Accelerated carbonation curing exposes newly cast cementitious material to concentrated carbon dioxide, promoting carbonate formation and densification of the near-surface pore network. The process can consume some of the carbon dioxide released during cement manufacture while also increasing early-age strength. The researchers built on their earlier work with the same mixtures by extending the assessment beyond chloride diffusion and depassivation. Their semi-empirical hybrid framework divides deterioration into a diffusion-controlled stage, Pd, ending when chloride at the reinforcement reaches an adopted threshold; a corrosion-controlled stage, Pco, ending when calculated corrosion current becomes sustained and measurable; and a mechanical-controlled stage, Pcr, ending when surface cracking reaches 0.2 millimeters. These are engineering transitions rather than isolated physical stages, because chloride ingress, corrosion and cracking continue to overlap.

For the diffusion stage, the team used Fick’s second law with chloride coefficients measured in earlier rapid chloride penetration tests on identical mixtures. The model assumed a 50-millimeter concrete cover, a surface chloride concentration of 1.8 percent by binder mass and a critical threshold of 0.4 percent by binder mass. The threshold is an engineering reference rather than a direct measurement of passive-film breakdown. At 28 days, the non-steady-state migration coefficients were 21.9 × 10^-12 square meters per second for ordinary Portland cement and 11.2 × 10^-12 after carbonation curing. For fly-ash mortar, the values were 7.0 × 10^-12 without curing and 7.9 × 10^-12 with curing; for the slag blend, they were 3.1 × 10^-12 and 5.3 × 10^-12, respectively. Thus, carbonation lowered the coefficient for ordinary cement but raised it for both blended systems, indicating reduced chloride transport resistance in the fly-ash and slag mixtures under the investigated curing conditions.

The electrochemical model coupled ionic conduction through the pore solution, anodic dissolution of iron, cathodic oxygen reduction and diffusion-limited oxygen supply. Chloride ingress changed the free-chloride concentration at the steel surface and therefore the anodic reaction kinetics. A two-dimensional finite element representation included the ribs of a 10-millimeter deformed bar so that geometric discontinuities could act as localized corrosion sites, approximating the non-uniform anodic and cathodic regions seen in reinforced structures. The model calculated the corrosion potential and local current densities while enforcing charge conservation. It did not prescribe corrosion current directly. Pco was defined as the time when the maximum local current density reached 0.5 microamperes per square centimeter, a criterion associated with the boundary between low-to-moderate and moderate-to-high corrosion activity. The predicted corrosion-current histories for the blended mixtures plateaued only slightly above this threshold, so Pco was particularly sensitive to the selected criterion.

To establish the final cracking criterion, the researchers tested reinforced mortar specimens that had been water-cured for one year. A constant 5-volt impressed current accelerated corrosion in 3 percent sodium chloride solution, while linear polarization resistance, electrochemical impedance and crack width were monitored. Cumulative steel loss was estimated from the applied charge using Faraday’s law. The specimens showed an initial period in which corrosion mass accumulated without visible cracking, followed by rapid crack growth; alternating-current resistance fell sharply and corrosion current rose as cracking developed. Near the 0.2-millimeter criterion, the measurements were scattered because corrosion products formed unevenly and some specimens developed several cracks instead of one dominant path. The researchers therefore used the mean mass loss associated with the target width, approximately 0.9 grams, as a common comparative threshold rather than a universal material constant. Six representative specimens were selected across three test batches because comparable single-crack behavior was difficult to obtain consistently.

The experimental threshold was also the main bridge to the 50-millimeter numerical cover. The test specimens had 20 millimeters of cover, so the model multiplied the threshold by 2.5, producing a target natural-exposure mass loss of 2.25 grams. This linear scaling assumed that fracture energy, reinforcement expansion and critical mass loss were directly proportional to cover depth. It simplifies nonlinear fracture and omits stochastic variation, and the accelerated test also produced corrosion products that may differ from those formed under natural chloride exposure. The researchers applied the same threshold to all six mixtures to support comparison, not absolute prediction. They also used literature-derived electrochemical constants rather than system-specific measurements and assumed complete Faradaic efficiency during accelerated testing, leaving uncertainty from possible side reactions unquantified. The model stopped at crack initiation, so it did not simulate how an open crack subsequently changes transport, resistivity or corrosion rate.

Across the modeled sequence, carbonation curing produced the same directional effect at Pd, Pco and Pcr within each binder system: it improved ordinary Portland cement and shortened the fly-ash and slag blends. For ordinary cement, the delay between reaching the chloride threshold and developing sustained corrosion was very short, indicating nearly immediate active corrosion after depassivation in the model. Blended systems remained passive-like for much longer and therefore retained substantially more life after chloride arrival. Their predicted Pco values were about 100 years for the uncarbonated fly-ash mortar and 104 years for the uncarbonated slag blend in the baseline analysis. Pcr occurred far later than Pd and Pco, showing that the interval of active corrosion before cover cracking can occupy a large share of total durability. Carbonation curing’s modest benefit for ordinary cement was therefore accompanied by a much larger difference between plain and blended binders.

Sensitivity analysis showed that the absolute stage times were highly dependent on the thresholds chosen to define them, while the comparative conclusions were more stable. Lowering the adopted chloride threshold from 0.4 to 0.3 percent shortened Pd by 19–30 percent, whereas raising it to 0.6 percent extended Pd by 41–85 percent. Changing the corrosion-current criterion from 0.4 to 0.6 microamperes per square centimeter shortened Pco by 17–27 percent or extended it by 76–78 percent for ordinary-cement systems. Several blended systems did not reach the higher criterion within the 151-year simulation window, illustrating the limited precision of Pco as an absolute measure. By contrast, a 20 percent change in the mass-loss threshold altered Pcr by approximately 12–19 percent, and varying the crack-width criterion from 0.1 to 0.3 millimeters changed Pcr by roughly 17–26 percent in the lower direction or 14–21 percent in the higher direction. For the perturbations that could be evaluated, the direction of carbonation-curing effects within each binder system was retained, although some rankings involving unreached or insufficiently long histories remained unresolved.

The framework’s practical value lies in extending durability assessment beyond a diffusion-only endpoint without requiring a computationally intensive fracture-mechanics model. Its finite element analysis handles electrochemical behavior and local rib geometry, while the accelerated test supplies a manageable cracking threshold. Still, the study does not demonstrate that carbonation curing will extend the life of an existing reinforced-concrete structure. The mortar lacked coarse aggregates and aggregate-paste interfacial zones that can change chloride pathways and crack confinement; exposure was represented as constant chloride without wetting and drying; the carbonated and uncarbonated layers were represented by one effective transport coefficient; and no natural-exposure validation was performed. The study therefore supports a narrow, comparative conclusion: under one day of sealed pre-curing followed by one day in 10 percent carbon dioxide at 55 percent relative humidity and 20 degrees Celsius, carbonation curing improved ordinary-cement durability in the modeled assessment but reduced chloride resistance and modeled service life in the tested fly-ash and slag blends. Recalibration with concrete-specific transport data and cracking thresholds, plus validation under natural exposure, is required before the method can guide absolute life prediction.

Subject of Research: Modeling accelerated carbonation curing effects on chloride ingress, steel corrosion and concrete cover cracking in mortar

Article Title: From chloride transport to cover cracking: A semi-empirical service life assessment of accelerated carbonation-cured concrete

Article References: Liyew, G., Alemu, A. S., Lee, H. K., Kim, J., Park, K.-T., Kim, G., & Kim, H.-K. (2026). From chloride transport to cover cracking: A semi-empirical service life assessment of accelerated carbonation-cured concrete. Case Studies in Construction Materials, 25, Article e06632. https://doi.org/10.1016/j.cscm.2026.e06632

Image Credits: AI Generated

DOI: 10.1016/j.cscm.2026.e06632

Keywords: accelerated carbonation curing, reinforced concrete, chloride ingress, steel corrosion, cover cracking, ordinary Portland cement, fly ash, blast furnace slag, service life prediction, finite element modeling, chloride, transport

The research team, led by Désiré Ndahirwa of UniLaSalle with colleagues including Hélène Lenormand, Hafida Zmamou and Nathalie Leblanc, focused on a question that has dogged the field for years. Wood ash contains reactive silica, alumina and calcium-bearing phases that can, in principle, behave like cement itself, reacting with water through hydraulic and pozzolanic pathways. In practice, however, pastes made from pure wood ash are weak and porous. Previous studies had shown that replacing moderate amounts of cement with wood ash often reduces compressive and flexural strength, and most work had examined substitution levels below 50 percent. Almost nothing was known about what happens at very high replacement levels, where wood ash dominates the mixture, and the stiffness of such materials, measured as Young’s modulus, had rarely been quantified despite its importance for structural design.

To close that gap, the researchers gathered four locally sourced wood ashes from the Normandy region of France. Two of them, designated WFA3 and WBA, came from the combustion of wood pellets and were delivered as wet sludge, requiring oven drying and crushing before use. The other two, WFA8 and WFA9, arrived as dry fine powders from a local heating plant operated by Coriance in Mont-Saint-Aignan, where boilers with power inputs of 6 and 8 megawatts burn forestry wood chips, bocage wood chips and pallet residues at temperatures between 900 and 1100 degrees Celsius. The team prepared seventeen paste formulations in total: four containing only wood ash, twelve blending 80 to 95 percent wood ash with 5 to 20 percent ordinary Portland cement, and a reference paste of pure cement, all compacted with a mini-Proctor device to boost density and cured for up to 28 days.

The chemical analysis alone revealed why wood ash is such a tricky raw material. The four ashes were dominated by calcium oxide, silica, potassium oxide and sulfur trioxide, but in wildly varying proportions. Sulfate contents reached 15.4 percent in WFA8 and 14 percent in WFA9, far above the 4 to 5 percent limit set by ASTM standards for pozzolans, and their combined pozzolanic oxides fell well below the required thresholds. In plain terms, these ashes do not qualify as conventional pozzolans, yet X-ray diffraction showed they carry crystalline phases such as portlandite, calcite, albite, alite, dolomite and sylvite that can still participate in binding reactions. The variability is a direct consequence of differing feedstocks, boiler designs and combustion temperatures, and it means each ash must be evaluated on its own terms rather than lumped into a single category.

When it came to mechanical performance, the effect of the small cement additions was unmistakable. Pure wood ash pastes managed compressive strengths of only 0.13 to 1.65 megapascals at 28 days, but raising the cement content to 20 percent lifted those values substantially. The best performer was the WFA8-based blend, which reached 5.49 megapascals, while WFA9 and WFA3 pastes achieved 4.01 and 3.87 megapascals respectively at the same dosage. The researchers attribute the gains to a richer supply of hydration products, including calcium silicate hydrate gel, portlandite and ettringite, formed as the cement’s tricalcium silicate reacts with water and progressively densifies the paste matrix. Stiffness told the same story: the modulus of elasticity, estimated from the linear portion of stress-strain curves, climbed with cement content, curing time and bulk density in three of the four ash families, with the highest values consistently recorded in mixtures containing 20 percent cement.

One ash refused to follow the script. The wood bottom ash, WBA, behaved atypically across every measurement. Its pastes lost compressive strength between 7 and 28 days at certain dosages, its modulus of elasticity peaked at 7 days and then declined, and thermogravimetric analysis found no detectable portlandite whatsoever. X-ray diffraction offered an explanation: the WBA pastes contained no alite, the calcium silicate phase that drives strength development in hydrating cement, and their dominant crystalline phases were calcite and quartz. Adding 20 percent cement pushed the estimated calcite content up from about 47 to 63.5 percent while quartz fell, a signature of carbonation of calcium silicate hydrate or the formation of complex, less efficient hydrate phases, both of which are associated with increased porosity and weaker binding.

Scanning electron microscopy added a visual dimension to the story. Under the microscope, pure cement paste appeared dense and well packed, bristling with the products of hydration, while pastes made from 100 percent wood ash showed loosely arranged particles riddled with interparticle voids, along with unreacted ash grains and dark fragments of unburnt wood. With 20 percent cement added, the microstructure tightened, porosity dropped and hydration products proliferated, with needle-like ettringite crystals, gel-like calcium silicate hydrates, plate-shaped portlandite and rhombohedral calcite all visible. The ashes from the heating plant, WFA8 and WFA9, consistently produced more compact matrices than the pellet-derived WFA3 and the bottom ash WBA, underlining how much origin and processing shape a material’s destiny.

The mineralogical detective work also turned up some genuinely unexpected chemistry. In the WFA8 blends, introducing cement promoted the formation of alunite, a potassium aluminum sulfate hydroxide phase not present in the unblended paste, alongside an array of compounds including syngenite, arcanite, serandite and harmotome. Thermogravimetric analysis at 7 and 28 days complemented the diffraction data, identifying calcium silicate hydrates and ettringite dehydrating between 50 and 200 degrees Celsius, AFm phases such as calcium monocarboaluminate and hemicarboaluminate decomposing between 200 and 300 degrees, portlandite dehydroxylating between 400 and 500 degrees, and carbonates releasing carbon dioxide from 500 to 800 degrees. The two techniques agreed closely, with the single discrepancy being ettringite in the bottom ash pastes, which thermal analysis detected but diffraction did not, likely because its concentration fell below the instrument’s detection limit.

What emerges from the study is a nuanced but practical message. Low doses of ordinary Portland cement, between 5 and 20 percent, can meaningfully upgrade pastes in which wood ash makes up as much as 95 percent of the solid content, provided the ash is fine and reasonably reactive. The improvements in stiffness, strength and microstructure are real, even if the resulting materials remain suited to low-strength applications such as lightweight binders rather than load-bearing concrete. The decisive variable, the authors conclude, is the variability of the ash itself: its origin, chemistry and mineralogy govern everything from phase development to porosity. That insight carries weight well beyond Normandy. With millions of tons of biomass ash generated annually and cement production responsible for a major share of global carbon dioxide emissions, even modest cement dosages that transform a landfill-bound waste into a functional building material represent a meaningful step toward circular, lower-carbon construction. The next challenge will be standardizing how ashes are characterized and selected, so that builders can trust what is in the bag before it ever reaches the site.

Subject of Research: The effect of low ordinary Portland cement content on the stiffness, microstructure and mineralogical composition of wood ash-based pastes

Article Title: Effect of low ordinary Portland cement content on stiffness, microstructure and mineralogical composition of wood ash pastes

Article References: Ndahirwa, D., Lenormand, H., Zmamou, H., Chenot, E., Potel, S., & Leblanc, N. (2026). Effect of low ordinary Portland cement content on stiffness, microstructure and mineralogical composition of wood ash pastes. Cleaner Engineering and Technology, 34, Article 101303. https://doi.org/10.1016/j.clet.2026.101303

Image Credits: AI Generated

DOI: 10.1016/j.clet.2026.101303

Keywords: wood ash, ordinary Portland cement, Young's modulus, compressive strength, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, calcium silicate hydrate, ettringite, porosity, biomass ash, sustainable construction