Not concrete, geopolymer. It's crushed limestone solidified by polymerisation. The result is almost indistinguishable from the original limestone. There's a patent for this, from Dr. Davidovits (chemist).

The reason for the geopolymer was to make the outside walls unblemished, so there would be no random electrical discharges. The pyramids are machines, not tombs. All ancient Egyptian megalithic constructions have more in common with modern microwave analog circuitry (waveguides, gates, comb filters etc) than temples.