Why next-gen steel plants will be designed around slag chemistry, not steel chemistry

• Steel slag needs to be engineered for carbonation
  
  
• Integrated slag valorisation will emerge as core business proposition
  
  

For more than a century, steelmaking has followed an unquestioned hierarchy: steel is the product, slag is the waste. Even when slag found secondary uses, as road aggregate, cement blending material, or ballast, it remained something to be tolerated, disposed of, or monetised marginally.

That hierarchy is now quietly but decisively breaking because of thermodynamics, carbon economics, and materials science. The reality emerging is this: slag is no longer a by-product to be managed; it is a deliberately engineerable material with intrinsic value. In many future steel plants, slag will increasingly become the primary designed output, with steel optimised around it.

Slag is not waste

At a fundamental level, steelmaking slag is already a highly processed material: it contains CaOSiOAlOMgO systems, has undergone high temperature reactions at 1,500-1,650C, limestone has already been decarbonated, and silicates and oxides have already reacted and partially equilibrated.

From a materials perspective, slag is pre-paid mineral processing. The steel plant has already invested thermal energy, chemical energy, fluxes, and process control. Throwing the slag away is equivalent to discarding a high-temperature chemical reactor output.

Historically this was acceptable because cement had cheap limestone, CO had no price, energy was centralised and abundant, and steel margins dominated the economics. None of those assumptions hold any longer.

Why slag can permanently store CO

Steelmaking slags (especially BOF/LD slags) contain reactive calcium and magnesium phases often described as free CaO, free MgO, and Ca-silicates. When exposed to CO in the presence of moisture, these phases can undergo mineral carbonation, forming stable carbonates:

• CaO CaCO
  
  
• MgO MgCO (typically slower than calcium reactions)
  
  

This is not offsetting. It is permanent CO storage.

However, carbonation is not a magical pump CO into hot slag process. In practice, effective carbonation requires a) sufficient surface area (crushing, granulation, and fines), b) a thin moisture film (critical for reaction kinetics), c) controlled CO partial pressure and contact time, and d) carefully managed temperature and liquid-to-solid ratios.

Too dry reaction is slow

Too wet reaction mechanisms change

Too high CO carbonate shells form and block further uptake

This is why slag chemistry and slag processing must be designed together.

What 'slag design- means in steel plant terms

Designing slag for carbonation and downstream use is not just chemistry. It is metallurgy + kinetics + process integration.

• Creating carbonatable phases without instability
  
  

Carbonation uptake primarily comes from a) free CaO and reactive Ca-silicates (CS like phases), and b) free MgO and Mg bearing silicates (slower, but relevant).

The challenge is balance:

• Too little reactive Ca/Mg poor CO uptake
  
  
• Too much free CaO/MgO expansion and soundness problems in cement/aggregate use.
  
  

Therefore, slag must be engineered to retain enough reactive phases for carbonation and avoid excessive free lime that compromises long-term stability.

• Making carbonation kinetically possible at scale
  
  

What actually makes carbonation work industrially are process levers, particle size and surface area. Finer slag carbonates faster, but too fine creates handling and dust issues.

• Moisture control: A thin water film dramatically accelerates carbonation; fully submerged systems behave differently.
  
  
• CO partial pressure: Elevated CO accelerates uptake, but excessive pressure causes surface passivation.
  
  
• Temperature, residence time, and L/S ratio must be tuned to avoid pore blocking and maximise conversion.
  
  

Steel chemistry still meets specifications. But within allowable bands, degrees of freedom are now used to optimise slag value. Examples: a) Lime saturation optimised for cement reactivity, not only sulphur removal. Traditionally, basicity was driven by refining needs. For downstream cement and SCM value, what matters is formation of reactive Ca-silicate phases and avoidance of excessive free lime that causes expansion.

This means BOF slag practice begins to consider cement performance, not just metallurgical sufficiency.

(b) MgO content designed for mineral stability, not only refractory life. MgO additions historically serve refractory protection. But free MgO causes late expansion, carbonation converts MgO into more stable carbonates, and controlled MgO improves both lining life and slag usability. The objective shifts from protect the lining to optimise final mineral stability.

c) FeO minimised not just for yield, but for slag usability. High FeO means yield loss, increase in slag density, interference with cementitious behaviour, and reduction of effective carbonation of Ca/Mg phases.

Minimising FeO now serves three purposes: 1. Steel yield 2. Energy efficiency and 3. Slag reactivity and value.

• d) AlO controlled for downstream cement performance. High AlO affects clinker chemistry, hydration kinetics, and setting behaviour. This is why steel slags often need pretreatment for cement use. A slag first mindset controls AlO proactively instead of correcting downstream.
  
  
• Slag as a carbon sink flips the value equation. Steel carries embedded CO liability. Slag can carry embedded CO credit.
  
  

Once lifecycle accounting, CBAM, and avoided emissions logic mature slag may generate more decarbonisation value per tonne than steel, steel margins alone will no longer justify plant economics, and integrated slag valorisation becomes a core business, not an add-on. At that point, slag is no longer secondary by definition. It becomes economically primary.

• Slag as the real entropy manager of steel plant. Steel plants lose energy through roof radiation, shell losses, waiting times, and batch discontinuities. Slag absorbs chemical reaction enthalpy, sensible heat and mixing energy. When slag is designed intentionally:
  
  
• Thermal gradients are managed
  
  
• Irreversible entropy generation is reduced
  
  
• Heat losses during waiting phases are minimized
  
  

Slag becomes a planned thermodynamic sink, not an accidental one.

• Steel plants become materials platforms
  
  

Once slag is treated as a primary engineered output, the steel plant evolves into a multi output materials hub:

• Metallic steel
  
  
• Cementitious binders
  
  
• Carbonated minerals
  
  
• Aggregates
  
  
• Recoverable heat
  
  

This changes: a) revenue structure, b) risk profile, c) ESG positioning and d) capital allocation logic.

This article is published by BigMint in collaboration with author Mr. R.V. Sridhar, Senior Independent Advisor, McKinsey & Co.