As the title suggests. If roman concrete supposedly has the capabilities to mend tiny cracks via chemical reaction, why isn’t it used with modern reinforcements to seal the pathways to the steel beams to protect it from oxygen and elements and prevent corrosion? Are there any major downsides to hot-mixed concrete, is it not as good as the studies make it out to be, or is it simply not viable due to cost and manufacturing process/storage requirements?
We are still finding out new things about concrete and new ways to make it better. The concrete we have now are a lot better then the Roman concrete. What you are talking about is quicklime chunks in the concrete, likely due to poor mixing quality of the concrete. These chunks does make the concrete weaker and more likely to crack. But the self-healing capability is that when you mix this quicklime with water it fills the crack making it water tight again. However this lime crack is even more brittle then the original concrete. So the chances of it cracking again is very high.
So for example if there is a hole eroded under your concrete driveway and you drive over it cracking the driveway. Then yes, the crack may “heal” itself after a few weeks. But if you drive over it again then it will crack again. And it will not heal the second time.