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, conducted by Yaser Rashidi and Asghar Habibnejad Korayem, tackled a durability problem that has resisted conventional solutions for decades. When sulfate ions invade cementitious materials, two primary damaging products form. Ettringite arises when alumina-bearing hydrates such as monosulfate and calcium aluminate hydrates react with incoming sulfate in the presence of calcium hydroxide and water. Gypsum forms simultaneously through cation-exchange reactions between sulfates and calcium hydroxide. Both phases occupy greater solid volume than the phases they replace, so the matrix expands, cracks, and then admits even more sulfate in a self-accelerating feedback loop. Over time the cohesion of the hydration products collapses, and strength and mass drain away.

Engineers have traditionally fought back along three fronts: limiting the reactive tricalcium aluminate content of cement as in ASTM C150 Type II and Type V sulfate-resistant cements, blending in supplementary cementitious materials such as fly ash, slag, metakaolin, and silica fume to consume calcium hydroxide pozzolanically, and reducing permeability through lower water-to-binder ratios or fine mineral additives that tighten the pore network. Each strategy helps, but none fully solves the problem, because residual calcium hydroxide remains available for gypsum formation, and even well-refined matrices still transport ions. Nanoparticles promised a fourth front, yet materials such as nano-silica, carbon nanotubes, and graphene oxide carry significant production costs and environmental burdens, and their effectiveness depends critically on achieving good dispersion.

That is where halloysite nanotubes stand out. With outer diameters of roughly 30 to 70 nanometers and lengths of 1 to 3 micrometers, these natural tubes have enormous specific surface area, around 640,000 square centimeters per gram in this study, and an aluminosilicate composition of about 48 percent silica and 35 percent alumina. They act on cement through three complementary mechanisms: physical filling of the fine interstitial spaces between cement grains, heterogeneous nucleation that gives hydration products abundant surfaces on which to grow, and gradual pozzolanic reactions that consume calcium hydroxide and generate additional calcium silicate hydrate gel. Unlike carbon nanotubes or graphene oxide, they require no energy-intensive synthesis, chemical oxidation, or high-temperature functionalization; they are simply mined and processed.

A crucial subtlety the researchers confronted is that nanoparticles tend to clump in the brutal environment of cement pore solution. With a pH of 13.1 to 13.5 and a high concentration of dissolved ions, the synthetic pore solution used in the study compresses the electrical double layers around particles, collapsing the electrostatic repulsion that keeps them apart. Without help, ultrasonication only temporarily breaks agglomerates apart; they re-form within hours. The team’s answer was polycarboxylate ether, or PCE, the same high-range water-reducing admixture routinely used to fluidize concrete. PCE molecules adsorb onto halloysite surfaces through their anionic functional groups while their long polyethylene glycol side chains form a steric barrier, physically preventing neighboring tubes from touching.

The dispersion experiments were elegant in their design. Suspensions matching the dosages used in the mortars were mixed 1:1 with synthetic pore solution and tracked by visual sedimentation, ultraviolet-visible spectroscopy, and dynamic light scattering. The results were unambiguous. Without PCE, the suspension’s normalized UV-Vis absorbance collapsed from 1.0 to just 0.04 within six hours, and its average hydrodynamic particle diameter ballooned from 972 to 2,750 nanometers as particles aggregated and settled. With PCE, absorbance stayed high and particle growth was minimal, with the two lower-dose suspensions growing only from about 409 to 513 and 486 to 578 nanometers respectively. The highest-dose suspension, corresponding to 3 weight percent HNTs, showed slightly reduced stability, an early warning sign that would echo in the mortar performance.

The mortars themselves told the rest of the story. Replacing 1, 2, or 3 weight percent of ordinary Portland cement with PCE-stabilized HNTs, at a fixed water-to-binder ratio of 0.4 and identical workability, the researchers exposed mortar bars and cubes to a 5 percent sodium sulfate solution for 182 days following ASTM C1012. The plain reference mortar expanded by 0.084 percent over 26 weeks, above the 0.05 percent threshold that ASTM C1157 classifies as non-resistant to sulfate. The 1 percent HNT mixture fared little better at 0.080 percent. But the 2 and 3 percent mixtures stayed below the threshold at 0.032 and 0.043 percent respectively, a genuinely high level of sulfate resistance achieved purely by a clay additive.

The mechanical evidence was equally compelling. Before exposure, compressive strengths rose from 42.6 megapascals in the reference to 52.9 megapascals with 2 percent HNTs. After 182 days of sulfate immersion, the reference mortar had lost 18.8 percent of its compressive strength and a striking 29.9 percent of its flexural strength, while its ultrasonic pulse velocity, a proxy for internal integrity, dropped 9.6 percent. The 2 percent HNT mortar lost only 5.2 percent compressive and 13.1 percent flexural strength, and its pulse velocity actually increased by 3.3 percent, meaning sulfate products filling refined pores left it more acoustically dense than before. Scanning electron microscopy revealed denser, crack-poor microstructures before exposure and far fewer needle-like ettringite crystals afterward. X-ray diffraction showed progressively weaker ettringite and gypsum peaks with increasing HNT content, and Fourier-transform infrared spectra confirmed the pattern: weaker sulfate-related bands at 1110, 670, and 550 inverse centimeters, and a stronger calcium silicate hydrate band at 982.

Why does a humble clay tube outperform? The authors attribute it to synergy. Chemically, diluting the clinker reduces the reactive aluminate supply, while pozzolanic consumption of calcium hydroxide starves both the gypsum and ettringite reactions of raw material. Physically, the tubes pack into pores, nucleate hydration, and even act as internal water reservoirs whose hydrophilic bores release moisture gradually to sustain curing. The result is a matrix whose pore connectivity is so reduced that sulfate ions simply cannot reach their targets in damaging quantities. The one caveat is dosage discipline: at 3 weight percent, agglomeration created weak zones, and every measured property slipped slightly below the 2 percent optimum, a direct macroscopic echo of the dispersion tests.

Perhaps the most consequential finding is the sustainability comparison. Compiled literature data show halloysite carrying a global warming potential of roughly 5 to 6.6 kilograms of carbon dioxide equivalent per kilogram, versus 28.55 for carbon nanotubes produced by chemical vapor deposition and 8.55 for graphene oxide, and an embodied energy of about 21 megajoules per kilogram versus 480 for CNTs and 120 to 140 for graphene oxide. At around 0.7 dollars per kilogram, HNTs cost less than a tenth of nano-silica and a tiny fraction of carbon-based alternatives. For a construction industry desperately seeking durable, low-carbon materials, the message is remarkable: a naturally abundant clay mineral, stabilized by an admixture already in every ready-mix truck, can match the sulfate resistance of engineered nanomaterials while leaving a fraction of their environmental and economic footprint. The next generation of sulfate-proof infrastructure may literally be dug from the ground.

Subject of Research: Use of halloysite nanotubes to enhance the sulfate resistance and sustainability of cementitious composites

Article Title: Sustainable halloysite nanotubes for sulfate–resistant cementitious composites: Physicochemical mechanisms and environmental advantages

Article References: Rashidi, Y., & Korayem, A. H. (2026). Sustainable halloysite nanotubes for sulfate–resistant cementitious composites: Physicochemical mechanisms and environmental advantages. Case Studies in Construction Materials, 25, Article e06582. https://doi.org/10.1016/j.cscm.2026.e06582

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06582

Keywords: halloysite nanotubes, sulfate attack, cement mortar, durability, nanomaterials, ettringite, gypsum, pozzolanic reaction, polycarboxylate ether, sustainable construction, pore refinement, embodied energy

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 geopolymer mix, designated GEO, is built from ground granulated blast-furnace slag and silica fume, activated by a sodium silicate and potassium hydroxide solution rather than by any Portland cement. It achieves a compressive strength of 152 megapascals, comfortably above the 120 megapascal threshold that defines the UHPC class, and incorporates 1.5 percent by volume of steel fibers. Its Portland cement reference, designated REF, reaches 165 megapascals. The two binders differ at the molecular level in a way that matters enormously for durability. Portland cement hardens into calcium silicate hydrate gel accompanied by portlandite, a calcium hydroxide phase that is notoriously vulnerable to acid dissolution and to the sulfate reactions that produce expansive ettringite crystals. The geopolymer instead forms a potassium-containing calcium aluminosilicate hydrate gel, C-(K)-A-S-H, in which portlandite is entirely absent. Quantitative X-ray diffraction of the hardened GEO binder confirmed the point: nearly 40 percent amorphous aluminosilicate gel, quartz and calcite as the main crystalline phases, and ettringite present only as a trace of about one percent by weight.

The laboratory program subjected prisms of both mixes to a year of immersion in five solutions: distilled water, five percent sodium sulfate, five percent magnesium sulfate, nitric acid held at pH 3, and a combined magnesium and sodium sulfate bath designed to mimic the cation pairing of real seawater, albeit at accelerated concentrations. The sodium sulfate results were striking. The Portland cement reference expanded by up to 0.16 percent, the classic signature of ettringite-driven sulfate attack, while the geopolymer actually contracted slightly, ending near minus 0.05 percent. With its aluminum locked into the gel structure and no monosulfoaluminate to react, the geopolymer simply lacks the raw material for the expansive reaction. Under nitric acid the story repeated itself: the geopolymer showed negligible dimensional change, whereas the reference swelled by more than 0.2 percent at its peak, consistent with acid attack on portlandite and the formation of secondary products.

But the trial also exposed a genuine vulnerability. In both magnesium-bearing solutions the geopolymer expanded beyond the 0.10 percent criterion that ASTM C1012 uses to flag sulfate-susceptible behavior, peaking at 0.29 percent in the combined bath, the condition most representative of seawater chemistry, where the reference actually contracted. The researchers attribute this to magnesium ions exchanging with calcium within the calcium-rich gel, destabilizing the binding phase, and to brucite precipitation at specimen surfaces. The lesson is precise: for this class of binder, magnesium rather than sulfate as such is the principal chemical enemy, a finding that gives future formulators a clear target, namely lowering the calcium content of the gel through partial substitution of the slag with fly ash or metakaolin.

Chloride resistance, the parameter that governs reinforcement corrosion in marine structures, told a more nuanced story. In the rapid chloride penetration test the geopolymer passed an average of 254 Coulombs, classified as very low penetrability, while the reference recorded 43 Coulombs, classified as negligible. Both sit far below the 1000 Coulomb boundary for low penetrability, and both are orders of magnitude better than the 2000 to 4000 Coulombs typical of ordinary structural concrete. The derived diffusion coefficients, around 6.9 times ten to the minus thirteen square meters per second for the geopolymer and 1.2 times ten to the minus thirteen for the reference, both fall in the characteristic UHPC range. The authors caution, however, that the test measures total ionic current, and the potassium-rich pore solution of the geopolymer conducts far more readily than a Portland cement pore solution, so the raw charge figures likely overstate the true gap in chloride transport between the two binders.

Electrical impedance spectroscopy provided a continuous, non-destructive window into the microstructure throughout the year. Monitoring the impedance of instrumented specimens across all exposure solutions, the team found that both materials showed steadily increasing bulk resistance over 365 days, a sign of continued pore refinement rather than degradation. The geopolymer’s impedance was essentially unaffected by sulfate solutions compared with the water control, while the reference showed reduced impedance in sodium sulfate, an early indicator of sulfate ingress into its pore network. Most intriguingly, in nitric acid the geopolymer’s impedance rose to the highest value of any specimen, suggesting that the acid selectively dissolves the most soluble surface phases and leaves behind a densified, protective aluminosilicate layer that impedes further attack.

Elevated temperature was the one arena where the geopolymer clearly lost ground. After heating to 600 degrees Celsius, the reference retained 90 percent of its ambient compressive strength, while the geopolymer dropped to 36 percent, a result the authors link to the calcium-rich gel behaving more like conventional hydrates than like the thermally robust, low-calcium gels of fly ash geopolymers, combined with a coarser pore network that permits steeper thermal gradients and microcracking. Yet the comparison carries an important caveat: at 600 and 900 degrees Celsius only a single reference specimen survived heating in testable condition, whereas every geopolymer specimen remained intact, with no explosive spalling even at 900 degrees, where the reference spalled severely. Since sustained exposure to 600 degrees is not a realistic offshore service condition, the finding defines a known limit of the formulation rather than a barrier to its intended use, and points to remedies such as pore refinement and hybrid fiber systems.

The decisive test came at HarshLab, a floating laboratory in the Cantabrian Sea operated by Tecnalia in Bilbao, Spain. Geopolymer prisms were deployed for twelve months across three exposure zones, atmospheric, splash, and fully immersed, some uncoated and some carrying bio-based coatings developed within the project for antifouling purposes. On retrieval, the material had lost nothing that matters. Flexural strength ranged from 14 to 17 megapascals across all groups, essentially unchanged from the 28-day reference value, and compressive strength remained between 150 and 167 megapascals, in several cases exceeding the original laboratory figure, apparently because continued geopolymerization in the moist marine environment offset any chemical degradation. Phenolphthalein spraying showed no carbonation anywhere, and silver nitrate colorimetry revealed no chloride penetration front even in permanently submerged specimens. Electrical resistivity after exposure, between 560 and 1610 ohm-meters, remained an order of magnitude above the threshold for negligible corrosion risk.

The convergence of independent indicators, mechanical retention, colorimetric evidence, and electrical resistivity, is what makes this study persuasive. Accelerated laboratory tests can exaggerate or misrepresent durability mechanisms, but here the field campaign confirmed the laboratory picture: the geopolymer sustained its full UHPC mechanical class through a year of real Atlantic service with no measurable transport-related degradation. The authors are candid about limitations, including the single production batch, the small number of specimens in some groups, the absence of pore-solution normalization in the impedance data, and the fact that the field campaign covered only the geopolymer, since the reference mix was assessed separately by a project partner. Twelve months is also short against the decades-long service life of an offshore structure, and the magnesium sensitivity observed in the laboratory remains microstructurally unverified.

Even so, the implications reach well beyond floating solar platforms, the immediate target of the NaturSea-PV project. Offshore photovoltaics need substructures that avoid land-use conflicts and benefit from water cooling of panels, but they demand concrete that can endure seawater, waves, and decades of chemical assault with a fraction of the embodied carbon of Portland cement. This study demonstrates that a geopolymer binder can deliver UHPC-class strength, proven marine durability, and clear advantages under sodium sulfate and acid exposure, while identifying magnesium sensitivity and high-temperature strength retention as the principal remaining weaknesses. With a patent application covering the exact formulation and follow-up work already mapped out, from lowering the gel’s calcium ratio to refining the pore structure, the cement-free concrete that spent a year in the Bay of Biscay and came back stronger may be one of the most credible routes yet toward genuinely sustainable marine infrastructure.

Subject of Research: Durability of a cement-free ultra-high performance geopolymer concrete compared with Portland cement UHPC under laboratory chemical exposure and real marine conditions

Article Title: Durability of a cement-free ultra-high performance geopolymer concrete: A laboratory comparison with Portland-cement UHPC validated by real marine exposure

Article References: Sleiman, Y., El Oifi, B., Martin, C., Saiyouri, N., & Sbartaï, Z. M. (2026). Durability of a cement-free ultra-high performance geopolymer concrete: A laboratory comparison with Portland-cement UHPC validated by real marine exposure. Case Studies in Construction Materials, 25, Article e06583. https://doi.org/10.1016/j.cscm.2026.e06583

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06583

Keywords: geopolymer concrete, ultra-high performance concrete, marine durability, sulfate attack, chloride ingress, Portland cement, floating photovoltaics, alkali-activated materials, HarshLab, NaturSea-PV, electrical impedance spectroscopy, sustainable construction