What is the ruling on building upon land that was purchased with money on which zakat was not paid, and which includes an unlawful amount?

If wealth reaches the nisab (definition) (minimum threshold) and remains for years without zakat (definition) being paid, then 2.5% of it must be paid for each past year, based on Allah’s Almighty saying: "And those who hoard gold and silver and do not spend it in the way of Allah – give them tidings of a painful punishment." As for land purchased for building (not for trade), there is no zakat on it. And unlawful wealth mixed with lawful wealth must be immediately removed to clear one's conscience.