It looks like the main problem with previous experiments with biochar added in cement was that the biochar did not bind well, making a crumbling result, but this article is about solving precisely that.
How important is binding in compression (which is what concrete is used for)? Not very.
Carbonate doesn't strengthen, it weakens. And if forming carbonate in biochar strengthens versus standard biochar, I would imagine forming oxide in biochar would strengthen even more.
The binding part is very important. It holds grains between them. For example that's the difference between a hard granite and a crumbly, sandy granite rock (that's called "decomposed granite"). It's day and night.
And I also believe them that the treated biochar is stronger than the untreated one. Maybe if the same process was used to treat the cement itself it would be a disaster, but here that's not the case.
What do you mean "forming oxide in biochar"?? The oxide of biochar is CO2. Are you trying to make fun of me heh...
But I could see it depositing in biochar as Calcium Oxide. A physical mixture, not a chemical reaction, is what I meant.