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

The work tackles a looming supply problem at its root. Alkali-activated materials, which harden when alkaline solutions dissolve aluminosilicate-rich precursors into binding gels, have long relied on ground granulated blast furnace slag, a steelmaking byproduct prized for its rapid reactivity and its ability to form dense calcium-aluminosilicate-hydrate gels. But global slag production sits at only about 330 million tonnes per year, and its share of total cement production has fallen from roughly 17 percent in 1980 to nearly 8 percent in 2014. As steelmakers decarbonize and slag gets snapped up for cement blending, researchers are racing to find locally available substitutes—and the Algerian team turned to materials that would otherwise clog landfills.

The first substitute, ceramic sanitary ware waste, is chemically ideal: it is rich in silicon dioxide (62.7 percent) and aluminum oxide (28.5 percent), the raw ingredients of aluminosilicate reaction networks. The second, eggshell powder, is almost pure calcium carbonate, with an oxide-equivalent calcium oxide content of 55.4 percent. Both were crushed and ground for one hour, with the ceramic powder emerging as the finest precursor at a median particle diameter of about 10 micrometers and the eggshell powder the coarsest at roughly 30 micrometers. Scanning electron microscopy revealed sharp, flaky ceramic fragments and porous, loosely packed eggshell platelets—morphologies that would prove decisive in how each material behaved in the fresh and hardened mortar.

Rather than testing mixes haphazardly, the researchers employed a central composite design, a response-surface statistical framework that maps the effects of two variables—ceramic and eggshell replacement levels, each varied from 0 to 25 percent of the slag content—using just nine experimental runs. Crucially, the alkalinity of the activator, a blend of 10-molar sodium hydroxide and sodium silicate, was held nearly constant across all formulations, at an alkali equivalent of about 17.1 percent. That design choice ensured that any differences in performance could be attributed to the precursor chemistry itself rather than to varying activator dosages, giving the study its statistical teeth.

The results revealed a striking trade-off between early and long-term performance. Ceramic waste slowed early strength gain, because its stable aluminum-oxygen-silicon bonds dissolve sluggishly in alkaline solution, delaying the release of reactive species. Yet by 28 days the picture had reversed: the mix containing 12.5 percent ceramic waste reached a compressive strength of 84.82 megapascals, exceeding the pure-slag reference at 80.41 megapascals, as the slowly dissolving ceramic steadily fed silica and alumina into the growing gel network and densified the microstructure. Eggshell powder behaved differently—moderate amounts boosted early strength through a filler effect, with one mix hitting 47.94 megapascals at just 7 days, but at 25 percent replacement strength collapsed to as low as 59.81 megapascals, a casualty of the eggshell’s low reactivity and its porous, irregular particles that introduce weak zones into the matrix.

Durability against carbonation—the process by which atmospheric carbon dioxide neutralizes the alkaline phases that protect steel reinforcement—emerged as the study’s most nuanced finding. Carbonation chemistry in alkali-activated systems differs fundamentally from that in Portland cement: in calcium-rich slag gels, carbon dioxide extracts calcium ions, progressively decalcifying the binding phase and shifting it toward brittle, silica-rich residues, while in low-calcium systems the pore solution is neutralized with less structural damage. After 28 days in a chamber at 3 percent carbon dioxide, the ceramic-rich mixes fared worst, with several showing near-complete alkalinity loss and strength losses up to 30.79 percent. The eggshell-containing mixes, by contrast, resisted degradation remarkably well: the best one lost only 12.05 percent of its strength, because the abundant calcium supplied by the eggshell buffered the carbon dioxide attack, stabilizing the gel structure and even plugging pores with newly precipitated calcium carbonate.

Spectroscopic and thermal analyses laid bare the molecular drama unfolding inside the carbonated samples. Infrared spectroscopy showed the main silicon-oxygen stretching band migrating to higher wavenumbers after carbonation, and Gaussian deconvolution revealed that the fraction of highly polymerized silicon units jumped from 31 to 47 percent in the pure-slag mix—an apparent increase in connectivity that actually signals degradation, as calcium-rich units are stripped away and replaced by condensed but mechanically feeble silica residues. Energy-dispersive X-ray analysis confirmed that calcium-to-silicon ratios in the binding gels plummeted from values as high as 4.98 down to roughly 0.7 to 0.82 after exposure, while carbon-to-silicon ratios soared, marking wholesale carbonate formation. In the pure-slag sample, the team even identified elongated sodium-carbonate crystals containing about 23 percent sodium and 56 percent carbon by atomic content—direct evidence of alkali ions migrating out of the destabilized gel and reacting with the invading carbon dioxide.

Feeding all measured responses into a multi-objective desirability optimization, the models converged on an ideal formulation containing roughly 19 percent ceramic waste and 16 percent eggshell powder, with a composite desirability of 0.876. This predicted optimum promises a 28-day strength of about 81.6 megapascals, porosity of 16.4 percent, and a carbonation-induced strength loss capped near 17.7 percent—figures the authors caution remain model predictions pending experimental validation. A sensitivity analysis across fifteen weighting scenarios barely moved the outcome, suggesting the optimum is robust. The environmental accounting, estimated using Bond’s law for grinding energy under both nuclear-based and gas-based electricity grids, placed the optimized binder’s embodied carbon at roughly 136 to 230 kilograms of carbon dioxide per cubic meter, averaging about 183 kilograms—well below ordinary Portland cement, calcium aluminate cement, calcium sulfoaluminate cement, and even limestone calcined clay systems when normalized by strength.

The implications ripple well beyond the laboratory. If a third of the slag in an alkali-activated binder can be swapped for bathroom ceramics and breakfast leftovers without sacrificing strength or durability, the approach could simultaneously relieve landfill pressure, hedge against future slag scarcity, and slash the carbon intensity of construction in regions where industrial byproducts are scarce but household waste is abundant. The authors are careful to note the caveats: the carbonation indicator used does not directly measure true corrosion risk, the optimum has not yet been experimentally cast, and the carbon analysis excludes transportation and collection. But as a proof of concept, the study makes a compelling case that the future of low-carbon construction may be hiding in demolition dumpsters and kitchen compost bins—waiting only for the right chemistry to set it in stone.

Subject of Research: Multi-performance optimization of alkali-activated mortars using ceramic and eggshell waste precursors with carbonation durability and carbon footprint assessment

Article Title: Multi-performance optimization of ceramic and biogenic wastes-based alkali-activated mortars with environmental benefits and carbonation durability assessment

Article References: Berkouche, A., Belkadi, A. A., Chiker, T., Cousture, A., & Aggoun, S. (2026). Multi-performance optimization of ceramic and biogenic wastes-based alkali-activated mortars with environmental benefits and carbonation durability assessment. Case Studies in Construction Materials, 25, Article e06554. https://doi.org/10.1016/j.cscm.2026.e06554

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06554

Keywords: alkali-activated materials, ceramic waste, eggshell powder, ground granulated blast furnace slag, carbonation, compressive strength, central composite design, desirability optimization, carbon footprint, sustainable construction, C-(N)-A-S-H gel, microstructural analysis

The research team, working with basic oxygen furnace (BOF) slag from a steel plant in Xinjiang, started from an observation that seems obvious in hindsight but has been overlooked in industrial practice: steel slag particles are not uniformly hard. When the researchers ran coarse slag through a planetary ball mill under deliberately mild conditions, which they call light ball milling, the material split into two distinct populations. A fraction of the particles crumbled readily into powder fine enough to pass through a 75-micrometer sieve, while the remaining hard particles were only reduced from large chunks to smaller ones without ever fragmenting into ultrafine material. By adjusting grinding time and energy input, the team could separate these two fractions and study each in detail.

The numbers behind this separation are striking. Light grinding for just ten minutes, using iron balls as the grinding medium so they could later be pulled away with an electromagnet, revealed that the easily pulverizable component makes up roughly 30 percent of the slag by mass. Within the 0.5 to 1 millimeter size range, more than 90 percent of the material is easily pulverizable, while larger particles from 1 to 20 millimeters still contain 12 to 25 percent of the soft fraction. The resulting powder is extraordinarily fine, with a median particle size between roughly 4 and 10 micrometers and a specific surface area approaching 2900 square meters per kilogram, comparable to ultrafine cements that normally demand intensive, energy-hungry grinding.

Why do some particles yield so easily while others resist? The answer lies in mineralogy, which the researchers probed with quantitative X-ray diffraction, X-ray photoelectron spectroscopy, and thermal analysis. The easily pulverizable fraction is dominated by dicalcium silicate, tricalcium silicate, tetracalcium aluminoferrite, magnetite, and magnesium oxide. The hard fraction, by contrast, is rich in the same calcium silicates but also contains free calcium oxide and iron(II) oxide, and it completely lacks detectable magnesium oxide. Spectroscopy of the iron states told the same story from a different angle: iron in the soft fraction is predominantly trivalent, at about 64.6 percent, while iron in the hard fraction is mostly divalent, at 63.4 percent. The team attributes this divide to the oxidation conditions during slag cooling. An oxidizing environment converts Fe(II) to magnetite, which suppresses the association between magnesium oxide and iron(II) oxide; under reducing conditions those phases combine into a rigid, poorly grindable framework, and free calcium oxide becomes encapsulated between silicate crystals.

The carbonation chemistry at the heart of this work exploits the fact that calcium silicate minerals react spontaneously with carbon dioxide and moisture, forming calcium carbonate and silica gel that bind particles together into a solid mass. Unlike ordinary hydration, carbonation of steel slag proceeds faster, delivers higher early strength, permanently sequesters CO2, and reduces alkalinity. Previous studies have used this reaction to make artificial gravel, sulfate-resistant blocks, plant-friendly porous concrete for ecological slope protection, and even materials that immobilize lead and zinc in contaminated soil. The obstacle has always been grinding: the very minerals that resist fragmentation, particularly dicalcium silicate, are locked inside hard particles, so producing fine slag powder conventionally means grinding everything, including the refractory majority, at enormous energy cost.

To compare performance, the researchers pressed each fraction into cylindrical specimens with water, compacted them at 2 megapascals, and cured them in a chamber filled with 99.9 percent CO2 at 0.1 megapascal. The hard fraction powder was prepared for fair comparison by intensive grinding to a specific surface area of about 105 square meters per kilogram, matching powders used in earlier studies. Strength measurements showed that both fractions follow the classic carbonation pattern in which strength rises with moisture content to a peak and then falls, but the optima differ sharply. The easily pulverizable powder reaches maximum carbonation strength at 16 percent moisture, substantially higher than the 10 percent optimum of the hard powder, a difference the authors attribute mainly to its much greater fineness, with mineralogy a possible secondary factor.

The performance gap that emerged was dramatic. Specimens made from the easily pulverizable fraction gained strength rapidly within the first three hours of carbonation and stabilized at 73.9 megapascals after 24 hours, roughly 1.5 times the 50.7 megapascals achieved by the hard fraction, despite actually containing a lower proportion of carbonation-active calcium silicates. Carbon dioxide uptake followed the same trend, with the soft fraction absorbing more CO2 at every curing duration. Thermal gravimetric analysis quantified the point: after just 3 hours of carbonation, the easily pulverizable fraction had already produced as much calcium carbonate, about 16.4 percent by mass, as the hard fraction generated after a full 24 hours. Electron microscopy explained the mechanical advantage. The calcium carbonate product in the soft fraction forms spindle-shaped crystals about 0.1 micrometers long, some 30 times smaller than the roughly 3-micrometer crystals in the hard fraction, and these fine products fill micropores more effectively, yielding a total porosity of only 17.5 percent versus 32.4 percent for the carbonated hard fraction.

The broader implication is a smarter, cheaper route to carbon-negative construction materials. Instead of grinding the entire slag mass, which risks over-grinding the hard particles into fragments that actually coarsen the powder and raise the carbon footprint, processors could lightly mill the slag, magnetically remove the grinding media, and sieve out the roughly 30 percent soft fraction as a ready-made ultrafine feedstock for carbonated cements. The team also showed that over-grinding can be detected by monitoring the growth rate of fine powder or the specific surface area as energy accumulates, giving operators a practical warning signal. The hard remainder, meanwhile, could find use as asphalt aggregate, where the authors note its selective removal may actually improve performance.

The researchers are careful about limits. Their slag came from a single BOF source, and they recommend validating the approach across different plants and slag chemistries. They also note that the study cannot fully disentangle how much of the soft fraction’s advantage comes from fineness versus mineral composition, and that definitive criteria for identifying over-grinding remain to be established, with mineralogical analysis or iron valence measurements proposed as future diagnostic tools. Still, the core message stands: steel slag is heterogeneous, that heterogeneity is exploitable with minimal energy, and exploiting it could turn one of the steel industry’s biggest waste streams into a workhorse for carbon sequestration in the built environment, a small but meaningful step toward China’s dual carbon goals and the global effort to decarbonize construction.

Subject of Research: Selective light ball milling of BOF steel slag to separate easily pulverizable, carbonation-active components from hard particles for low-carbon cementitious materials

Article Title: Enhanced carbonation performance of pulverizable components in BOF steel slag separated by light ball milling: Comparative analysis of physical properties and process implications

Article References: Yu, C., Zheng, G., Han, D., Cui, J., Chen, W., Ran, W., Cheng, L., & Wu, Z. (2026). Enhanced carbonation performance of pulverizable components in BOF steel slag separated by light ball milling: Comparative analysis of physical properties and process implications. Case Studies in Construction Materials, 25, Article e06501. https://doi.org/10.1016/j.cscm.2026.e06501

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06501

Keywords: steel slag, carbonation, ball milling, CO2 sequestration, low-carbon cement, calcium carbonate, mineralogy, construction materials, basic oxygen furnace slag, grinding energy, calcium silicate, sustainable building