I think the pyramids were cast using a mixture of limestone and granite. They might have built wooden frames (dyes) to cast the mixture, then once set, burnt the wood. There have been wooden structures found inside the great pyramid. This way, the building of the pyramid is easily explained. Lots of people lugging buckets up and down the structure to pour mixture into a dye. I can't conceive of any other way it could've been done.

They might have used heat or a chemical reaction (like concrete) to liquefy and solidify the mixture.