The researchers point out that pouring concrete would have spared the ancient builders from using steep ramps to push stones to the summit of the pyramids.
Zahi Hawass, chief of Egypt's Supreme Council of Antiquities, dismissed the theory as "unlikely."
He noted that concrete was widely used at the pyramids in modern restoration work, suggesting that team may have taken samples from these modern cuts.
But Barsoum rejected such criticism.
"I would have to be a complete and utter fool to confuse Portland cement to what we saw," he said.
David Walker, a Columbia University geologist, said that Barsoum and colleagues have a strong case when considering the mineralogical constitution of the block chips they examined.
"Both sides in this controversy have good points. Some blocks are definitely natural and some are not," Walker said, adding that the mystery over how the ancient Egyptians may have poured concrete is "all the more intriguing."
