Why concrete matters
Concrete is the most widely used construction material on Earth. The UK construction industry uses approximately 100 million tonnes of it annually. The carbon problem lies primarily in the cement - specifically Portland cement (CEM I) - which requires heating limestone to 1,450°C. This process releases CO₂ both from the energy used and from the chemical reaction (calcination) itself.
A typical CEM I Portland cement has an embodied carbon of 0.912 kgCO₂e/kg. For a standard RC 32/40 concrete mix, this translates to approximately 0.132 kgCO₂e/kg - which sounds small until you consider the volumes used in construction.
Cement replacements: GGBS and PFA
The most established strategy for reducing concrete carbon is replacing a proportion of Portland cement with supplementary cementitious materials (SCMs):
GGBS (Ground Granulated Blast-furnace Slag): A by-product of iron manufacturing. Replacing 50% of cement with GGBS reduces the concrete carbon by approximately 33%, giving around 0.089 kgCO₂e/kg. It also improves durability and resistance to chloride and sulphate attack.
PFA (Pulverised Fuel Ash / Fly Ash): A by-product of coal power stations. A 30% replacement reduces carbon by approximately 22%, giving around 0.103 kgCO₂e/kg. Availability is declining as coal plants close, so long-term reliance on PFA is risky.
Emerging alternatives
Beyond traditional SCMs, several innovative approaches are gaining traction:
LC3 (Limestone Calcined Clay Cement): Uses calcined clay and limestone to replace up to 50% of clinker. Being actively trialled in the UK.
Carbon capture and utilisation: Some manufacturers are injecting captured CO₂ into concrete during mixing, where it mineralises permanently.
Geopolymer concrete: Replaces cement entirely with alkali-activated binders. Early stage but promising for precast applications.
Biochar addition: Small additions of biochar can sequester carbon within the concrete while maintaining performance.
Design strategies to reduce concrete volume
Beyond material substitution, design decisions can dramatically reduce the volume of concrete needed:
Post-tensioned slabs: Can reduce slab thickness by 20–30%, directly cutting concrete volume.
Voided slabs: Technologies like BubbleDeck or Cobiax use void formers to reduce concrete in areas where it's structurally unnecessary.
Timber hybrid structures: Using CLT or glulam for upper floors while retaining concrete for foundations and lower storeys.
Optimised foundation design: Ground investigations that reduce conservative over-design can cut foundation concrete by 20%+.
Key Takeaways
- Cement is the carbon culprit - focus on cement replacement strategies
- 50% GGBS replacement gives ~33% carbon reduction with performance benefits
- PFA availability is declining as coal plants close - plan alternatives
- Design optimisation (voided slabs, post-tensioning) can reduce volume by 20–30%
- Emerging tech like LC3 and carbon capture could transform the industry
Ready to calculate your project's carbon impact?
Use our Carbon Calculator to specify materials and discover lower-carbon alternatives.
Open Carbon Calculator