Is it permissible to use cement blocks in the laḥd (a niche in the side of the grave) and to cover the graves of the deceased, knowing that cement is a material touched by fire, and does this contradict the prohibition mentioned in the ḥadīth regarding the use of what has been touched by fire in the graves of Muslims?

The prohibition of using materials touched by fire in Muslim graves was reported in Musannaf Ibn Abi Shaybah with a weak chain of transmission. However, scholars dislike materials touched by fire as an optimistic gesture that the deceased will not be touched by fire. Therefore, if it is possible to use materials not touched by fire in burial, that is preferable. But if it is not possible, then there is no harm in using cement to protect the deceased from floods or predators.