There is no restriction to praying on any medium, as long as it is "Tahir" (طاهر). There were no carpets in mosques for at least 40 years after Hijrah (الهجرة). The Prophet (saaw) had a sandy floor (or actually no floor). Omar Ibn ElKhattab (ra) introduced gravel to the mosque's floor, and it wasn't until the era of the Omayyad state that carpets were introduced in mosques.
There were also recorded cases for companions praying in mud during rains. In short, pray on any floor you wish.
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