[PhD Jury] Accelerated carbonation of Municipal Solid Waste Incinerated Bottom Ash for the design of concrete

A few hours of CO₂ treatment can reproduce many of the changes that take months when incinerator bottom ash ages naturally. But how much of that treated ash can go into concrete? The answer from @Imad Eddine KANJO’s PhD research surprised me. It depends less on the replacement rate than on how much metallic aluminum is left in the ash.

I chaired Imad’s PhD defense at the @University of Liège. He presented his work on replacing quarried aggregate with carbonated bottom ash from municipal waste incineration.

Imad tested two ash batches with similar physical properties. With the first, the concrete kept good workability and strength even when the treated ash replaced all of the natural coarse aggregate. The second had about three times more residual metallic aluminum. In the alkaline cement paste, that aluminum released hydrogen, and the concrete swelled, microcracked and lost strength. The damage began at 50% replacement and grew at 100%. Standard physical tests did not show any difference between the two batches.

This matters for circular construction. Europe is now standardizing circular construction through prEN 18177. Imad’s results show why secondary materials need to be checked batch by batch, beyond a recycled-content claim. For concrete producers, that means measuring the metallic aluminum before deciding how much aggregate to replace.

The carbon results show how much is at stake. Under optimized conditions, the treatment mineralized about 15 kg of CO₂ per metric ton of ash. In the modeled mix, replacing all the natural coarse aggregate lowered cradle-to-gate emissions from 248 to 235 kg CO₂e per m³, about 5%. Cement remained the largest source of emissions. In our own work on embodied carbon benchmarks and net zero carbon buildings, material choices keep coming back as the decisive factor in upfront carbon. A 5% saving on one mix is modest. Applied across a national building stock, together with less quarrying, it adds up, as long as the treated ash meets the performance requirements of the application.

Congratulations, Imad, on your successful defense. I was glad to share the discussion with your supervisor @Luc Courard and fellow jury members @Kinda Hannawi, @Stéphanie Staquet, @Bertrand François and @Grégoire Léonard. I also thank the partners of the CARBOC project (REMIND Wallonia portfolio) who supported the research.

Kanjo, I. E. (2026). Accelerated carbonation of Municipal Solid Waste Incinerated Bottom Ash (MSWI-BA) for the design of concrete [Doctoral thesis, ULiège - Université de Liège]. ORBi-University of Liège. https://orbi.uliege.be/handle/2268/349185

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