I’ve been saying concrete for decades. It makes sense, being a a result of the necessary annual dredging of the river. Over time you could build up huge structures as a part of normal life. Every spring, the flood brings the raw materials to your doorstep.
The idea of exhausting a kingdom rounding up slaves, cutting stone with soft copper tools, moving them with non existent trees etc, over hundreds of years, is absurd.
Around the time I read Davidovits’ book (20 or so years ago) I saw a documentary on gold mining in Brazil. The miners lugged 40 pound bags of soil, rock, and whatnot out of their claims, climbing ladders, and processed these where that could reasonably be done. The claims were something like 8 feet on a side, adjacent to one another, with nothing but common paths in and out of the giant pit. These miners, over 20 years, took what had a been a hill and — 40 pounds at a time — turned it into a hole deeper than a concave reflection of the vanished hill.