Sand Replacement vs Cement Replacement with ΔC

ΔC improves particle packing in the paste, and that opens up two distinct ways to design a lower-carbon mix. Both use the same drop-in liquid admixture, and the difference is what the biochar displaces. Picking the right one is mostly a question of how much risk the project can absorb.

The two strategies

The mental model: SR adds storage without changing your binder; CR allows for further carbon reduction by displacing cement.

How they compare on carbon

Both lower the mix's global warming potential (GWP), but by different amounts because they pull different levers.

• Sand Replacement delivers a solid reduction, on the order of a 10–20% GWP cut, almost entirely from stored biogenic carbon, while leaving the cementitious system alone.
• Cement Replacement goes further, into the 20%+ range, because it stacks avoided cement emissions on top of the stored carbon.

How they compare on strength

The strength data below is from a lab study comparing both strategies against a control mix. Both approaches have also been tested commercially — Cement Replacement at CarbonRun, Sand Replacement at Mi'kma'ki Place in Membertou.

Neither strategy asks you to trade away performance. In head-to-head testing against a control mix:

• SR matched the control early and finished ahead at 28 days, because the biochar improves packing without diluting the binder.
• CR held early-age strength despite less cement and also came in at or above the control by 28 days.

That's the opposite of the usual low-carbon trade-off, where cutting carbon slows strength gain.

Lab results for ΔC-CR mixes vs control at 7 and 28 days:

• Control
• 4% ΔC-CR
• 5% ΔC-CR
• 6% ΔC-CR

A 7% dose of ΔC enables ~9% less cement, ~50 kg CO₂e stored, same fresh properties, ~10% higher strength, and lower GWP.

How to choose

Use this as a starting point:

• Choose SR when the project is risk-sensitive, the mix is dialed in, or you want an easy first trial. You keep the binder system exactly as-is and simply introduce carbon storage.
• Choose CR when the target demands a bigger reduction and there's room to re-engineer the mix slightly. You get avoided emissions plus storage.
• Start with SR, graduate to CR. A common path is to prove ΔC with a low-risk SR pour, then move to CR once the mix behaviour is trusted. Several field pours that began conservatively had strength headroom to push toward deeper cuts.

The common thread

Whichever you pick, ΔC doses like any other liquid admixture with no plant retrofit, and the total powder volume stays balanced, so the mix stays familiar to the crew placing it.

• See the headline proof on the ΔC page.
• Read why stored carbon counts in biogenic carbon permanence.
• Model your mix with the ΔC estimator.