Is zakat obligatory on a sum of money paid to reserve cement that has not yet been received, and the time for paying zakat has become due?

The cement you purchased presents two scenarios: The first is that the sale contract has been concluded, and you intended to sell it. In this case, it is considered a trade commodity, and its zakat (definition) is due by valuation, even if you have not taken possession of it. Zakat is due on it every year, unless there is an impediment, otherwise, the zakat for all years is paid upon possession. The same applies if you purchased it to build a building intended for trade. The second scenario is that you paid the money but the purchase contract has not been concluded. In this case, zakat is due on the money paid because it is still in your possession.