Due to relocating the shower and tub in my bathroom remodel, there is now an unused shower drain under the center of my jacuzzi tub. This is a concrete slab floor with PVC pipe drains.

What is the best way to seal off that drain? The plumber said to just fill it with sand. He said the trap will prevent the sand from moving down the line.

I am thinking something more air and water tight just in case of sewer gas or water backup.

Should he find a PVC plug and glue in or is it okay to dump concrete into the drain? I think the plumber wants to use sand because the plug may stick up and interfere with the mortar bed under the tub.