The Low-Carbon Dam

The concrete in the Son La dam, on the Black River in northwest Vietnam, was built on sixty kilograms of cement per cubic metre. Structural concrete uses five times that or more. The rest of the binder, a hundred and sixty kilograms of it, was fly ash, and not fresh fly ash either. It was reclaimed from ponds at a power station where it had lain for two decades or more, made usable only after a process was developed to remove the unburnt carbon it carried. Nearly three-quarters of the cementing material in a 138-metre dam was, in effect, someone else's waste.

That was not a compromise forced by cost. It was the point. Roller-compacted concrete, the method that builds a dam the way you build a road, in thin layers spread and rolled rather than poured, runs on a fraction of the cement of ordinary concrete and leans on fly ash to make up the binder. And at Son La the fly ash was doing real structural work, not just filling space: it reacts with the lime the cement releases as it hydrates and becomes part of the hardened paste, which is how a mix this lean in cement still hardens into a 138-metre dam.

The reason this was ever done has nothing to do with carbon. A large dam is a mass of concrete, and cement generates heat as it sets; pack enough of it together and the core heats up, expands, and then cracks as it cools against the cooler face. The oldest defence is less cement and more fly ash, which reacts slower and cooler. At Gibe III in Ethiopia, the tallest roller-compacted dam in the world at 250 metres, the designers held the cement between seventy and a hundred and twenty kilograms per cubic metre and, for the first stretch of the dam, used a blast-furnace-slag cement in place of ordinary Portland, for exactly that reason.

One of the main advantages of this type of cement is that the low heat of hydration greatly contributes to avoiding thermal cracks in the massive concrete.

Pietrangeli et al., Design of the Highest RCC Dam (Gibe III, H = 250 m), Studio Pietrangeli, 2015.

What changed is that the world caught up to why this matters beyond cracking. Cement is the most carbon-intensive part of concrete, and concrete is the most-used material human beings make.

After water, concrete is the most abundant resource in the world.

Global Cement and Concrete Association.

Making it is what emits. Cement production accounts for around seven per cent of all human carbon dioxide emissions, and the hard part is where that carbon comes from. Roughly half to two-thirds of it is not from the fuel that fires the kiln but from the limestone itself: heat calcium carbonate to turn it into cement and it gives up its carbon dioxide as a matter of chemistry, before any fuel is burned. Renewable power cannot switch that away. The one lever that works everywhere is to make less clinker, the fired ingredient, and Son La's fly ash is that lever in its purest form. Each tonne of Portland cement carries on the order of eight hundred kilograms of carbon dioxide, most of it from the stone rather than the fuel, so a binder that is nearly three-quarters fly ash carries a fraction of the carbon of one made mostly from fired clinker. No dam of this kind publishes a carbon figure per cubic metre, but the direction of the emission factor is not in doubt: less clinker, less carbon.

None of this is news to Indian practice. India is the second-largest cement producer on Earth, and close to three-quarters of its cement is already blended with fly ash or slag, which holds down its clinker use and the carbon that comes with it. That is exactly the low-clinker lever the rest of the world is now reaching for. The blended cements the world is turning toward, Portland pozzolana and Portland slag, have been the Indian default for decades. And Indian roller-compacted dams have run the same low-clinker playbook. Teesta Low Dam Stage-IV in West Bengal was built at eighty-five kilograms of cement against a hundred and thirty-five of fly ash, more than sixty per cent of its binder the by-product. The low-carbon concrete the world is now racing toward, Indian concrete has in many ways already been living.

The lesson: The greenest cement is the cement you never make, and the dam engineer has been placed to prove it for half a century. Roller-compacted concrete with high fly ash or slag is not an experimental green material. It is standard mass-concrete practice, chosen first to keep the dam from cracking and now doubly justified by the carbon it never emits. Two cautions come with it. Fly ash and slag react slowly, so the strength has to be verified at ninety and a hundred and eighty days rather than twenty-eight, and the mix designed around that curve. And the material has to be the right one, low-carbon fly ash or properly ground slag, tested to IS 3812 and IS 16714, not whatever the nearest plant happens to be dumping. Get those right and the lowest-carbon concrete on the site is also the concrete holding back the reservoir.

Read more: Low-Carbon RCC Dams: Reducing Cement Content Without Compromising Durability →