Light Ball Milling Splits Steel Slag Into a Fraction That Carbonates Far Better

Steel is one of the most recycled materials on Earth, yet its production leaves behind an enormous and awkward legacy: steel slag. In China alone, output of this stony byproduct of steelmaking now exceeds 120 million tons per year, piling up in landfills, occupying land, and posing environmental risks because of its high alkalinity. At the same time, the construction industry is desperate for low-carbon alternatives to ordinary cement, whose calcination at 1450 degrees Celsius makes cement production responsible for roughly 8 percent of global carbon dioxide emissions. A new study published in Case Studies in Construction Materials suggests that a surprisingly simple sorting trick, based on nothing more than a gentle spin in a ball mill, could unlock the carbon-storing potential hidden inside slag while slashing the energy cost of processing it.

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