I was already aware of the geopolymer theory, but I had no idea it had been developed to such a degree.
This is a longer vid about Tiwanaku and Pumapunku: https://youtu.be/rf9qK9QTlq0?feature=shared

And this is a short demonstration video about the geopolymer process:  https://youtu.be/znQk_yBHre4?feature=shared

This certainly makes the most sense to me of any theory, including (gasp!) ancient aliens.