What should one do if they cast the pebbles at the Jamarat with stones collected from Muzdalifah, then later read that the throwing is not valid if the stones are made of cement that does not contain gravel, and they do not remember whether their stones contained cement or not?

The majority of scholars stipulate that the pebbles for the Jamarat must be made of stone. However, the Hanafi school of thought permits stoning with anything that originates from the earth. As long as the thrower has taken the pebbles from where they are found and thrown them without believing that it is not a stone, then the outward appearance of his stoning is valid. A doubt that arises after stoning the Jamarat or any other act of worship is not to be given any consideration.