I say, and with God's success: If the brick is not built with it, meaning it is moved and transferred from its place, if the impurity is on the side where the worshiper is, then prayer is not permissible on it. However, if the impurity is on the side that is next to the ground, then prayer is permissible on it; because it is like the ground, so the condition for it is the purity of the place of prayer only. See: 'Umdat al-Ri'ayah p.1: 139, Naf' al-Mufti p.153, and Al-Bahr al-Ra'iq 237, and God knows best.