligrams per liter, roughly twenty times the blank system. Under optimized conditions, sodium gluconate released up to 98.5 percent of the available calcium from the solid, dissolving so much gypsum that only 5.4 grams of residue remained from an initial 240 grams of phosphogypsum. Electron microscopy showed the characteristic plate-like gypsum particles almost completely dismantled, while X-ray diffraction confirmed the near-total removal of gypsum and a relative enrichment of insoluble quartz. The gluconate anion, the study notes, forms stable soluble complexes with calcium ions, shifting the gypsum dissolution equilibrium and driving continued release.

Then came the twist. When the leachates were carbonated, the leachate with the most dissolved calcium performed the worst at consuming it. The ammonium chloride system, starting from a moderate 2,376 milligrams per liter of dissolved calcium, plummeted below 50 milligrams per liter within a single day. Sodium acetate and ammonium acetate followed similar trajectories, and after fourteen days all three had achieved calcium consumption efficiencies of 98.7 to 99.0 percent. The sodium gluconate leachate, despite beginning with 15,315 milligrams per liter, crawled down slowly, still holding 2,177 milligrams per liter after two weeks, for a final consumption efficiency of just 85.8 percent. Its pH also stayed persistently high, remaining near 11.8 at the end of the experiment while the other systems settled around neutral.

The authors attribute this behavior to lingering calcium-gluconate complexation. Even though the total dissolved calcium was enormous, much of it was locked in soluble complexes rather than existing as free calcium ions, reducing the ion activity product needed to trigger calcium carbonate precipitation and delaying supersaturation and nucleation. In other words, a high calcium concentration on paper does not guarantee that the calcium is chemically available to react with carbonate. The study is careful to note that carbon dioxide was supplied from a static gas phase without stirring or sparging, so gas-liquid mass-transfer resistance may also have shaped the apparent kinetics, but all leachates faced the same conditions, making the comparison fair.

The solid products told the same story from a different angle. Every leachate produced calcite, the crystalline form of calcium carbonate, confirmed by X-ray diffraction, infrared spectroscopy, and thermogravimetric analysis. But the quality of the products diverged sharply. The water, ammonium chloride, sodium acetate, and ammonium acetate systems yielded solids that were overwhelmingly carbonate, with apparent calcium carbonate equivalents of 93.56 to 96.90 percent by weight. The sodium gluconate product was a different beast entirely: quantitative X-ray diffraction showed crystalline calcite made up only about 20 percent of the recovered solid, with the remainder predominantly X-ray amorphous, accompanied by an unusual silicon-oxygen signature and a distinct thermal profile featuring substantial low-temperature mass loss.

Translated into climate terms, the differences are meaningful. Normalized to the initial phosphogypsum mass, the water, ammonium chloride, sodium acetate, and ammonium acetate routes mineralized an estimated 3.96, 14.36, 15.87, and 20.00 kilograms of CO2 per tonne of phosphogypsum, respectively. The sodium gluconate route, judged only by its quantified crystalline calcite, incorporated 72.51 kilograms of CO2 per tonne into calcite, a far higher figure that reflects its extraordinary calcium supply. Yet the authors caution that this headline number masks the complexity of the product and the substantial calcium left stranded in solution, which matters enormously if the goal is clean, well-defined carbonate for construction fillers or cementitious applications.

The broader lesson is a methodological one that could ripple through the field. Most phosphogypsum carbonation studies optimize a single leaching-carbonation system and report calcium recovery or carbonate yield, without asking whether more calcium release actually translates into better carbonation. By systematically decoupling the two stages, this study shows they can even be inversely related. Ammonium chloride emerges as the balanced performer, offering moderate calcium release, near-complete consumption, and clean calcite-dominated products, while sodium gluconate demonstrates that powerful complexing agents can extract calcium superbly yet sabotage its subsequent use. For engineers designing carbonation-based materials from industrial waste, the criterion for a good recycled calcium source must therefore expand from how much calcium a leachate contains to how much of that calcium can actually be consumed and converted into well-defined carbonate solids.

There remain open questions. The authors acknowledge that a complete calcium-sulfur-carbon material balance was not established, and they call for future work with phase-resolved elemental accounting to track where every element ends up. Impurities in phosphogypsum, including soluble phosphorus, fluoride, organic matter, and heavy metals, may also interact with calcium phases and carbonate nucleation in ways not yet fully resolved, and the sample from a single Guizhou source may not represent phosphogypsum from other phosphate ores. Still, the core message stands: in the race to turn a six-billion-tonne waste mountain into a carbon sink, chemistry that looks strongest at the extraction stage may quietly undermine the storage stage, and only a full-chain view, from dissolution to precipitation, can identify the leachates truly fit for locking CO2 into stone.

Subject of Research: Activator-assisted calcium release from phosphogypsum and its aqueous carbonation for CO2 mineralization

Article Title: Comparative study of phosphogypsum-derived leachates as recycled calcium sources for carbonation: Linking Ca release, consumption and carbonate product formation

Article References: Shi, Y., Guo, Q., Li, S., Zhou, X., Yan, K., & Liu, P. (2026). Comparative study of phosphogypsum-derived leachates as recycled calcium sources for carbonation: Linking Ca release, consumption and carbonate product formation. Case Studies in Construction Materials, 25, Article e06557. https://doi.org/10.1016/j.cscm.2026.e06557

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06557

Keywords: phosphogypsum, mineral carbonation, CO2 sequestration, calcium leaching, calcite, sodium gluconate, ammonium chloride, industrial waste valorization, gypsum dissolution, carbonate precipitation, low-carbon construction materials, recycled calcium sources