Blended cements reduce embodied carbon, but they gain strength more slowly — and on a fast-track frame, early strength is the programme. The answer is not to avoid them, but to plan for them.

We now agree target strengths, striking times and curing methods with the designer before the mix is specified. Maturity sensors in the slab give us real strength data, so formwork is struck on evidence rather than on the calendar.

On a sample project, this approach kept a four-day floor cycle while reducing the cement content of the frame. The lesson: carbon targets work best when the people pouring the concrete are in the room when they are set.