Blended Cements and the Carbon Question

Most of cement's carbon footprint is not the kiln fuel. It is the chemistry. Turning limestone into clinker releases carbon dioxide from the limestone itself, and no amount of renewable energy at the plant changes that. The only reliable way to cut emissions per tonne of cement is to use less clinker per tonne of cement.

Why blended cements are the near-term lever

PPC replaces clinker with fly ash, a by-product of coal combustion. PSC replaces it with granulated blast-furnace slag, a by-product of steelmaking. Both take an industrial waste stream and put it to structural use, displacing the single most carbon-intensive component of the mix.

The environmental argument and the engineering argument point the same way — which is unusual, and worth noticing.

The engineering is not a compromise

• Denser, less permeable concrete, because the pozzolanic reaction fills capillary pores.
• Better resistance to sulphate and chloride attack, extending the service life of the structure.
• Lower heat of hydration, reducing thermal cracking in mass pours.
• Continued strength gain well beyond 28 days, where OPC has largely plateaued.

The one genuine trade-off is early strength. Blended cements develop strength more slowly in the first seven days, which matters on a fast formwork cycle and matters not at all on a foundation that will cure for a month before it carries load.

The durability multiplier

There is a second-order effect that gets less attention than it deserves. A structure that lasts eighty years instead of forty halves the embodied carbon of its own replacement. Specifying a grade that resists the exposure conditions it will actually face is, over a building's life, a larger emissions decision than the clinker factor of any single bag.

Choosing the right cement for the environment is good engineering. That it is also the lower-carbon choice is a convenience the industry should use more deliberately than it does.