Scientists replaced 40 percent of cement with rice husk cinder, limestone crushing waste, and silica sand, giving concrete a rubber-like quality, six to nine times more crack-resistant than regular concrete. It self-seals, replaces cement with plentiful waste products, and should be cheaper to use.

[u/mvea]

https://newatlas.com/materials/rubbery-crack-resistant-cement/

[u/aprendemos]

It’s also worth noting that we cannot make concrete without sand (including the newest kind of concrete described in the post), and the process of gathering sand is terrible for the environment. Humans use more sand than any other resource except for air and water. The sand in concrete has to be water-derived sand, like the kind found on the bottom of the ocean or the banks of rivers. We can’t use desert sand to make concrete, as the edges of each grain are too smooth to be useful. So, in order to build new modern buildings and cities, countries are decimating their environments to access water-derived sand. We are destroying riverbanks, causing terrible flooding and decimating fish populations. We are digging up entire islands that are uninhabited by humans and mining beaches until erosion becomes problematic in the surrounding areas. You can probably guess that these issues are especially unregulated in countries like India and China that are constructing new buildings at dizzying rates.

There’s no easy solution. Cities are not possible without concrete. Concrete makes human lives safer and better, and currently, concrete isn’t physically possible without sand. Enforced regulations in all countries are essential, but that is easier said than done. People in affluent counties can renovate instead of building new homes form scratch and can get used to living in smaller homes/hotels/offices rather than trying to make every space a maximum luxury.

Here’s a summary of the book that describes this whole sand issue in depressing and fascinating detail: https://www.npr.org/2018/08/05/635748605/the-story-of-sand-in-the-world-in-a-grain

[u/Chagrinnish → u/aprendemos]

The sand in concrete has to be water-derived sand, like the kind found on the bottom of the ocean or the banks of rivers. We can’t use desert sand to make concrete, as the edges of each grain are too smooth to be useful.

That's not correct and/or you're contradicting yourself. River/ocean sand is smooth like "desert" sand; it's desirable when you're pumping or pouring large areas of concrete as it makes the mixture more flowable. That's not the highest strength sand to use for concrete however; masonry joints or castings will specify "sharp sand" as the jagged grains lock together better.

[u/aprendemos → u/Chagrinnish]

From the article: "The No. 1 thing that we use sand for is making concrete. And desert sand is too round to work in concrete. Desert sand has been worn down through thousands of years of erosion by wind tumbling and tumbling it and tumbling it. So the grains - the actual grains themselves end up kind of rounded with their edges and corners broken off, whereas sand that you find in riverbeds and on beaches and at the bottom of the ocean is more angular. So it locks together much better to form concrete. ...imagine trying to build something out of a stack of marbles, as opposed to trying to build something out of a stack of little, tiny bricks."

[u/Chagrinnish → u/aprendemos]

The author is comparing round sand and sand that is less round. The surface of either desert or river sand is still smooth and while river might be less round overall it's still a poor sand to use with concrete. Also note that the author is not an engineer; he's a journalist with a single book to his name.

The only reason why river sand is valuable is that it's easy to collect. With a shovel and a dump truck you're on your way to selling sand, and given that most cities are near a river there's always a nearby source where shipping the sand is going to be the highest cost overall -- up to 10x the cost of the sand itself. Its value in concrete still pales in comparison to sharp sand created by crushing rock but such an operation (a quarry, crushing equipment, and a lot of time) requires a much greater investment in capital than shoveling it out of a river.