Mortar Under Acrylic Shower Base


I'm re-doing a shower using an acrylic/fiberglass base. The instructions say to put "mortar cement" under the base. Makes sense, there were lumps of grey rock-like substance under the one I pulled out. Problem is, I can't translate "mortar cement" into anything I can buy at HD, etc.
My searches tell me to use the same mortar as laying bricks, or to NOT use the same stuff, use regular concrete mix, use regular mix with sand added, don't use regular mix as it will wreck the plastic, etc etc etc. I think I know less now than when I started.
Can somebody tell me what it should say on the bag at the building center? Brand names are fine, if it isn't available around here at least I can ask for equivalent product.
Thanks, Red
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You are looking for "Thin-set Mortar" and you will find it in the tile section, as it is used to set tiles.
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EXT wrote:

mortar-bedded shower bases and plastic tubs I ever saw go in were bedded in plain old mortar mix. You definitely don't want concrete, lest a stone poking up worry its way through the shower base.
ISTR on TOH a year or two back, they showed people using gypcrete, and even a plastic foam that set up rock hard, instead. It doesn't need that much strength like a sidewalk would, it is just to keep the shower base from flexing as people stand and move in it.
-- aem sends...
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Thin-set is only as runny as you make it by adding too much water. To bed a shower base, you need something that is soft enough to conform to the bottom shape so that it supports the base completely when it hardens. There will not be much room under the base, so this is where thinset comes in, as it will harden and maintain its strength even in very thin layers. If a thick bedding is needed, then masonry mortar "may" work
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wrote:

I used thinset, mixed to a "low slump" consistancy and just "worked" the base in.

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My base flexed a bit when it was on the cement floor. Called the manufacturer and he recommended using expandable foam--the kind that is used to seal leaks etc. and comes in a spray can. I inserted the small nozzle extension in all the open spaces under the base and let it fill until it started coming out. Let it harden and then cut off all the stuff that had expanded outside of the base. It's been several years now without a problem and the base is solid--no movement at all. MLD.
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