Calcined London Clay (CLC) as SCM — HS2 / Skanska Costain Strabag (ExClay)

Product · Materials Science

Product · InnDex 28 · Evidence provided · High specification risk

Pozzolanic powder from heated HS2 tunnel spoil (London Clay) to partially replace Portland cement in concrete.

London Clay excavated during HS2 tunnelling is calcined at 750–900°C and ground to produce a supplementary cementitious material (SCM). It reacts with Portland cement hydration products, reducing clinker content and embodied carbon per tonne by ~97% versus virgin cement. Lab trials confirm 28-day compressive strengths of 35–57 MPa and calcination CO₂ intensity of 24–42 kg/tonne.

ExClay calcines London Clay excavated during HS2 tunnelling at 750–900°C to produce a supplementary cementitious material that partially displaces Portland cement, achieving ~97% lower CO2 per tonne in the calcination step versus virgin clinker and demonstrating 35–57 MPa 28-day compressive strength across 13 lab trial batches. The on-project resource logic is elegant — the excavation waste stream and the carbon reduction opportunity arise from the same activity — and confirmed pozzolanic reactivity puts this one step ahead of most waste-valorisation claims. One piece of evidence is provided, but the honesty of the record is that this remains at REAL 1b feasibility stage with Innovate UK funding only; no structural deployment in permanent HS2 works has been confirmed, no BS/EN qualification pathway is established, and calcination process scalability beyond lab batching has not been demonstrated. Long-term durability characteristics — chloride ingress resistance, carbonation, sulfate attack — are absent from published data. The deeper constraint is commercial geography: this material is time- and site-limited to HS2 excavation, and no pathway to sustained commercial production for other projects is outlined. For a specifier, ExClay is a credible innovation signal for the broader calcined clay SCM trend (which is more broadly proven, as Holcim's ECOPlanet demonstrates), but it is not a product a project team can specify or procure today.

Strengths

• Diverts millions of tonnes of excavation spoil from landfill, converting site waste into resource
• Reduces concrete embodied carbon by ~97% in the calcination step versus Portland clinker production
• Addresses dual problem on same project: exploits waste stream while cutting carbon on structures generated by that excavation
• Demonstrated pozzolanic reactivity and compressive strength in controlled lab batches (13 trials)

Considerations

• Strength gain is slower and lower-bound than Portland cement alone; 28-day strengths (35–57 MPa) suggest requirement for higher replacement ratios or extended cure times (moderate)
• Material properties and performance depend entirely on source clay mineralogy; London Clay variability may limit geographic replicability and standardisation (moderate)
• Calcination energy input (thermal load at 750–900°C) and CO₂ accounting must be verified against full lifecycle; renewable energy assumption not stated (moderate)

Risks

• No confirmed structural deployment in permanent HS2 works as of October 2024; remains at feasibility stage (REAL 1b) with Innovate UK funding only (high)
• Regulatory acceptance and performance standards for CLC in UK structural concrete not yet established; lack of independent third-party qualification or BS/EN pathway evident (high)
• Calcination process scalability, kiln infrastructure, and supply chain logistics for spoil-to-powder conversion not demonstrated beyond lab trial scale (high)
• Long-term durability and durability-related performance (chloride ingress, sulfate attack, carbonation) in CLC-blended concrete not reported in available evidence (moderate)
• Material is site-specific and time-limited to HS2 excavation; no pathway to sustained commercial production or use on other projects outlined (moderate)

Performance

• Cement replacement rate (tested): 50–70% by weight
• 28-day compressive strength (optimised mixes): ~50 MPa
• CLC calcination CO2 intensity: 24–42 kg CO2/tonne (vs ~800 kg/tonne for clinker)
• Trial batch scale: ~200 kg CLC produced; 13 concrete batches

Reality check

Lab feasibility is credible (peer-reviewed trials, multiple partners). Calcination CO2 figures appear sound (lower than clinker due to lower reaction temperature). However, no field trials in structural concrete reported; no durability data beyond 28 days; no long-term performance in HS2 conditions; no confirmed commercial scaling pathway or cost model published. Innovate UK funding signals credibility but does not guarantee deployment. Source is authoritative (HS2 Learning Legacy) but does not claim working concrete in permanent structures. Claim maturity: bench to feasibility, not pilot or deployment.

#circular_economy #spoil_reuse #embodied_carbon #supplementary_cementitious_material #concrete #waste_diversion

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