What makes concrete stronger is the amount of rebar you put in it.
Actually it’s the ratio of Portland cement, water and aggregate. With some additional additives such as Fibermesh strands.
I have been a supervisor at two different concrete batch plants for over 25 years, thank you.
Oh and yes rebar is fundamentally required in many cases, but the Romans didn’t have it.
Actually, what does not kill you will make you stronger.