Until reasonably recently when new evidence came to light most people believed that the pyramids were built by slaves.
There must have been some conviction behind this because until the 90's in the UK at least it was taught as fact in schools. Having a look around though I can't see what the original justification or evidence for this comes from.
Yeah, I was taught this as well.Looks like it was sort of derived from Herodotus and a lively imagination of the judeo-christian tradition.
AN ACCOUNT OF EGYPT
By Herodotus
" Down to the time when Rhampsinitos was king, they told me there was in Egypt nothing but orderly rule, and Egypt prospered greatly; but after him Cheops became king over them and brought them to every kind of evil: for he shut up all the temples, and having first kept them from sacrifices there, he then bade all the Egyptians work for him. So some were appointed to draw stones from the stone-quarries in the Arabian mountains to the Nile, and others he ordered to receive the stones after they had been carried over the river in boats, and to draw them to those which are called the Libyan mountains; and they worked by a hundred thousand men at a time, for each three months continually. Of this oppression there passed ten years while the causeway was made by which they drew the stones, which causeway they built, and it is a work not much less, as it appears to me, than the pyramid; for the length of it is five furlongs and the breadth ten fathoms and the height, where it is highest, eight fathoms, and it is made of stone smoothed and with figures carved upon it. For this they said, the ten years were spent, and for the underground he caused to be made as sepulchral chambers for himself in an island, having conducted thither a channel from the Nile. For the making of the pyramid itself there passed a period of twenty years; and the pyramid is square, each side measuring eight hundred feet, and the height of it is the same. "
Josephus also writes at the end of the first century CE that the Hebrews were enslaved and forced to build the pyramids in Egypt, so what was written in the Bible seems to have morphed into this by that stage:
And having in length of time forgotten the benefits they had received from Joseph; particularly the crown being now come into another family; they became very abusive to the Israelites; and contrived many ways of afflicting them: for they enjoyned them to cut a great number of channels for the river, and to build walls for their cities, and ramparts, that they might restrain the river, and hinder its waters from stagnating, upon its running over its own banks: they set them also to build pyramids...
Antiquities 2, Chapter 9. - http://penelope.uchicago.edu/josephus/ant-2.html