A team led by Chaojie Wang and Chengchao Guo set out to answer a deceptively simple question: how strong is the interface between polymer-stabilized soil and concrete, and how quickly does that strength fade when the interface is submerged? The researchers used a two-component permeable polymer grout, mixing a polyether polyol-based component with an isocyanate-rich component in equal masses. The liquid penetrates soil pores under controlled grouting pressure, then cures into a solid that binds soil grains together and glues them to concrete surfaces. This chemistry, based on urethane and urea linkages, gives the material its high bonding strength and low viscosity, but those same chemical bonds are vulnerable to hydrolysis, the slow reaction in which water molecules break chemical links apart.

The experimental program was meticulous. Soil excavated from a construction site in Zhengzhou, Henan Province, was air-dried, sieved, and re-wetted to controlled moisture contents of 6 and 11 percent. Concrete cubes of three strength grades, C30, C40, and C50, were cast, cured for 28 days, and then precision-cut with a CNC machine into half-cubes whose cut faces formed the test interfaces. The researchers machined grooves into some of these faces to create four levels of surface roughness, quantified using the joint roughness coefficient scale developed by Barton and Choubey, ranging from perfectly smooth to a JRC of 19.5. Soil was compacted in layers onto the concrete surfaces inside a custom mold, and a purpose-built constant-pressure grouting rig, monitored by a digital pressure sensor sampling at 1 hertz, injected the polymer until penetration was complete.

Once cured, the specimens faced their ordeal. After seven days of natural hardening, they were submerged in water for 7, 21, or 56 days, then sheared along the polymer-stabilized-soil-to-concrete interface in a servo-controlled direct shear machine under normal stresses of 200, 400, and 600 kilopascals. The resulting shear stress-displacement curves told a consistent story across every combination of concrete grade, soil density, grouting pressure, and roughness. Each curve rose elastically, peaked, softened, and settled to a residual plateau, the classic signature of a quasi-brittle interface that fails suddenly rather than stretching plastically.

The numbers reveal a striking three-stage decay pattern. The most dramatic loss of strength came early: within the first seven days of immersion, peak shear strength dropped by just over 20 percent on average, and the decline was steepest of all in that opening week. Between 7 and 21 days the erosion of strength continued but slowed, and between 21 and 56 days the curves flattened into a plateau. After 56 days underwater, the peak shear strength had fallen by roughly 30 percent overall, with a worst case of 35.15 percent. Crucially, the interface never collapsed entirely. The residual bonding capacity stabilized at approximately 60 to 65 percent of the original dry strength, suggesting that the polymer possesses an intrinsic water resistance that prevents complete bond loss.

Normal stress turned out to amplify the damage. Under identical immersion times, specimens sheared at higher normal stress lost a larger fraction of their strength than those tested at lower stress, because water lubricates the sliding surfaces more effectively when they are pressed together harder. The same pattern appeared in the residual strength, which is governed almost entirely by sliding friction once the bond has failed. After 56 days of immersion, residual shear strength had decayed by about 35 percent, exceeding the peak-strength decay, with a maximum of 38.44 percent. Fitting the data to the Mohr-Coulomb framework showed why: the interfacial friction angle, a measure of how much the rough surfaces resist sliding, proved more water-sensitive than cohesion, the chemical-glue component of strength. After 56 days, the friction angle at peak strength had decayed by up to 36.57 percent, and at residual strength by as much as 40.67 percent, while cohesion losses peaked near 26 percent.

Microscopy explained the mechanism. Scanning electron microscope images of the stabilized soil showed that water dissolved protruding polymer films, loosened surface soil particles, and smoothed the once-rough interface, eliminating the mechanical interlocking that had helped resist shear. ImageJ analysis of the micrographs revealed that apparent porosity climbed from about 25 percent in unimmersed samples to just under 27 percent after 56 days, with most of the growth occurring early. Mercury intrusion porosimetry confirmed the trend: the porosity of grouted soil rose from 21.49 percent before immersion to 26.49 percent after 56 days, while average pore size grew by 43 percent. Yet even the degraded soil remained far denser than the ungrouted silt, whose porosity of nearly 50 percent dwarfed anything measured after treatment. The water damage concentrated in pores between 7 and 200 micrometers, carving the fine through-cracks and voids that directly weaken the bond.

The failure surfaces themselves shifted character with soaking. In dry conditions, shear failure typically tore through the interior of the stabilized substrate, leaving soil smeared across the concrete, a mode the authors call type A. With longer immersion, less soil adhered to the concrete after failure, and the fracture plane migrated to the polymer-soil interface itself, the cleaner type B mode. Quantitatively, the proportion of soil clinging to the concrete surface fell steadily with immersion time and stabilized, while higher normal stress pushed failures back toward the substrate. The team also established a practical quality-control clue: specimens with total porosity above roughly 23 to 24 percent were far more likely to fail at the interface, and porosity correlated linearly with strength decay regardless of normal stress.

To turn these observations into a design tool, the researchers derived a three-stage damage constitutive model that captures the full bond-slip curve: a smooth rising branch, an S-shaped softening segment after the peak, and a frictional residual plateau described by the Mohr-Coulomb law. Fitted to the experimental data with the Levenberg-Marquardt algorithm, the model achieved coefficients of determination between 0.9275 and 0.9944 across all immersion times and stress levels, and it correctly reproduced the post-immersion shift toward greater ductility, in which the interface deforms further before reaching its diminished peak.

The authors are candid about the limits of their work. The five test combinations came from a preceding orthogonal design rather than a full factorial matrix, vertical displacement during shearing was not measured, and the hypothesis that strength stabilizes because accessible hydrolysable bonds are consumed early, leaving hydrophobic-protected links intact, rests on microscopy and literature rather than direct spectroscopy. Still, the practical message is clear. For trenchless tunnel repairs in water-rich ground, the finding that roughly 60 to 65 percent of interfacial strength survives 56 days of immersion offers a conservative starting point for design, while the porosity threshold gives inspectors a measurable warning sign. Water attacks the glue quickly, but it does not dissolve it completely, and knowing exactly where the decay stops may be the most valuable number this study delivers.

Subject of Research: Shear strength degradation and damage modeling of the polymer-stabilized soil to concrete interface under water immersion

Article Title: Research on the shear strength and damage constitutive model of interface between polymer stabilized soil and concrete under water immersion

Article References: Wang, C., Ding, L., Diao, Y., Zhang, X., Guo, C., Wang, F., & Du, X. (2026). Research on the shear strength and damage constitutive model of interface between polymer stabilized soil and concrete under water immersion. Case Studies in Construction Materials, 25, Article e06528. https://doi.org/10.1016/j.cscm.2026.e06528

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06528

Keywords: permeable polymer grouting, tunnel reinforcement, soil-concrete interface, direct shear test, water immersion, shear strength decay, Mohr-Coulomb parameters, damage constitutive model, mercury intrusion porosimetry, scanning electron microscopy, hydrolysis, geotechnical engineering

The choice of organism is central to the strategy. Galdieria sulphuraria, a unicellular red alga of the class Cyanidiophyceae, thrives in conditions that would kill nearly all competing microbes: acidic environments with pH values between 1 and 5 and temperatures up to 56 degrees Celsius. In this study, the alga was grown at pH 1.8 and 45 degrees Celsius in complete darkness, drawing energy from organic substrates rather than photosynthesis. This heterotrophic mode of growth offers clear advantages for industrial waste valorization. Because the extreme culture conditions suppress bacterial and fungal contaminants, the process can run under non-sterile conditions, dramatically reducing costs compared with conventional fermentation. At the same time, the organism tolerates the complex and variable chemistry of real industrial residues, a persistent stumbling block for cleaner cultivation systems.

The raw materials came from a German shrimp processing facility and included wastewater, sludge, and minced shrimp heads and shells. Compositional analysis by near-infrared spectroscopy revealed that shrimp residues were remarkably protein dense, containing 57 percent protein and 11 percent lipids by weight, while the sludge contained 28 to 32 percent protein, roughly 13 percent carbohydrates, and 2 to 8 percent fat. The wastewater, with only 0.85 percent solids and negligible free amino nitrogen and phosphate, served not as a nutrient source but as a dilution medium. To unlock the nutrients locked inside the solid residues, the team tested a series of hydrolysis strategies: enzymatic treatment with the acidic protease Protease S-02, enzymatic treatment with Glucoamylase AN, a combination of both enzymes, and a chemical pretreatment with 1 percent sulfuric acid at 90 degrees Celsius followed by enzymatic digestion.

The hydrolysis results carried an important and somewhat sobering message. Free amino nitrogen recovered from sludge hydrolysates remained low, at only 1 to 2 milligrams per gram of sludge, regardless of the treatment applied. Shrimp residues performed far better, yielding 10 to 15 times more free amino nitrogen when digested with protease, the enzyme cocktail, or acid-assisted enzymatic hydrolysis over 48 hours. Strikingly, control experiments without any enzymes showed that a large share of the recoverable nutrients, around 15 milligrams of free amino nitrogen per gram of shrimp residue and 100 to 150 milligrams of phosphate per gram of both residues, could be released simply by solubilizing and mixing the materials. The yields were also notably below the 50 to 90 percent recovery figures often reported for proteolytic hydrolysis of fish and meat processing wastes, a discrepancy the authors attribute to limited protein accessibility, suboptimal mixing and solids concentrations, or a mismatch between the chosen protease and its substrate.

With hydrolysates in hand, the researchers turned to cultivation trials. In shaken flask cultures held for seven days in the dark, Galdieria sulphuraria grew exponentially in sludge hydrolysate concentrations of 25 to 50 percent by volume, reaching a maximum growth rate of 1.36 per day at the 50 percent level supplemented with 2.5 grams per liter of glucose. This rate matches values previously reported for heterotrophic growth of the species. However, the windows of tolerance were narrow. At 75 percent sludge hydrolysate, growth was inhibited, and shrimp hydrolysate supported growth only at concentrations of 25 percent or less. The culprit appears to be organic acids: experiments showed that acetate concentrations of 0.3 grams per liter and above blocked growth, while lower levels were metabolized. The mechanism is well understood in acidophilic microbiology. Below the pKa of acetic acid, the molecule becomes protonated, diffuses across the cell membrane, and releases its proton inside the cell, acidifying the cytoplasm and disrupting metabolism.

To push biomass concentrations higher, the team moved to fed-batch cultivation in a 5-liter bioreactor. Cultures were started with a mixture of 15 percent sludge hydrolysate and 10 percent shrimp hydrolysate plus 10 grams per liter of glucose, and on days three and four were fed an additional 300 to 400 milliliters of a richer solution containing 70 percent sludge hydrolysate, 30 percent shrimp hydrolysate, and 22.5 grams per liter of glucose, delivered by peristaltic pump. The initial batch phase produced consistent growth rates of 1.19 to 1.25 per day, corresponding to doubling times between 13.3 and 14.0 hours, and biomass reached 6 to 8 grams per liter. In the best-performing run, continued growth after feeding pushed the biomass concentration to nearly 10 grams per liter, with the produced cells containing 47 to 62 percent protein and 7 to 8.6 percent lipids.

The pigment phycocyanin, a natural blue colorant with established markets in food and nutraceutical industries, accumulated to as much as 8 milligrams per gram of dry biomass during the initial exponential growth phase, falling to around 4 milligrams per gram in stationary phase in one culture. Yet another culture produced only 1 to 2 milligrams per gram despite similar nutrient conditions. This variability pointed the researchers toward a deeper question: whether growth depends not just on how much nitrogen is available, but on which nitrogen compounds are present. Because the nitrogen species in residue hydrolysates are difficult to identify, the team ran a systematic screening, supplying the alga with 50 millimolar solutions of individual amino acids and comparing growth against ammonium sulfate, the standard nitrogen source.

The screen revealed pronounced substrate specificity. Ammonium sulfate supported a growth rate of 0.95 per day, and only two amino acids came close: alanine at 0.87 per day and proline at 0.80 per day, marking them as highly suitable nitrogen donors. Glutamic acid, aspartic acid, leucine, arginine, glycine, and methionine supported measurable but weaker growth between 0.59 and 0.73 per day, while tryptophan, lysine, phenylalanine, isoleucine, threonine, serine, histidine, and valine performed poorly, with rates between 0.16 and 0.48 per day. Lysine and cysteine permitted no or only weak, delayed growth. The practical implication is significant: residues must be selected not only for their total nitrogen content but for the amino acid composition of that nitrogen, since the alga cannot universally exploit amino acids. The low phycocyanin yield in one fed-batch culture may reflect exactly this gap, where free amino nitrogen was abundant but the right amino acids were missing.

Taken together, the study maps both the promise and the constraints of a decentralized, circular bioeconomy in which aquaculture farms recycle their own residues into protein-rich algal feed or pigment feedstocks. The authors identify three dominant control points: the limited and variable release of nitrogen from residues, growth inhibition at high hydrolysate concentrations driven by organic acids such as acetate, and carbon limitation caused by glucose depletion even under fed-batch operation. Because much of the phosphate and some free amino nitrogen are released by mixing alone, pretreatment must be tailored to each substrate and enzyme combination to remain cost-effective. If those parameters can be mastered, the extreme lifestyle of Galdieria sulphuraria, its tolerance of heat, acidity, and contamination, could transform the waste streams of the world’s fastest growing food sector into a reliable source of protein and natural colorants, closing a nutrient loop that has until now leaked into rivers and coastal waters.

Beyond protein and phycocyanin, the biomass produced in such a process carries additional commercial value. Galdieria sulphuraria is known to accumulate highly branched glycogen, a form of storage carbohydrate of interest for nutritional applications, and its protein fraction is reported to offer a favorable amino acid profile suitable for animal feed and human nutrition. Because the alga was cultivated heterotrophically in the dark, productivity is not constrained by light availability or photobioreactor geometry, allowing the high cell densities typical of stirred-tank fermentation to be approached.

The fed-batch strategy used here also illustrates a broader principle of residue-based bioprocessing. Rather than exposing the culture to a single hydrolysate at full strength, the researchers diluted inhibitory components during start-up and then supplied nutrients progressively, keeping the alga in exponential growth while limiting the accumulation of organic acids. This kind of staged feeding mirrors established practice in industrial fermentation, where substrate toxicity is managed through controlled dosing rather than batch addition.

Decentralization is another notable aspect of the concept. Since the residues originate at shrimp processing facilities and aquaculture farms, a compact cultivation unit operating at acidic pH and elevated temperature could, in principle, run on-site without sterilization equipment, converting waste into feed or pigment precursors where it is generated. The remaining challenges are largely quantitative: stabilizing hydrolysate composition across batches, matching nitrogen quality to the alga’s substrate specificity, and balancing carbon dosing against acetate inhibition. Addressing these control points will determine whether laboratory yields of roughly ten grams per liter of protein-rich biomass can be translated into an economically viable, closed-loop component of the aquaculture industry.

Subject of Research: Heterotrophic cultivation of Galdieria sulphuraria on aquaculture residue hydrolysates for protein- and phycocyanin-rich biomass production.

Article Title: Valorization of Aquaculture Residues by Heterotrophic Cultivation of the Extremophilic Microalga Galdieria Sulphuraria for Protein- and Phycocyanin-Rich Biomass Production

Article References: Kleps, C., Händel, N., Schönfelder, S., Baum, L., Ogurek, M., & Pleissner, D. (2026). Valorization of Aquaculture Residues by Heterotrophic Cultivation of the Extremophilic Microalga Galdieria Sulphuraria for Protein- and Phycocyanin-Rich Biomass Production. Waste and Biomass Valorization. https://doi.org/10.1007/s12649-026-03778-7

Image Credits: AI Generated

DOI: 10.1007/s12649-026-03778-7

Keywords: Galdieria sulphuraria, aquaculture residues, phycocyanin, microalgae, waste valorization, circular bioeconomy, hydrolysis, heterotrophic cultivation, shrimp waste, protein-rich biomass, bioprocess engineering, Valorization