Part of the problem is "ancient Roman concrete" is the epitome of selection bias. We basically know why it's strong, and can basically recreate it, but it would take a very long period of process optimization and corrosion testing to get a consistent product we're sure has a long lifespan.
If you want something to last 1000 years, you may be better off cutting it out of a hard, stable stone (like granite).
How long? 1 year? 10 years? 100? 1000?
If it's not the last option, isn't it worth figuring out?
Actually, you don't have to imagine it. Go watch the show, "Peripheral". This idea of long-lasting concrete was actually what I found myself thinking about just last night watching it, in the scenes of future London.
Or is the tailoring of the mix to the environmental conditions going to be a more significant factor than the components?
There are actually lots of artificial materials that weather more slowly than rebar-reinforced portland-cement concrete. But buildings are huge — a small house commonly contains many tons of building materials, and a large building contains thousands of tons — so the overriding consideration for building materials is that they be cheap as dirt.
In fact, buildings — then and now — are to a very large extent literally made of dirt. And rocks. Bricks? That's clay, which is a kind of dirt. Mortar? That's sand, which is a kind of dirt, glued together with typically a small amount of ordinary portland cement with an even smaller amount of additives. Drywall? That's gypsum, which is the kind of dirt White Sands National Monument is made out of (as well as, in its mineral form, many stunning ancient Egyptian vases) although a significant amount of modern gypsum is sourced from flue-gas desulfurization. Plaster? That's sand glued together with cooked gypsum, except when it's sand glued together with quicklime, which is made by cooking chalk. And of course adobe is sand glued together with unfired clay.
The issue is that costs that seem totally reasonable when you're dealing with gram or kilogram quantities of a material become totally unmanageable when you need a hundred tonnes of it. You can use glass, wood, or steel (the cheapest metal) in more modest amounts, but they generally can't replace OPC.
So, there's a pretty short list of materials that are both strong enough and cheap enough to use as building cements, and which can additionally be synthesized on a building site between grains of sand (as opposed to, say, in a furnace isolated for oxygen, or at 500°). You have:
· the above gypsum, quicklime, ordinary portland cement ("OPC"), and unfired clay;
· tar (asphalt);
· geopolymer cements (milled metakaolin polymerized with waterglass);
· waterglass itself;
· Sorel cement (magnesium oxychloride);
· magnesium oxysulfate;
· calcium aluminates; and
· the phosphates of aluminum, magnesium, and occasionally calcium or even zinc.
Of these, some have worse weathering properties than OPC: unfired clay, Sorel cement, magnesium oxysulfate, and (if cured improperly, in a famous nationwide disaster in Spain a few decades ago which wasn't discovered for years) calcium aluminates.
But there are all kinds of extremely stable materials that would last much, much longer than concrete in the weather, but which are just too expensive to make buildings out of, or which nobody knows how to make big enough pieces of. Soda-lime glass (though foamed glass and fiberglass have uses), aluminum, titanium, aluminum bronze, epoxy "granite", gold, silver, polytetrafluorethylene, sapphire, yttria-stabilized zirconia, yttria, carborundum, silicon, the rubber used for car tires, and so on.
Grossly overbuild the structure, so that it is lightly stressed. Don't use any reinforcing material within the concrete (much of the damage that happens to a modern reinforced concrete structure is the result of the internal reinforcement swelling due to corrosion). Ensure that the structure doesn't freeze (water that penetrates the concrete and freezes will cause cracking).
Note that following these limitations would rule out the vast majority of modern concrete structures.
Raise the bar and use diamond.
I used to be in the CVD diamond world. On my honeymoon nearly 30 years ago, my wife and I were hiking in the Himalayas, in Sikkim.
My Dad loved diamonds and used them in his work (geophysical drilling, using diamond bits). He passed a year before my marriage. I wanted to leave a rem meminisse eum a in his honor.
So I grew a 2mm thick x 2.5cm diameter diamond disc. I used some nucleation art to make his first name appear as a texture variation in the faceted growth surface. I left it on a protected ledge at ~ 19,000 feet, with a note that just said “Thanks, Dad”.
That disc, or its fragments, will be around until that mountain gets covered by a lava flow or subducted into the mantle.
Concrete? A thousand years? Let's aim higher. :-)
Structures that lasted from ancient times to today were either made out of giant slabs of stone or had maintenance done on them repeatedly over the years.