The material at the center of the study is called self-protected underwater mortar, or SPUM. Rather than loading the mortar with viscosity-modifying chemicals, the method relies on an underwater protective agent, a water-soluble polymer dissolved in the surrounding water at a concentration of 0.5 percent by mass. The polymer, a linear ionic macromolecule with a weight-average molecular weight of at least 7.5 million, accumulates at the boundary between the fresh paste and the water, forming a stable protective film. This layer suppresses the dispersion of cementitious material into the water without increasing the viscosity of the mortar itself, which means the mix can still flow freely into place. Earlier work has shown that this approach can retain around 90 percent of its compressive strength when cast in seawater compared with dry-land casting, an impressive figure for underwater construction.

The motivation for the new study comes from one of the most demanding environments on Earth: the Qinghai-Tibet Plateau. Hydraulic structures there are often cast underwater and then spend their service lives in water-level fluctuation zones, where seasonal freezing and thawing batter the material year after year. To make matters worse, the plateau is dotted with saline lakes whose surface and groundwater carry high concentrations of sulfate ions. Sulfate attack is a notorious durability problem because sulfate ions react with cement hydration products to form expansive compounds such as ettringite and gypsum, generating internal stresses that crack the material from within. When sulfate attack and freeze-thaw cycles act together, previous research has shown they can accelerate damage far faster than either agent alone.

To probe these effects, the team prepared three versions of the same mortar mix, which used ordinary Portland cement partially replaced with 10 percent fly ash and 30 percent tuff powder, a locally available mineral admixture, along with a polycarboxylate superplasticizer. The first batch, designated SPUM-LE, was cast and cured in a conventional land environment. The second, SPUM-TW, was cast and cured in water containing 0.5 percent of the protective polymer. The third, SPUM-SS, was cast and cured in water containing both the polymer and 5 percent sodium sulfate, and was later exposed to freeze-thaw cycling in the same sulfate solution. The researchers measured slump flow, compressive and flexural strength at 3, 7 and 28 days, and then subjected all specimens to 100 rapid freeze-thaw cycles spanning temperatures from minus 18 to 5 degrees Celsius, tracking mass loss and relative dynamic elastic modulus along the way.

The rheology results showed immediately how much the surrounding medium matters. The land-cast mortar spread the farthest and fastest, while the version cast in the polymer solution lost 11.6 percent of its slump flow and needed 66.8 percent longer to reach a 200-millimeter spread, largely because the viscous solution and added buoyancy resist the flow of fresh mortar. The sulfate-polymer bath was harsher still: slump flow dropped by 33.8 percent and the spread time ballooned by 772.5 percent, a consequence of the denser liquid further suppressing the driving force for spreading. These numbers matter because flowability is the entire selling point of the self-protection concept, and the study shows that the chemistry of the water being poured into can quietly erode that advantage.

Strength development told a more subtle story. At 3 days, the underwater specimens were dramatically weaker than their land-cast counterpart, with the polymer-only batch reaching just 8.73 megapascals against 19.22 megapascals on land. The sulfate batch fared slightly better early on, apparently because sulfate ions initially accelerated hydration. But that early boost did not last. By 28 days, the land mortar reached 34.50 megapascals, the polymer-only batch 27.00 megapascals, and the sulfate batch only 23.40 megapascals, giving underwater-to-land compressive strength ratios of 0.80 and 0.60 respectively. The explanation lies at the interface: the polymer film that protects the mortar from washout also slows the transport of water to cement particles, retarding hydration, while accumulating gypsum and other sulfate products coat unhydrated cement and further impede the reaction. Flexural strength proved even more sensitive, with the sulfate-exposed batch actually declining from 6.29 to 5.55 megapascals between 7 and 28 days.

Freeze-thaw performance split the specimens in an unexpected way. The land-cast mortar held up best, losing only 1.29 percent of its mass and retaining 84.61 percent of its dynamic elastic modulus after 100 cycles. The polymer-only underwater mortar suffered the worst surface damage, shedding 5.30 percent of its mass as scaling and particle detachment, while still retaining 76.09 percent of its modulus. The sulfate-exposed batch showed the opposite pattern: modest mass loss of 3.99 percent, partly masked by white sulfate deposits accumulating on the surface, but the steepest internal degradation, with its dynamic elastic modulus falling to just 67.92 percent. In other words, one material fell apart from the outside in, while the other crumbled from the inside out. Sulfate ions penetrating the specimen during cycling formed expansive products that drove internal microcracking even as deposited crystals compensated for surface mass loss.

Microscopy and porosity measurements explained why. Scanning electron microscopy revealed dense, continuous matrices in the land-cured specimens but looser structures with visible micropores in the underwater batches, and abundant needle-like sulfate products in the sulfate-exposed material at 28 days. Mercury intrusion porosimetry added a twist: after 100 freeze-thaw cycles, the underwater specimens actually showed lower total porosity and fewer large harmful pores than the land-cast material, because continued hydration filled their more open pore systems. Yet they still performed worse. The lesson is that pore structure measured after the fact can be misleading; what governs freeze-thaw durability is the initial pore structure before cycling and the cumulative damage that accumulates within it. A loose starting framework gives hydration products plenty of room to fill, but that same looseness lets ice crystals and expansive reactions inflict damage that no amount of later pore-filling can undo.

To turn these observations into something engineers can use, the team fitted their damage data to a Weibull statistical model, using both mass loss and dynamic elastic modulus as damage indicators. The fits were strong, with coefficients of determination reaching 0.99 for the modulus-based model. Converting laboratory cycles into field years using an acceleration factor of 12 and an estimated 100 natural freeze-thaw cycles per year in the Lhasa region, the model predicted service lives of roughly 23.7 years for the land-cast mortar, 10.9 years for the polymer-only underwater mortar and 13.3 years for the sulfate-exposed batch under the more conservative mass-loss criterion. The authors caution that these figures are engineering estimates, since laboratory acceleration can never perfectly reproduce field conditions, and they note that the coupled nature of casting, curing and freezing makes it difficult to isolate the contribution of each stage.

The broader significance of the work is twofold. For engineers planning hydropower and hydraulic projects in cold, chemically aggressive regions, it provides hard numbers on a construction method that could otherwise be adopted on faith, showing that self-protected underwater mortar can genuinely work but that its durability ceiling depends heavily on the water it is born into. For materials science more broadly, it is a vivid demonstration that durability cannot be judged from a single snapshot of microstructure. A material that looks denser after testing may in fact be more damaged than one that looks coarser, because the true story lives in the sequence of events, the initial defects, the migrating ions and the accumulating cracks, rather than in the final photograph.

Subject of Research: Freeze-thaw durability and sulfate resistance of self-protected underwater mortar cast with an underwater protective agent

Article Title: Effects of underwater protective agent and sulfate exposure on the freeze-thaw resistance of self-protected underwater mortar

Article References: Zhang, J., Liu, Z., Shen, D., Lang, T., Li, Y., Sui, H., & An, X. (2026). Effects of underwater protective agent and sulfate exposure on the freeze-thaw resistance of self-protected underwater mortar. Case Studies in Construction Materials, 25, Article e06601. https://doi.org/10.1016/j.cscm.2026.e06601

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06601

Keywords: self-protected underwater mortar, underwater protective agent, freeze-thaw cycles, sulfate attack, concrete durability, Qinghai-Tibet Plateau, cement hydration, pore structure, Weibull damage model, hydraulic engineering, anti-washout, service life prediction

The research team, led by Tongning Cao and colleagues including Han Wang, Qi Dong, Fengjuan Wang, Zhiyong Liu, Chenyun Yu, Bin Peng, and Jinyang Jiang, set out to close a persistent gap in concrete durability science. Most laboratory studies of sulfate attack have been performed under simple immersion, wet-dry cycling, or atmospheric pressure conditions. Those regimes capture shallow-marine and terrestrial exposure reasonably well, but they ignore the defining feature of the deep-sea environment: sustained hydrostatic pressure ranging from several to tens of megapascals. At such pressures, the state of pore water, the morphology of the pore network, and the very mechanisms of ion transport inside concrete may be fundamentally altered, making conventional durability predictions unreliable for deep-water structures.

To probe these coupled effects, the researchers built a custom hydrostatic pressure-sulfate erosion apparatus capable of simultaneously controlling pressure, temperature, and sulfate exposure. The system, rated to a maximum of 25 megapascals with a pressure resolution of 0.1 megapascals, centers on a one-liter high-pressure reaction vessel pressurized by nitrogen gas. Cubic cement paste specimens with a water-to-binder ratio of 0.4 were cast, cured for 28 days, and then sealed with epoxy resin on five faces, leaving a single 20 by 20 millimeter face exposed. This clever geometry forced sulfate ions to travel along one well-defined direction, simplifying the interpretation of the depth-resolved measurements that followed.

Specimens were exposed to a 5 percent sodium sulfate solution at three gauge pressures: 0, 1.2, and 2.4 megapascals, for a total of 150 days. The solution was refreshed every 15 days to maintain aggressive conditions. At 60, 90, 120, and 150 days, replicate specimens were retrieved, sectioned into 2-millimeter-thick slices along the exposure direction, and treated with isopropanol to halt hydration. All sample handling took place in a nitrogen-controlled glove box to prevent carbonation from contaminating the chemistry. One portion of each powdered slice was analyzed for sulfate ion content by gravimetric barium sulfate titration, while the other was subjected to quantitative X-ray diffraction with Rietveld refinement, using aluminum oxide as an internal standard, to measure the amount of ettringite, also known as AFt, formed at each depth.

The results reveal a striking pressure dependence. At a depth of 9 millimeters, the sulfate concentration after 150 days rose from 1.6 percent under atmospheric pressure to 2.7 percent at 2.4 megapascals. Deeper in the material the contrast became even more dramatic: at 19 millimeters, the sulfate content after 150 days was 0.5 percent at 0 megapascals, but 0.9 percent at 1.2 megapascals and 1.2 percent at 2.4 megapascals. In other words, moderate pressures that are routine in coastal and shallow offshore engineering already boost sulfate penetration by roughly 50 to 80 percent, and the enhancement grows with both depth and time. Ettringite formation followed the same pattern, accumulating preferentially near the exposed surface and decaying inward, with concentrations at 5 millimeters reaching 1.69 percent at 0 megapascals, 2.6 percent at 1.2 megapascals, and 3.1 percent at 2.4 megapascals after 120 days.

Microscopy and porosimetry explained why. Field-emission scanning electron microscopy showed the corrosion products as interwoven needle-like and rod-like crystals, the classic habit of ettringite, while energy-dispersive spectroscopy confirmed the enrichment of calcium, aluminum, sulfur, and oxygen in ratios matching the mineral’s stoichiometry. Notably, ettringite crystals clustered around portlandite, suggesting that as sulfate ions consume calcium from the pore solution, portlandite dissolves to replenish it, sustaining the reaction and progressively degrading the matrix. Mercury intrusion porosimetry then quantified the structural toll: total porosity climbed from 14.93 percent at 0 megapascals to 19.02 percent at 1.2 megapascals and 25.81 percent at 2.4 megapascals, with the pore-size distribution shifting toward larger, better-connected channels.

This pore-structure evolution sets up a vicious feedback loop. Pressure-enhanced sulfate transport delivers more reactant deeper into the material, accelerating ettringite formation, which coarsens and connects the pore network. The modified network in turn raises the effective diffusion coefficient, allowing still faster sulfate ingress. According to Fick’s law, this increase in effective transport capacity strengthens concentration-gradient-driven migration, producing deeper penetration and higher interior sulfate contents. The authors’ simulations capture this nonlinear amplification: after 150 days, predicted sulfate concentrations at 20 millimeters were 112 percent higher at 1.2 megapascals and 290 percent higher at 2.4 megapascals than under atmospheric pressure. From a crystallization-pressure perspective, the elevated sulfate supply also drives the pore solution to supersaturation sooner, triggering earlier ettringite crystallization and shortening the material’s latent expansion period.

To turn these observations into a predictive tool, the team constructed a coupled sulfate transport-reaction model grounded in Fick’s second law and mass conservation. The model treats sulfate ions as either free in the pore solution or consumed by reaction with calcium and aluminate-bearing hydration products to form ettringite, following the stoichiometry that one mole of AFt requires two moles of external sulfate. The effective diffusion coefficient is computed from porosity, tortuosity, and the Stokes-Einstein estimate of sulfate diffusivity in water, with a phenomenological pressure term linking hydrostatic stress to porosity change through the circumferential tensile stress around idealized cylindrical pores. The equations were solved numerically with the finite volume method on hexahedral grids, accelerated by a multigrid V-cycle scheme implemented in MATLAB.

Validation against the 1.2 megapascal experiments showed good agreement between predicted and measured sulfate and ettringite profiles across the full 150-day exposure and the entire penetration depth, with minor discrepancies attributed to the finite thickness of the sliced samples. The framework provides, for the first time, an experimentally supported quantitative link between hydrostatic pressure, sulfate transport, reaction kinetics, and pore-structure evolution in cementitious materials. The authors caution that the study used cement paste under controlled laboratory conditions rather than full concrete in service, and that further validation across broader materials, environments, and timescales is needed before deep-ocean application. Even so, the message for engineers is clear and urgent: durability assessments for submerged and deep-buried structures that ignore hydrostatic pressure may substantially underestimate how fast sulfate attack proceeds, and how soon the ocean’s quiet chemistry begins to dismantle the concrete we trust to hold it back.

Subject of Research: Effect of hydrostatic pressure on sulfate transport and ettringite formation in cement paste

Article Title: Effect of hydrostatic pressure on transport-reaction processes of cement paste under sulfate attack

Article References: Cao, T., Wang, H., Dong, Q., Wang, F., Liu, Z., Yu, C., Peng, B., & Jiang, J. (2026). Effect of hydrostatic pressure on transport-reaction processes of cement paste under sulfate attack. Case Studies in Construction Materials, 25, Article e06588. https://doi.org/10.1016/j.cscm.2026.e06588

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06588

Keywords: sulfate attack, hydrostatic pressure, cement paste, ettringite, concrete durability, deep-sea environment, ion transport, porosity, XRD-Rietveld, transport-reaction model, marine engineering, pore structure

The research team, led by Hui Huang and Cong Zhang, prepared three mixtures under an identical binder–aggregate–water framework: a plain mortar control, a PP-fiber-reinforced composite, and a PVA-fiber-reinforced composite. Both fiber types were 6 millimeters long and added at a volume fraction of 1.0 percent, so that fiber dosage would not confound the comparison. The fibers themselves, however, are chemically and mechanically very different. The PP fiber, with a diameter of 20 micrometers, has a tensile strength of 800 megapascals and an elastic modulus of 5 gigapascals, and its surface is hydrophobic. The finer PVA fiber, at 14 micrometers, is roughly twice as strong at 1,800 megapascals and nearly three times stiffer at 14 gigapascals, and its hydrophilic chemistry bonds more readily with cement hydration products. Those differences turned out to matter enormously.

The specimens, 70.7-millimeter cubes, were cured for 28 days and then subjected to a rapid water-freezing and water-thawing regime in which temperatures swung between −18 °C and 5 °C every four hours, with the samples fully immersed. Measurements were taken after 0, 25, 50, 75, 100, 125, and 150 cycles. Before exposure, the plain mortar was actually the strongest material, at 65.03 megapascals of compressive strength, compared with 61.92 megapascals for the PVA composite and 54.61 megapascals for the PP composite. But strength at the start is not the same as strength that lasts.

After 150 freeze–thaw cycles, the ranking had reversed in the way that matters for durability. The fiber-reinforced composites retained 82.29 percent (PP) and 84.50 percent (PVA) of their initial compressive strength, while the unreinforced mortar retained only 66.45 percent. Mass loss told a parallel story: the mortar reached 5 percent mass loss by 125 cycles through continuous surface scaling, whereas both fiber mixtures degraded more slowly in a three-stage pattern—slow loss in the first 50 cycles, acceleration between 50 and 100 cycles as cracks connected, and a slowdown thereafter. Even after 150 cycles, the fibers still reduced surface material loss by more than 59 percent relative to plain mortar, with PVA consistently outperforming PP.

Perhaps the most striking finding was that stiffness, not strength, is the more sensitive fingerprint of freeze–thaw damage. The apparent elastic modulus, derived from the slope of the compressive stress–strain curve between 10 and 30 percent of peak stress, fell by 32.9 percent for PP-FRCC and 29.2 percent for PVA-FRCC over 150 cycles—nearly double the corresponding strength losses. Because the modulus reflects the load-transfer continuity of the internal microstructure, it registers the accumulation of pores, microcracks, and debonded interfaces long before catastrophic strength loss appears. For engineers assessing aging infrastructure, this suggests that stiffness-based or non-destructive measurements could serve as an early-warning system for frost damage.

Scanning electron microscopy revealed why the two fibers behaved differently. Using calibrated image analysis with two semi-quantitative descriptors—a visible defect area ratio and an interfacial deterioration ratio—the researchers quantified what they saw in the micrographs. The visible defect area ratio climbed from 1.8 to 8.2 percent in mortar, from lower starting values to 6.8 percent in PP-FRCC and 5.6 percent in PVA-FRCC. More telling was the interface: the fraction of visibly deteriorated fiber–matrix boundary rose from 3.0 to 21.2 percent for PP-FRCC but only from 2.7 to 17.4 percent for PVA-FRCC. The hydrophobic PP surface, chemically stable and inexpensive, gradually separated from the matrix under repeated hydraulic and crystallization pressures, while the hydrophilic PVA fiber maintained more continuous contact with surrounding hydration products.

Those microscopic differences echoed upward into failure behavior. Under splitting tensile loading, PVA-FRCC specimens displayed primary or deflected splitting cracks in 17 of 21 observations (81.0 percent), whereas PP-FRCC showed localized or branched damage features in 15 of 21 (71.4 percent). The tensile-to-compressive strength ratio, a marker of balanced degradation, stayed remarkably stable for PVA-FRCC—fluctuating within a coefficient of variation of just 0.83 percent—while mortar’s ratio swung the most. In compression, fiber-bridged cracking kept the reinforced cubes intact even after 150 cycles, while the plain mortar failed brittly with severe spalling and a conical crushing zone at its core.

To connect these observations mechanically, the team built a three-dimensional mesoscale model in ABAQUS, representing the composite as a cementitious matrix, randomly distributed equivalent fibers, and an equivalent pore phase whose volumetric expansion stands in for freeze–thaw damage. Calibrated against the measured compressive response at each cycle stage, the model reproduced the monotonic strength decline well at early and intermediate stages, with discrepancies under about 4 percent through 75 cycles. By 150 cycles the gap widened to roughly 17 percent for both composites—a limitation the authors attribute to the simplified pore-expansion representation and the absence of explicit fiber–matrix bond-slip and pull-out. Still, the simulated damage fields were revealing: equivalent strain, stiffness degradation, and compressive damage, initially scattered around isolated weak regions, progressively coalesced into connected damage paths as freeze–thaw pre-damage accumulated, mirroring the experimentally observed shift from distributed fine cracking to localized through-cracks.

Two-way analysis of variance confirmed that material type, cycle number, and their interaction significantly shaped mass loss, compressive strength retention, and stiffness retention, with strikingly large effect sizes—partial eta squared above 0.9 for the cycle effect on mass loss. For splitting tensile strength retention, the interaction was not significant, indicating that tensile degradation followed overall material and cycle effects rather than a material-specific trajectory. Together, the statistics reinforce the central message: fiber reinforcement does not merely slow damage, it changes the pathway by which deterioration proceeds.

The practical implications reach cold-region roads, bridges, repair overlays, and any concrete that may sit water-saturated through winter. Adding just 1 percent by volume of either fiber substantially curbed surface scaling and strength loss, with PVA offering slightly better residual strength, stiffness retention, and interfacial stability under this particular mixture design, while PP delivered comparable protection at lower cost. The authors are careful to note that the comparison is bounded by their specific mix proportions, cube geometry, and exposure regime, and that dosage optimization, salt frost exposure, flexural testing, and quantitative porosity measurements remain for future work. Even so, the study delivers a rare, unified account of frost damage—from the swelling pore to the debonding fiber to the cracking cube—and a compelling case that the humble fiber–matrix interface may be the most important square micrometers in frozen concrete.

Subject of Research: Freeze–thaw damage evolution and durability of polypropylene- and polyvinyl alcohol-fiber-reinforced cementitious composites

Article Title: Freeze–thaw damage evolution of PP- and PVA-fiber-reinforced cementitious composites: Mechanical degradation, interfacial deterioration and mesoscale modeling

Article References: Huang, H., Zhang, C., Liu, X., Xu, X., Jia, B., Gu, S., & Wang, W. (2026). Freeze–thaw damage evolution of PP- and PVA-fiber-reinforced cementitious composites: Mechanical degradation, interfacial deterioration and mesoscale modeling. Case Studies in Construction Materials, 25, Article e06594. https://doi.org/10.1016/j.cscm.2026.e06594

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06594

Keywords: freeze–thaw cycles, fiber-reinforced cementitious composites, polypropylene fibers, PVA fibers, concrete durability, interfacial deterioration, mesoscale modeling, elastic modulus degradation, scanning electron microscopy, compressive strength retention, cold-region infrastructure, damage localization

The threat itself operates through two intertwined mechanisms. The first is physical: sulfate solutions penetrate the porous network of cement through capillary action and osmosis, and as temperature and humidity fluctuate, dissolved sulfates reach supersaturation and crystallize inside capillary pores, voids, and micro-cracks. Growing crystals press relentlessly against pore walls, and once that pressure exceeds the internal cohesion of the matrix, pores expand and coalesce, the framework loosens, and surfaces begin to crack, peel, and powder. This damage opens fresh pathways for more sulfate ingress, creating a self-reinforcing cycle of erosion, deterioration, and accelerated erosion. The second mechanism is chemical and far more destructive. Sulfate ions react with calcium hydroxide to form gypsum and with calcium aluminates to generate ettringite—two expansive products that swell to roughly twice and one-and-a-half times their original volume, respectively. In the cramped interior of hardened cement, this expansion generates internal stresses that break chemical bonds between hydration products, raise porosity, propagate cracks, and ultimately strip away compressive strength, rupture strength, and interfacial bonding capacity.

Earlier work had already hinted that nanomaterials could help. Studies cited by the team showed that 1.5% nano-SiO₂ boosted the rupture strength of fly ash-cement mortar by 90%, while 0.5% raised compressive strength by 67.1% to 118.2%, and that 1% nano-CaCO₃ refined concrete’s microstructure enough to mitigate late-stage expansion damage. But nanomaterials carry a notorious drawback: their ultrafine particle size and enormous specific surface area make them prone to agglomeration during mixing, which raises water demand, degrades workability, and can undermine the very strength they are meant to enhance. The new research set out to resolve this tension by systematically comparing both nanoparticles across multiple dosages, combining macroscopic measurements of mass, compressive strength, and rupture strength with microscopic characterization by scanning electron microscopy (SEM), X-ray diffraction (XRD), and nuclear magnetic resonance (NMR).

The experimental design was rigorous and methodical. Seven mortar groups were prepared with a water-to-cement ratio of 0.55: an unmodified control, three groups with nano-SiO₂ at 1%, 3%, and 5% by mass of cement, and three groups with nano-CaCO₃ at the same dosage levels. The nano-SiO₂ particles measured about 20 nanometers with a specific surface area of 80 square meters per gram, while the nano-CaCO₃ particles were roughly 40 nanometers with 30 square meters per gram. After 28 days of standard curing and drying at 80 degrees Celsius, the 40 by 40 by 160 millimeter prisms were fully immersed in a 5% sodium sulfate solution at 20 degrees Celsius, with the solution renewed every 30 days to maintain stable concentration and pH. Specimens were retrieved at 30, 60, 90, 120, 150, and 180 days for testing, with three parallel specimens averaged for each measurement.

The mass evolution told a striking story. All groups first gained weight as invading sulfate reacted with hydration products to form pore-filling ettringite and gypsum, then lost weight as excessive expansion triggered micro-cracking, surface spalling, and detachment of erosion products. The nano-SiO₂ group at 3% dosage peaked at a 4.39% mass gain at 90 days and still retained a 3.70% mass change at 180 days, losing only 0.69% after its peak. By contrast, the 5% nano-CaCO₃ group reached a slightly higher peak of 4.55% but then shed a full 3.55% of its mass in the same window, ending at roughly 1%. After 180 days, the optimal nano-SiO₂ group’s mass change was 122.9% higher than that of the optimal nano-CaCO₃ group—a quantitative demonstration that the mass gained by nano-SiO₂ modification reflects a dense, penetration-resistant matrix, whereas the mass gained by heavily dosed nano-CaCO₃ stems from expansive ettringite that later causes severe spalling.

Compressive and rupture strength followed a three-stage trajectory of initial rise, mid-term plateau, and final decline, with peaks generally appearing between 30 and 90 days as ongoing hydration gave way to expansion damage. The 5% nano-SiO₂ group posted the highest peak compressive strength of 55.59 megapascals at just 30 days, while the 3% nano-SiO₂ group peaked at 51.47 megapascals. Crucially, strength retention revealed the deeper advantage of nano-SiO₂: at both the 1% and 3% dosage levels, nano-SiO₂-modified specimens retained more of their peak strength than their nano-CaCO₃ counterparts, with the 1% nano-SiO₂ group preserving 75% of its peak compressive strength after 180 days compared with 65.4% for the equivalent nano-CaCO₃ group. In rupture strength, the 3% nano-SiO₂ group held a retention rate of 44.4% versus 34.2% for 3% nano-CaCO₃, and its residual strength of 1.82 megapascals clearly exceeded the control group’s 1.65 megapascals. The team attributes this resilience to nano-SiO₂’s dual action: its pozzolanic reaction chemically consumes calcium hydroxide—the very ingredient sulfate ions need to form gypsum—while its micro-aggregate filling effect physically blocks ion transport channels.

The microscopic evidence sealed the argument. X-ray diffraction showed that in the unmodified control, ettringite and gypsum peaks surged between 90 and 120 days, reached maximum intensity by 150 to 180 days, and were joined by thaumasite, a notoriously destructive sulfate-bearing phase, marking the onset of accelerated deterioration. The 1% nano-SiO₂ group, by contrast, showed the lowest ettringite and gypsum peak intensities of all groups throughout the 90-to-180-day window and no obvious thaumasite signal at all, because the pozzolanic reaction had stripped the system of the calcium hydroxide needed to fuel attack reactions. Scanning electron microscopy at 10,000-fold magnification made the damage hierarchy visible: after 180 days, the control mortar displayed interconnected crack networks, spalling, and a failed hydration skeleton, while the 3% nano-SiO₂ specimen maintained a complete, continuous gel skeleton with only trace surface attack products. The overall degradation ranking ran from worst to best as control, 5% nano-CaCO₃, 3% nano-CaCO₃, 1% nano-CaCO₃, 1% nano-SiO₂, 5% nano-SiO₂, and 3% nano-SiO₂.

Nuclear magnetic resonance added a non-destructive, quantitative dimension by tracking pore size distribution through the relaxation behavior of hydrogen nuclei in pore water. At the outset, all nano-modified samples already showed lower initial porosity than the control’s 11.41%, thanks to nanoparticle filling. Porosity then fell during the first 90 days as expansive products temporarily plugged capillary channels—the 5% nano-SiO₂ group plummeting from 10.16% to a remarkable 3.97%—before rebounding after 120 days as expansion stress cracked the matrix. By 180 days, the control’s porosity had climbed back to 11.12%, nearly erasing its early densification, while the nano-modified specimens showed far smaller rebounds. Classifying pores by the widely used Wu Zhongwei system, the researchers found that the fractions of harmful pores (50 to 200 nanometers) and severely harmful pores (above 200 nanometers) grew most dramatically in the control group and most slowly in the 5% nano-SiO₂ group, whose T₂ spectra shifted least and whose harmless pore fraction remained the highest throughout.

The study’s practical verdict is nuanced: the optimal dosage depends on which performance indicator matters most. For nano-SiO₂, 3% proved best for mass stability while 5% excelled in compressive strength, rupture strength, and porosity, leading the team to recommend a 3% to 5% window. For nano-CaCO₃, 1% was optimal for mass and mechanical properties, and although 5% minimized porosity, SEM images showed clustered ettringite enrichment at that dose, with expansion stress initiating micro-cracks that no amount of physical filling could offset—so 1% stands as the overall recommendation. The fundamental difference lies in mechanism: nano-SiO₂ fights sulfate attack on two fronts, chemically through pozzolanic conversion of calcium hydroxide into dense calcium-silicate-hydrate gel and physically through pore refinement, while nano-CaCO₃ works mainly through nucleation and filling. For engineers designing structures destined for sulfate-laden coastlines, saline soils, or aggressive industrial environments, the message is clear: a few well-chosen nanometers of silica, added at just the right fraction, can mean the difference between a structure that crumbles within decades and one that endures for generations.

Subject of Research: Sulfate resistance of cement-based materials modified with nano-SiO₂ and nano-CaCO₃ nanoparticles

Article Title: Optimum dosage and long-term sulfate resistance of cement-based materials modified by nano-SiO₂ and nano-CaCO₃

Article References: Hao, E., Yuhang, L., Zhang, D., Wang, J., Ya, G., Wang, X., & Cao, Y. (2026). Optimum dosage and long-term sulfate resistance of cement-based materials modified by nano-SiO₂ and nano-CaCO₃. Case Studies in Construction Materials, 25, Article e06585. https://doi.org/10.1016/j.cscm.2026.e06585

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06585

Keywords: sulfate attack, nano-SiO2, nano-CaCO3, cement-based materials, concrete durability, ettringite, gypsum, pozzolanic reaction, pore structure, NMR, XRD, SEM

The science behind the threat begins with chemistry. Steel bars embedded in concrete are protected by a passive oxide film sustained by the high alkalinity of the pore solution. Carbon dioxide from the atmosphere diffuses into the concrete and reacts with the cement matrix, a process called carbonation that lowers the pH and dissolves that protective layer. Once the carbonation front, which advances from the surface inward, reaches the steel, depassivation occurs and active corrosion begins. The corrosion products occupy far more volume than the original steel, generating tensile stresses that crack the concrete cover, degrade the bond between steel and concrete, and ultimately erode the structure’s stiffness and ductility.

How fast this front moves depends on a delicate interplay of material and environmental factors: the concrete’s water-to-cement ratio, its porosity and compressive strength, the ambient temperature, the relative humidity, and the concentration of carbon dioxide in the air. The new research, published in Case Studies in Construction Materials by Chiara Pinheiro Teodoro, Rogério Carrazedo and Emilio Bastidas-Arteaga, adopts a climate-dependent carbonation model in which carbonation depth accelerates with rising temperature and elevated carbon dioxide, but slows when relative humidity exceeds roughly 65 percent. Paradoxically, the most dangerous humidity range sits between about 25 and 65 percent, where pores are moist enough for carbon dioxide to react but dry enough for gas to diffuse rapidly.

That sensitivity is precisely what makes climate change so consequential. Under most future Shared Socioeconomic Pathways, both temperature and atmospheric carbon dioxide are projected to rise throughout Brazil, while relative humidity declines across much of the country, with the strongest reductions expected in the Amazon region. The team used a dataset spanning 1961 to 2100 at a spatial resolution of 0.2 degrees, covering five emission scenarios from the ambitious SSP1-1.9 to the fossil-fueled SSP5-8.5. In many locations, the projected drops in humidity fall squarely into the critical range that accelerates carbonation, compounding the effects of warming and rising carbon dioxide.

To translate these climate signals into engineering risk, the researchers ran Monte Carlo simulations with Latin hypercube sampling, generating 100,000 samples for each combination of location, construction period, climate scenario, exposure condition and aggressiveness class. The limit state function compared the concrete cover depth against the predicted carbonation depth, treating cover, diffusivity, ageing and urban carbon dioxide factors as random variables with realistic variability. The results were striking: for structures built in 2000 under the NB 1:1978 code, the probability of depassivation exceeded 50 percent within 40 years at nearly all of the 500 sampled locations, whereas structures built in 2020 under the stricter NBR 6118:2014 mostly stayed below that threshold over the same service life.

The contrast reveals how much design codes matter. Brazil’s concrete standard has evolved from the permissive NB 1 rules of 1940 through 1978, which prescribed covers as thin as 1.5 centimeters indoors and set no water-to-cement limits, to the current NBR 6118:2023, which mandates covers up to 5 centimeters and water-to-cement ratios as low as 0.45 for the harshest environments. Structures designed under the older codes combine shallow cover with high diffusivity, leaving them doubly exposed as the climate warms. The study found that buildings erected before 2003 systematically show higher depassivation probabilities, and that the differences among future emission scenarios widen over time, meaning long-term durability decisions should never rest on a single climate projection.

Geography matters just as much as construction date. The machine learning analysis, which trained a Random Forest model on simulations from 500 representative locations, showed that inland regions with intermediate humidity and elevated temperatures are the most vulnerable. Manaus, with high temperatures and humidity often between 40 and 70 percent, shows markedly higher depassivation probabilities than Porto Alegre, where cooler temperatures and humidity above 65 percent suppress carbonation. Coastal cities such as Fortaleza and Recife benefit from persistently high humidity, although the authors caution that their results there capture only the carbonation contribution, since chloride-induced corrosion, often more severe near the sea, was outside the scope of the study.

The Random Forest model itself proved remarkably powerful, achieving a coefficient of determination near 0.999 on a random test split and an average of 0.941 in a stricter spatial validation that excluded entire geographic regions from training. It also trained in about 18 minutes, whereas the artificial neural network alternative required more than 24 hours with lower accuracy. This efficiency means engineers can now estimate corrosion initiation probability for virtually any location, construction year, exposure class and climate scenario in seconds, turning what was once a computationally prohibitive reliability analysis into a practical design tool.

The stakes are far from academic. A 2024 investigation following a bridge collapse in northern Brazil reported that at least 736 bridges in the country are in poor or critical condition, and field studies have repeatedly found that actual concrete covers in Brazilian buildings frequently fall short of design specifications, with one survey reporting nearly half of measured covers non-compliant. The new study’s national statistics show a median 50-year depassivation probability of about 2.8 percent for pre-2025 structures, dropping to 1.5 percent for those built under the stricter modern code, but with regional medians reaching 14.2 percent in the South and sharp variability across exposure classes.

The authors argue that Brazil’s current four-class exposure scheme is too coarse to capture the country’s enormous climatic diversity, and that future revisions of the durability provisions should become region-specific and climate-informed. Their framework, which could be extended to include chloride ingress and corrosion propagation, points toward a shift from prescriptive rules to performance-based design, in which cover depths and concrete quality are tailored to local climate futures rather than one-size-fits-all tables. As global emissions continue to climb, the invisible chemistry unfolding inside Brazil’s concrete may become one of the most expensive and overlooked consequences of a changing climate.

Subject of Research: Probabilistic assessment of carbonation-induced reinforcement depassivation in Brazilian reinforced concrete structures under climate change

Article Title: Probabilistic assessment of carbonation-induced depassivation in Brazilian reinforced concrete structures under climate change

Article References: Teodoro, C. P., Carrazedo, R., & Bastidas-Arteaga, E. (2026). Probabilistic assessment of carbonation-induced depassivation in Brazilian reinforced concrete structures under climate change. Case Studies in Construction Materials, 25, Article e06584. https://doi.org/10.1016/j.cscm.2026.e06584

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06584

Keywords: reinforced concrete, carbonation, corrosion, climate change, Brazil, depassivation, Monte Carlo simulation, machine learning, random forest, durability, design codes, SSP scenarios

UHPFRC is an extraordinary material by any measure. Its ultra-dense matrix dramatically restricts chloride penetration, while embedded steel fibers give it compressive strengths above 150 megapascals and tensile strengths far beyond those of conventional concrete. Earlier experiments have shown that a 60-millimeter UHPFRC layer can more than triple the load capacity of a severely corroded beam, and that U-shaped jackets encasing a beam on three sides can dramatically boost stiffness. Yet a critical complication has gone largely unexamined: not all UHPFRC behaves the same way after it cracks. The French standard NF P18-470 formally separates the family into strain-softening, low strain-hardening, and high strain-hardening grades. Strain-hardening UHPFRC keeps gaining stress after first cracking, spawning networks of fine, distributed micro-cracks and a pseudo-ductile plateau. Strain-softening UHPFRC, though still mechanically superior to ordinary concrete, sheds stress progressively once past its peak, relying on fewer, wider cracks.

Which of these behaviors a given mix displays depends on fiber type, mix proportions, admixtures, and curing conditions, and the consequences for structural rehabilitation are far from academic. Prior experimental campaigns produced apparently contradictory results: one study found that strain-hardening UHPFRC enhanced the ductility of corroded beams, while another reported that strain-softening layers actually reduced it. Because most existing research on this strengthening technique has been experimental, and because numerical modeling of corrosion-induced degradation remains notoriously difficult—with researchers split over how to represent rust expansion, bond loss, and interfacial behavior—the field lacked a reliable computational tool to systematically separate these effects. The new study, published in Case Studies in Construction Materials, was designed to close that gap.

Lead author Zhukai Tang, together with Wai-Meng Quach, Ran Feng, and Zhiyuan Chen, built a nonlinear finite element framework in ABAQUS that tackles the three hardest problems head-on. First, nonlinear spring elements spaced at 50-millimeter intervals connect reinforcement nodes to the surrounding concrete, capturing the bond-slip deterioration that corrosion causes; the springs follow calibrated constitutive laws in which maximum bond stress falls with corrosion ratio according to established models for corroded reinforcement. Second, the interface between the UHPFRC jacket and the substrate concrete is governed by a traction-separation law, whose normal and shear strengths of 19.2 and 5.9 megapascals were extracted directly from slant-shear and double-shear interface tests performed by the team, with damage initiating at a plastic displacement of 0.241 millimeters. Third, the researchers implemented two distinct post-peak tensile constitutive models for UHPFRC, one strain-hardening and one strain-softening, carefully tuned so that both materials shared an identical tensile strength of 11.4 megapascas under the same steel fiber conditions, 15-millimeter-long fibers at 0.2 millimeters diameter and a 2 percent volume fraction. This clever control variable isolates the pure effect of post-cracking behavior on structural response.

The model itself was grounded in real experiments. The team validated it against 24 corroded, UHPFRC-strengthened beams from two published four-point bending campaigns. In the first, six strengthened beams and three unstrengthened controls were exposed to 360 days of wet-dry cycling in artificial seawater, producing 2 to 3 percent corrosion of the bottom reinforcement and stirrups before being retrofitted with 15, 30, or 45 millimeter layers of strain-hardening UHPFRC with a compressive strength of 153.7 megapascals. In the second, 18 beams were corroded by impressed current to controlled ratios of 10 to 23 percent in the bottom bars, then strengthened with strain-softening UHPFRC in bottom-face or U-shape configurations at thicknesses from 20 to 60 millimeters. Across all 24 specimens, the simulated-to-experimental load ratios ranged from 0.90 to 1.08, averaging 0.994 with a standard deviation of just 0.0504, an accuracy rare in corrosion modeling, where stochastic deterioration usually wrecks convergence.

With the model validated, the researchers ran 48 simulations of simply supported beams, sweeping UHPFRC thicknesses from 10 to 60 millimeters in 10-millimeter increments, comparing bottom-face and U-shape jacket configurations, varying corrosion ratios from 5 to 30 percent, and switching between the two tensile constitutive laws. The results are striking. For uncorroded beams, increasing the strain-hardening UHPFRC thickness from 10 to 60 millimeters raised peak load capacity by 8.2 to 79.6 percent with bottom-face strengthening, and by a remarkable 27.8 to 273 percent with U-shape jackets. The strain-softening variant followed similar trends, gaining 4.5 to 59.2 percent and 26.3 to 222.2 percent respectively. In the most extreme case, a 60-millimeter strain-hardening U-shape jacket boosted flexural capacity by 273 percent, yield load by 200.1 percent, and lifted the cracking load from 19.7 kilonewtons to 210.2 kilonewtons—more than a tenfold improvement in the load at which the beam first cracks.

The mechanism behind these gains is partly geometric and partly material. Thicker jackets increase the section depth and moment of inertia, while UHPFRC’s superior tensile capacity carries stress across cracked zones and unloads the corroded steel. But the fiber-bridging effect proved especially potent for crack control: fibers mechanically anchor micro-cracks, and energy dissipated during fiber pull-out delays the coalescence of macro-cracks. Crack resistance improved even faster with thickness than raw strength did. Failure modes also evolved with the jacket. Bottom-face strengthened beams shifted from tightly concentrated midspan flexural cracking toward a more uniform flexural-shear crack pattern as thickness grew, while U-shape beams tended toward a single dominant flexural crack at peak load, a signature of UHPFRC’s crack resistance concentrating deformation once fibers began pulling out. Interfacial contact stress analysis showed the highest stresses at the jacket ends, flagging those zones as debonding initiation sites, but the U-shape configuration’s side arms relieved end-of-span stress concentrations and made debonding significantly less likely than in bottom-face strengthening.

The most consequential finding, however, concerns the two tensile behaviors. Although both materials shared the same tensile strength, the performance gap widened with thickness: peak load differences between strain-hardening and strain-softening versions grew from 5.6 to 25.2 kilonewtons for bottom-face beams, and from 2.0 to 68.5 kilonewtons for U-shape beams, as thickness increased from 10 to 60 millimeters. Yield strength gaps followed the same trajectory. The strain-hardening material’s extended hardening phase means fibers slip over a longer strain range before pull-out or fracture, sustaining load even as micro-cracks accumulate. Corrosion amplified the contrast further: as corrosion ratios climbed to 30 percent, ductility of strain-softening-strengthened beams degraded much more sharply, because those beams lean heavily on the steel reinforcement for deformation capacity, precisely the component corrosion destroys. Strain-hardening jackets preserved ductility far better. Interestingly, strain-softening beams showed marginally higher initial stiffness, attributed to their stiffer pre-cracking tensile response—small consolation given their deficits elsewhere.

Beyond the simulations, the team distilled a closed-form theoretical model for predicting moment resistance of strengthened beams, accounting for the different stress blocks that strain-hardening and strain-softening UHPFRC produce across the cracked section. Validated against the 48 simulated beams plus test data from four independent experimental studies, the formulation achieved a mean prediction-to-test ratio of 1.012 with a standard deviation of 0.097, giving designers a practical hand-calculation tool for the first time that explicitly distinguishes post-peak material classes. The authors are candid about limitations: corrosion was modeled as uniform rather than pitting, stirrup corrosion was excluded, the flexural-shear mechanism transition was not captured, and the traction-separation law awaits full sensitivity analysis. The model is therefore best suited to moderate corrosion and monotonic loading, with non-uniform corrosion, coupled degradation, and long-term performance flagged for future work.

The practical message for infrastructure owners is clear and actionable. Bottom-face UHPFRC strengthening is the more economical option and still delivers large capacity gains, making it attractive for routine rehabilitation budgets. But when maximum strength, long-term corrosion resistance after retrofit, and preserved ductility are the priorities—as they usually are for critical marine structures—the U-shape jacket in a strain-hardening UHPFRC is the superior choice, and the study shows that specifying the material’s post-cracking class is not a detail but a decisive engineering parameter. As coastal concrete assets worldwide age into their corrosion-prone decades, tools like this model give engineers a quantitative basis for choosing not just whether to strengthen, but exactly how.

Subject of Research: Finite element analysis of corroded reinforced concrete beams strengthened with strain-hardening and strain-softening UHPFRC layers

Article Title: Flexural behavior of corrosion-damaged RC beams strengthened with strain-hardening and strain-softening UHPFRC: Numerical study

Article References: Tang, Z., Quach, W.-M., Feng, R., & Chen, Z. (2026). Flexural behavior of corrosion-damaged RC beams strengthened with strain-hardening and strain-softening UHPFRC: Numerical study. Case Studies in Construction Materials, 25, Article e06486. https://doi.org/10.1016/j.cscm.2026.e06486

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06486

Keywords: UHPFRC, reinforced concrete, corrosion, finite element analysis, structural strengthening, flexural behavior, marine infrastructure, bond-slip, strain hardening, ductility, concrete durability, rehabilitation