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Short answer: No material is a universal winner for ocean alkalinity enhancement (OAE). In a 31-day coastal experiment published by Li et al. in Marine Pollution Bulletin in 2026, fly ash released alkalinity faster than the tested olivine and basalt, while steel slag produced a sharp early pH rise followed by alkalinity consumption through carbonate precipitation. Lime is a separate, processed route: limestone is calcined into quicklime and hydrated lime, which can deliver alkalinity more readily but adds energy use and process emissions.
Those findings are material- and site-specific. A sound choice requires comparing dissolution kinetics, durable net carbon removal after processing and transport, trace contaminants, secondary mineral reactions, ecological exposure, scalability and monitoring—not simply selecting the material with the highest short-term alkalinity reading.
What ocean alkalinity enhancement is trying to do
OAE increases the alkalinity of seawater so it can absorb more atmospheric carbon dioxide. As carbon dioxide dissolves, it participates in the carbonate system and is converted mainly into bicarbonate and carbonate ions. In principle, that stores carbon in dissolved form for long periods while reducing the acidity associated with additional carbon dioxide.
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For that reason, material comparisons should report total alkalinity, dissolved inorganic carbon, pH and solid phases over time, together with a clearly defined life-cycle boundary. The same mineral name can describe feedstocks with different particle sizes, impurity levels and reactivity.
Results at a glance
The table summarizes the directly reported figures from Li et al.’s 2026 31-day coastal comparison. The two carbon figures are hypothetical deployment projections for coastal China, not field measurements or achieved commercial removal rates.
| Material | Observed behavior in the 31-day comparison | Study projection or process detail | Main uncertainties and hazards |
|---|---|---|---|
| Olivine | Alkalinity enhancement of 12 μmol/kg/day under the tested conditions | Hypothetical coastal-China projection: 0.57 ± 0.06 Tg CO2 per month | Slow, condition-dependent dissolution; possible nickel, chromium and iron release; sediment and grain-size effects |
| Basalt | Negligible alkalinity release during the tested period and conditions | No comparable deployment projection stated in the cited comparison | Long dissolution timescales and uncertainty about field-scale coastal performance |
| Fly ash | Alkalinity release of 30 μmol/kg/day under the tested conditions | Hypothetical coastal-China projection: 1.24 ± 0.05 Tg CO2 per month | Source-specific contaminants, possible chromium and phosphorus release, and eventual surface passivation |
| Steel slag | Early pH increase of approximately 1.5 ± 0.3 pH units, later stabilizing near a 0.7 ± 0.4 increase | Secondary carbonate precipitation consumed alkalinity in the experiment | Transient pH response may not produce durable removal; precipitation can slow further reaction |
| Lime-derived alkalinity | Not tested alongside the four materials above in the cited experiment | Limestone is heated above 800 °C to make CaO, then hydrated to Ca(OH)2 | Calcination energy and emissions; outcome depends on heat source, capture system, transport and dosing |
The reported rates are not interchangeable commercial specifications. They came from one set of samples and conditions, and the projected teragrams per month depend on modeling assumptions that must be examined before reuse.
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Olivine: a natural silicate with a slow, variable reaction
Olivine is a magnesium-iron silicate proposed for marine enhanced rock weathering. Its chemical formula gives it theoretical alkalinity potential, but theoretical capacity is not the same as alkalinity that actually reaches seawater. Dissolution depends on grain size, mineral surface area, seawater chemistry, temperature, mixing, hydrodynamics and whether particles settle into permeable or reactive sediments.
In the 2025 review by Geerts and colleagues, coastal field evidence for olivine remained limited. Sediment type was identified as a particularly important control: particles in one seabed environment can react very differently from particles dispersed in the water column or deposited in compact sediment. A short tank or coastal trial therefore cannot establish a universal field rate.
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Olivine also requires element-specific environmental screening. Iron-bearing grains can release iron, while nickel and chromium are important potential contaminants. Zhuang and colleagues’ 2025 perspective describes possible migration and interactions with manganese oxides in sediments and calls for assessment and mitigation before large-scale use. The risk cannot be inferred from the mineral name alone; deposits differ in trace-metal content, weathering history and beneficiation.
When olivine might be considered
- When a well-characterized local feedstock has acceptable nickel and chromium leaching results.
- When the deployment site has hydrodynamics and sediments that support measurable dissolution without concentrating particles in sensitive habitat.
- When a monitoring plan can distinguish genuine alkalinity and carbon-system changes from dilution, mixing or carbonate precipitation.
Basalt: familiar rock, limited short-term release in the comparison
Basalt is another natural silicate candidate, but its mineral assemblage generally weathers more slowly than a processed alkaline material. In the 2026 coastal experiment it produced negligible alkalinity release during the test window. That result describes the tested basalt, particle preparation and conditions; it does not prove that every basalt source behaves identically or that longer deployments would show no release.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchBasalt can appear attractive because it is abundant and typically contains fewer of the specific concerns associated with ultramafic olivine. Even so, feedstock assays, particle-size distribution, sediment behavior and life-cycle energy for quarrying and grinding remain necessary. If the intended strategy depends on rapid alkalinity delivery, the observed short-term behavior is a warning that a large mass may be required before a measurable signal appears.
Lime: faster delivery through an energy-intensive processing route
“Ocean liming” commonly starts with limestone rather than a naturally reactive powder. Heating limestone above 800 °C drives off carbon dioxide and produces calcium oxide (CaO), or quicklime. Adding water converts CaO into calcium hydroxide, Ca(OH)2, which can dissolve and add alkalinity more rapidly than untreated limestone.
The processing step changes the life-cycle accounting. Kilns require substantial heat, and calcination itself releases carbon dioxide from the limestone. Net removal therefore depends on the kiln’s energy source, whether process emissions are captured, the amount of material that dissolves, transport distance and the fate of the added calcium and carbonate species. Calling lime “low carbon” without those boundaries would be misleading.
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The cited government evidence review and life-cycle literature describe the pathway but do not provide a directly comparable lime trial in Li et al.’s olivine, basalt, fly-ash and steel-slag experiment. Lime should therefore not be ranked numerically against those materials using that study. A fair comparison would use the same seawater, dose, particle-size specification, duration and full life-cycle assumptions.
Operational issues specific to lime
- Quicklime is caustic and reacts exothermically with water, so storage, hydration and dosing require industrial controls.
- Localized over-dosing can create high-pH zones before mixing; diffuser design and staged addition matter.
- Fine particles can increase handling, inhalation and transport burdens even when they improve dissolution.
- Any carbon accounting must include quarrying, calcination, hydration, transport, monitoring and residual solids.
Fly ash: rapid alkalinity in the test, but highly source-dependent
Fly ash is a combustion byproduct whose composition varies with fuel, boiler conditions, pollution-control equipment and post-processing. In Li et al.’s experiment it released alkalinity faster than the tested natural silicates, and the authors’ life-cycle discussion credited byproduct status and avoided upstream production emissions as potential advantages.
That advantage is conditional. Avoided-emission claims depend on the system boundary and on how emissions are allocated between the primary industrial process and the ash. Transport, drying, milling, screening and any stabilization can materially change the result. Not every ash stream is suitable for marine exposure.
The study flagged possible chromium and phosphorus release and eventual surface passivation. A coating that forms on ash particles can reduce later dissolution, so an initial alkalinity rate should not be extrapolated indefinitely. Before deployment, each batch needs chemical assays and leaching tests under the salinity, pH and redox conditions expected at the site.
Minimum checks for an ash feedstock
- Document the combustion source, collection system, storage history and any beneficiation.
- Measure major oxides, trace metals and nutrients, including chromium and phosphorus.
- Run seawater leaching tests over time rather than relying on a one-time extract.
- Test whether particle coatings or secondary minerals reduce alkalinity release.
- Set rejection limits for batches that fail local marine-protection requirements.
Steel slag: strong initial pH response, followed by a different chemistry
Steel slag contains reactive calcium- and magnesium-bearing phases, so it can change pH quickly. The 2026 comparison observed the largest early pH increase among the tested materials, followed by a decline toward a smaller stable increase. The investigators linked this pattern to secondary carbonate precipitation that consumed alkalinity.
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This sequence matters because a pH spike can look impressive while the carbon pathway remains uncertain. Carbonate precipitation may store carbon in a solid phase, but it can also remove dissolved alkalinity that would otherwise support continued atmospheric carbon dioxide uptake. The relevant question is not “How high did pH rise?” but “What happened to total alkalinity, dissolved inorganic carbon and the precipitated minerals over time?”
Slag chemistry is strongly tied to furnace feedstocks and processing. Free lime, metals and other phases vary among plants and batches. Marine use therefore requires leaching characterization, particle controls, ecological exposure modeling and a plan for secondary solids that may settle on sediments.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why the reported numbers cannot be treated as a league table
The comparison’s alkalinity figures are rates measured under a defined coastal test, expressed per kilogram of seawater per day. They are useful for showing relative behavior in that experiment, but they do not state how many tonnes of material would be needed at a real site, how evenly the material would mix or how much of the signal would persist.
The two teragram-per-month values are model projections for a hypothetical coastal deployment in China. They are not records of material delivered, carbon dioxide measured or removal credits issued. Scaling a laboratory rate requires assumptions about water volume, dosing, residence time, particle settling, dissolution, carbonate precipitation and monitoring uncertainty.
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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsComparisons also change when life-cycle emissions are included. Grinding a hard mineral, heating limestone, transporting a low-density ash or handling slag can each dominate a project’s footprint. Byproduct accounting must specify what upstream emissions are avoided and why that allocation is appropriate.
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Environmental safety depends on the feedstock and the receiving site
A low aggregate heavy-metal pollution index in one experiment does not establish universal ecosystem safety. The relevant exposure depends on the concentration of each element, its chemical form, leaching rate, sediment retention, food-web transfer and the sensitivity of local organisms.
Site assessment should cover water-column habitats, benthic communities, fisheries, protected areas, sediment permeability and background metal concentrations. Olivine’s nickel and chromium concerns, ash’s possible chromium and phosphorus release, and slag’s variable metal inventory require different analytical panels. Baseline sampling is needed before dosing so that natural seasonal changes are not mistaken for an impact.
Local permitting may also treat a material as a waste, industrial residue, mineral product or marine discharge, with different testing and documentation requirements. A project should obtain regulatory advice before moving bulk material, not after a field release.
A defensible material-selection workflow
- Define the carbon claim. Decide whether the objective is alkalinity addition, atmospheric carbon dioxide removal, or both. Set a durability period and state which emissions count.
- Characterize the exact feedstock. Record origin, mineralogy, particle-size distribution, moisture, major elements and trace contaminants. Do not substitute a generic product for the tested sample.
- Measure dissolution in representative seawater. Use the intended salinity, temperature, mixing, sediment type and dose. Track total alkalinity, dissolved inorganic carbon and pH through time.
- Test secondary reactions. Identify carbonate precipitation, surface passivation, metal adsorption or release, and changes in phosphorus or other nutrients.
- Complete a transparent life-cycle assessment. Include quarrying or collection, grinding, calcination or hydration, storage, transport, deployment and monitoring. Document byproduct allocation and any carbon-capture assumptions.
- Screen ecological exposure. Combine leaching data with local hydrodynamics, sediment transport and organism sensitivity. Model worst-case accumulation rather than relying only on average concentrations.
- Set monitoring and stop rules. Define control sites, sampling frequency, analytical detection limits, trigger concentrations and conditions that pause or end dosing.
What a monitoring, reporting and verification plan should measure
- Carbonate chemistry: total alkalinity, dissolved inorganic carbon, pH, temperature, salinity and oxygen.
- Material fate: suspended and settled solids, particle-size changes, coating formation and mineral phases.
- Contaminants: dissolved and particulate metals, nutrients and any feedstock-specific compounds in water and sediment.
- Ecology: plankton, benthic organisms, indicator species and community changes against matched controls.
- Mass balance: tonnes delivered, distribution, dissolution fraction, exported alkalinity and uncertainty intervals.
- Life-cycle records: electricity and fuel use, transport distances, processing losses and monitoring resources.
Short deployments should be presented as experiments or pilots. Durable removal claims require repeated measurements, independent quality assurance and uncertainty estimates that include unmeasured or slowly reacting material.
What the current evidence supports
The 2026 coastal comparison supports a narrow conclusion: the tested fly ash released alkalinity faster than the tested olivine and basalt, while steel slag followed a rapid-pH-then-precipitation pathway. It does not establish fly ash as the universal best material, prove that steel slag delivers durable removal, or validate any of the projected monthly removals at industrial scale.
Olivine remains a condition-dependent option with limited coastal field evidence and specific metal concerns. Basalt’s short-term response was negligible in the cited test. Lime can provide a deliberately processed, faster-reacting alkalinity source, but its calcination burden must be included. Industrial byproducts may avoid some upstream production emissions, yet their chemistry and contaminants are inseparable from the source process.
The most credible choice is therefore site-specific: select the feedstock that meets dissolution, contaminant, life-cycle and ecological thresholds under the receiving site’s conditions, then verify the result with a monitoring plan capable of detecting both carbon benefits and harm.
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