Great Pyramid stone blocks probably cast in limestone

"This is not my day job." So begins Michel Barsoum [professor in the Department of Materials Science and Engineering at Drexel University] as he recounts his foray into the mysteries of the Great Pyramids of Egypt. As a well respected researcher in the field of ceramics, Barsoum never expected his career to take him down a path of history, archaeology, and "political" science, with materials research mixed in...

The widely accepted theory-that the pyramids were crafted of carved-out giant limestone blocks that workers carried up ramps-had not only not been embraced by everyone, but as important had quite a number of holes...

[A]fter extensive scanning electron microscope (SEM) observations and other testing, Barsoum and his research group...found that the tiniest structures within the inner and outer casing stones were indeed consistent with a reconstituted limestone. The cement binding the limestone aggregate was either silicon dioxide (the building block of quartz) or a calcium and magnesium-rich silicate mineral.

The stones also had a high water content-unusual for the normally dry, natural limestone found on the Giza plateau-and the cementing phases, in both the inner and outer casing stones, were amorphous, in other words, their atoms were not arranged in a regular and periodic array. Sedimentary rocks such as limestone are seldom, if ever, amorphous.

The sample chemistries the researchers found do not exist anywhere in nature. "Therefore," says Barsoum, "it's very improbable that the outer and inner casing stones that we examined were chiseled from a natural limestone block."