Imagine we wanted to make an exact copy of the pyramids in Egypt, the great pyramid specifically.
Never mind building them without machines. Do you think mankind, not just the current residents of Egypt, but do you think in this world there exists the technical know how and political will to even be able to do it?
You could make concrete of any quality, it only depends on price. Concrete used, f.e. in building NPP reactor zones is magnitude orders better than any roman concrete you could find out. It will last for hundreds of millenia if not thousands.
Really, concrete checmistry is much more interesting and, what is more important, useful mystery than all pyramides of the Earth together. It is really still only partially studied, and not all processes going in concrete have explanation, despite our use of concrete for millenias.
Rebar does not rust inside concrete, because it is protected by alkaline environment of concrete. If you follow guidelines and do not allow rebar to stick out and keep at least 5cm between rebar and concrete surface it will last forever even in underwater constructions.
Egyptian astronomer need only to find directions to the sunrise and sunset of any day, then find a middle. Today we will name it vector algebra and sum of two vercotrs pointing to sunrise and sunset will be vector directed precisely to the axis of earth rotation, we don't really have to. People learned to precisely half things long before Egypt.
In any case, all that pyramides things is undoubtely fascinating, but I can't imagine that anybody here never heard about pyramides and didn't read similar posts thousands of times already.
Funny, that this constant repeating that Egyptians build pyramides with assumption that we could not do that now is even more puzzling then pyramides themselves. Like some cult that demands constant praising pyramids. IDK, choose some date (may be "encoded" in pyramides), make that day a "Egyptian pyramides day" and celebrate it. Would be a nice archeological holiday.
Our ancestors did a tons of interesting and amazing things and we don't know how exactly they did it. But all that things have no any relevance today, and nobody need to repeat them. They without a doubt have significance in historical sense, and deserve place in books and articles, but it is just senseless to repeat them again and again like they are something in line with current conspiracies.
I'm all for discussing anything weird and strange from bigfoot and UFO to pyramids and greek fire, but only in presense of some new information. Repeating all that old and well-known stuff everybody already read in hundreds of books and thousands of posts is extremely weird and looks like some strange conspiracism cult. We assemble together and tell each other exactly same old stories round and round. And it is even do not bring anything for soul or thought, unlike, say, church service or just listening beloved music. It's fucking weird.