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/danielravennest]

For those not familiar with concrete, it typically is made from gravel, sand, cement, and water. The water turns the cement powder into interlocking crystals that bind the other ingredients together.

There are a lot of recipes for concete, but the typical "ordinary Portland Cement" concrete is made with a cement that starts with about 5 parts limestone to 1 part shale. These are burned in a high temperature kiln, which converts them chemically to a product that reacts with water.

Lots of other materials will do this too. The ancient Romans dug up rock that had been burned by a volcano near Pozzolana, Italy. The general category is thus called "Pozzolans". Coal furnace ash and blast furnace slag are also rocks that have been burned. They have long been used as partial replacements for Portland Cement. Rich husk ash and brick dust are other, less common, alternative cements.

Note: Natural coal isn't pure carbon. It has varying amounts of rock mixed in with it. That's partly because the coal seams formed that way, and partly because the mining process sometimes gets some of the surrounding bedrock by accident.

Portland Cement got its name because the concrete it makes resembled the natural stone quarried in Portland, England at the time.

[u/ImFamousOnImgur → u/danielravennest]

I did a paper in undergrad about Roman concrete. Their recipe was no joke. It’s a big reason why their stuff is still standing to this day.

Coliseum? Yup. Roman concrete. Oh and you know how some of the walls collapsed after an earthquake in 1500 something? Yeah those were the sections that were built by a different architect and he didn’t use the same materials.

[u/noelcowardspeaksout → u/ImFamousOnImgur]

For the Pantheon they used different grades of concrete made with different additives depending on the qualities they required. The dome has pumice included to make it light for example. It has stood for around 2000 years without being rebuilt.

Edit: Pantheon

[u/ImFamousOnImgur → u/noelcowardspeaksout]

Yup. It’s quite amazing the amount of knowledge they had. A lot of that knowledge was lost when the empire fell.

They think the secret to the quality was the volcanic rock used, and if I recall, it was especially good at setting underwater even.

[u/Opeewan → u/ImFamousOnImgur]

There's a bit more to it than that, salt plays a big part in it:

https://www.nature.com/news/seawater-is-the-secret-to-long-lasting-roman-concrete-1.22231

[u/Opeewan → u/[deleted]]

Obviously they didn't and either came up with their recipe through trial and error or it was a lucky coincidence.

[u/darklorddanc → u/[deleted]]

Well, pretty much all concrete does get stronger in a very noticeable way if you ever have to remove it. The difference between concrete that is a year old and thirty is very obvious if you have to remove it. Concrete that has been setting for one year is relatively easy to remove or grind compared to older concrete. They probably just measured it by observation. And they probably developed a common protocol just like we have for when you can put concrete into full use at 4, 10 and 40 days by observation and familiarity and simple experience. What works and what doesn’t. If something these guys worked on failed they weren’t working on 15 other things so they could focus on stuff and see what presented itself as far as cause and effect.

[u/10MeV → u/darklorddanc]

We had a family cottage with a concrete step/porch that was probably over 50 years old. A sledgehammer mostly bounced off of it. That concrete might as well have been granite!

[u/TooFarSouth → u/10MeV]

So what we need to do when we build new roads, it seems, is construct them with concrete and then open them in 50 years. Road work today moves much too quickly!

[u/[deleted] → u/TooFarSouth]

Some roads are made with concrete and last a long time, it's just very expensive and dependent on weather (big temp. changes are bad)

[u/10MeV → u/TooFarSouth]

Brilliant!

[u/Memetic1 → u/10MeV]

And now I want a concrete house... Damn consumerism.

[u/[deleted] → u/darklorddanc]

Thanks. It's not something I have much experience with

[u/G_Morgan → u/darklorddanc]

As somebody who's just renovated an old masonry stone wall + concrete mortar house to install stuff like proper rafters I can attest to this. Concrete just keeps getting stronger the more you dry it. Getting ~200 year old concrete off a wall is an exercise in frustration without power tools.