What type for mortar does one buy to set a plastic tub into to make it a bit more solid underfoot? This would be on top of a plywood subfloor.

Thanks

What type for mortar does one buy to set a plastic tub into to make it a bit more solid underfoot? This would be on top of a plywood subfloor.

Thanks
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I doubt it makes much difference. If you are only doing one tub, go ahead with whatever you find cheap, perhaps concrete mix.