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How the Great Pyramids of Giza were built has remained an enduring mystery. In the mid-1980s, Davidovits proposed that the pyramids were cast in situ using granular limestone aggregate and an alkali alumino-silicate-based binder. Hard evidence for this idea, however, remained elusive. Using primarily scanning and transmission electron microscopy, we compared a number of pyramid limestone samples with six different limestone samples from their vicinity. The pyramid samples contained microconstituents (μc's) with appreciable amounts of Si in combination with elements, such as Ca and Mg, in ratios that do not exist in any of the potential limestone sources. The intimate proximity of the μc's suggests that at some time these elements had been together in a solution. Furthermore, between the natural limestone aggregates, the μc's with chemistries reminiscent of calcite and dolomite—not known to hydrate in nature—were hydrated. The ubiquity of Si and the presence of submicron silica-based spheres in some of the micrographs strongly suggest that the solution was basic. Transmission electron microscope confirmed that some of these Si-containing μc's were either amorphous or nanocrystalline, which is consistent with a relatively rapid precipitation reaction. The sophistication and endurance of this ancient concrete technology is simply astounding.