Cement replacement options
CEM I (100% Portland cement, baseline) — typical 0.95 kgCO2e/kg cement. CEM II/A (6–20% replacement with GGBS, PFA, limestone) — 0.78 kgCO2e/kg. CEM II/B-S (21–35% GGBS) — 0.62 kgCO2e/kg. CEM III/A (36–65% GGBS) — 0.45 kgCO2e/kg. CEM III/B (66–80% GGBS) — 0.30 kgCO2e/kg. Higher replacement = lower carbon but longer set time + lower early strength. For typical residential foundations + slabs, CEM III/A (50% GGBS) optimal balance — 50% carbon saving, ~25% slower early strength (acceptable for non-critical pours). Concrete C32/40 GGBS-replacement equivalent £128–£148/m³ vs CEM I £115–£132/m³.
Practical specification
Order: 'concrete C28/35 with CEM III/A binder' or 'GGBS replacement 50%'. Suppliers in London with low-carbon stock: Aggregate Industries Ecocrete + Cementition; Tarmac Topcrete; Hanson EcoPlus; Breedon Cemfree (Cemfree is GGBS-activated, near-zero cement — 90%+ carbon saving but specialist use). All deliverable to standard mixer trucks. Lead time: standard mix same-day if pre-ordered; specialty Cemfree 2–5 day lead time. Pour temperatures: CEM III/A acceptable down to 5°C ambient; below 5°C use admixture (Sika ViscoCrete) to maintain workability + setting.
Use-case suitability
Excellent fit: foundations (mass + reinforced), ground slabs, basement walls (CEM III particularly resistant to sulphate attack — better than CEM I in London clay groundwater), retaining walls, large pours where slow set + lower heat-of-hydration desirable. Less suitable: rapid-strike formwork (cores, fast columns) — CEM I better for early strength; sub-zero ambient pours; thin sections where rapid drying needed. Concrete suppliers will advise on mix design for specific application. Building Control + structural engineer approval routine — CEM III recognized in BS 8500 + Eurocode 2 with same strength classes.
