I came across this fascinating article.
The Ancient Egyptians built their great Pyramids by pouring concrete into blocks high on the site rather than hauling up giant stones, according to a new Franco-American study. The research, by materials scientists from national institutions, adds fuel to a theory that the pharaohsï¿½ craftsmen had enough skill and materials at hand to cast the two-tonne limestone blocks that dress the Cheops and other Pyramids. Despite mounting support from scientists, Egyptologists have rejected the concrete claim, first made in the late 1970s by Joseph Davidovits, a French chemist. The stones, say the historians and archeologists, were all carved from nearby quarries, heaved up huge ramps and set in place by armies of workers. Some dissenters say that levers or pulleys were used, even though the wheel had not been invented at that time. Until recently it was hard for geologists to distinguish between natural limestone and the kind that would have been made by reconstituting liquefied lime. But according to Professor Gilles Hug, of the French National Aerospace Research Agency (Onera), and Professor Michel Barsoum, of Drexel University in Philadelphia, the covering of the great Pyramids at Giza consists of two types of stone: one from the quarries and one man-made. ï¿½Thereï¿½s no way around it. The chemistry is well and truly different,ï¿½ Professor Hug told Science et Vie magazine. Their study is being published this month in the Journal of the American Ceramic Society.
The pair used X-rays, a plasma torch and electron microscopes to compare small fragments from pyramids with stone from the Toura and Maadi quarries.
They found ï¿½traces of a rapid chemical reaction which did not allow natural crystalisation . . . The reaction would be inexplicable if the stones were quarried, but perfectly comprehensible if one accepts that they were cast like concrete.ï¿½
The pair believe that the concrete method was used only for the stones on the higher levels of the Pyramids. There are some 2.5 million stone blocks on the Cheops Pyramid. The 10-tonne granite blocks at their heart were also natural, they say. The professors agree with the ï¿½Davidovits theoryï¿½ that soft limestone was quarried on the damp south side of the Giza Plateau. This was then dissolved in large, Nile-fed pools until it became a watery slurry.