Lime-ash floors were an economical form of floor construction from the 15th century to the 19th century, for upper floors in parts of England where limestone or chalk were easily available. They were strong, flexible, and offered good heat and sound insulation.
Lime-ash is the residue found at the bottom of a wood-fired lime kiln, consisting of waste lime and wood ash. These kilns became common in the early 1400s and continued to be used until newer technology replaced them in the late 1800s. Lime-ash could also be made in coal-fired kilns. In areas where gypsum was common they were known as plaster floors. Isaac Ware in his A Complete Body of Architecture (1756) remarks on “the beauty of floors of plaster mixed with other ingredients”, comparing them with those of granite.”
Note: you could use the same method to make slab floors on grade. Add a moisture barrier under the slab to prevent wicking of moisture.