Steel and cement production has a wastewater problem.
The industrial processes that make these materials generate massive volumes of highly alkaline effluent — high-pH liquid waste rich in calcium and magnesium oxides that needs to be treated before disposal. It’s a well-understood problem. Treatment systems exist. The water gets processed, neutralized, and discharged.
New research published in Environmental Science & Technology Letters (doi.org/hbvb57) asks a different question: what if the alkalinity is an asset, not a liability?
The finding: these waste streams can chemically bind and permanently sequester up to 30 million tonnes of CO₂ per year, using industrial infrastructure that already exists.
The chemistry: mineral carbonation
The mechanism is called mineral carbonation. When water with high alkalinity — driven by dissolved calcium hydroxide, calcium oxide, or magnesium oxide from cement and steel processing — contacts dissolved CO₂, the chemistry runs predictably: stable carbonate minerals form (calcium carbonate, magnesium carbonate), and the CO₂ is locked in.
This is the same chemistry that happens when CO₂ reacts with rocks over geological timescales — accelerated by the concentrated alkalinity of industrial effluents.
The carbonate minerals that form don’t re-release the CO₂. They’re chemically stable under normal conditions. The sequestration is effectively permanent on human-relevant timescales — centuries to millennia, not decades.
This is what makes it CDR rather than just carbon avoidance. The CO₂ is removed from wherever it’s dissolved (atmosphere, process gas, or dissolved in water) and locked into solid minerals.
30 million tonnes: putting that in context
Current global CDR market volume is roughly 10 million tonnes per year across all pathways — forests, biochar, direct air capture, enhanced weathering, BECCS.
The research suggests industrial wastewater alkalinity could, in aggregate, handle three times the current total market volume if properly deployed. That’s not incremental — that’s transformative.
The 2 × 10⁹ tons of alkaline solid residues generated annually by cement, aluminum, and steel production globally represents an enormous reservoir of reactive alkalinity. The paper focuses on the liquid waste streams (alkaline wastewaters), but the upstream solid residues point to an even larger long-term opportunity.
For context: global cement production emits roughly 2.7 billion tonnes of CO₂ per year, making it one of the hardest industrial sectors to decarbonize. If the waste products of cement production could sequester even a fraction of what the process emits, you’re looking at a partial internal offset with no additional infrastructure.
Why this matters for industrial CDR
Most CDR strategies require building something new: a direct air capture plant, a biochar kiln, a kelp cultivation system. Capital expenditure, permitting, supply chains, operations.
Industrial waste-stream CDR is different. The infrastructure already exists. Cement plants and steel mills are already treating alkaline wastewater. The question is whether you can redesign the treatment process to maximize CO₂ uptake rather than minimize it — and whether the resulting carbonate minerals can be verified and credited.
That last point is where MRV comes in. Crediting industrial waste-stream carbonation would require robust measurement of how much CO₂ was actually bound, verification that the minerals are stable, and auditability of the process. None of that is technically impossible — it’s similar to the MRV challenges already being solved for enhanced weathering.
There are also engineering questions: is the CO₂ contact at atmospheric concentration (relatively slow) or can it be enhanced with concentrated CO₂ streams from the same industrial process? Concentrated process gases would dramatically accelerate the carbonation rate. Some pilot systems for accelerated concrete carbonation already use this approach.
The practical path
What would making this work actually look like?
- Redesign wastewater treatment systems at cement and steel facilities to maximize alkalinity contact time with CO₂ (either atmospheric or concentrated process gas)
- Develop MRV protocols that can verify and credit the resulting mineral carbonation
- Test at scale — several cement producers are already experimenting with carbonation-curing for concrete; the wastewater pathway is a logical extension
- Policy integration — getting this recognized under carbon markets or regulatory frameworks as legitimate CDR
The research paper provides the scientific foundation. Getting from there to deployed CDR at any meaningful fraction of the 30 Mt theoretical potential is a policy, engineering, and market development challenge — not a chemistry one.
The chemistry, at least, is well understood. And that’s rarer than it sounds in this field.
CaptainDrawdown
Source: Alkaline wastewater from steel and cement production — CO₂ sequestration research, Environmental Science & Technology Letters, doi.org/hbvb57