Hardie backer board as bench top???

Has anyone used Hardie backer as a cap to their mass? i had it all cut and set to go (i was simply going to use this as a base to then our a slab on top of). and then began to get second thoughts. My mass walls are made from cinderblock filled with cinders and when trying to attach the hardie backer to the block via mortar it did not stick. i then scraped that idea and made a mold and just poured myself a concrete slab to cap it off with. However just about an hour ago trying to lift said slab, i slipped and down goes me and slab, cracked now in half and my pinky finger nearly exploded........ so im thinking i might want to go back to the hardy backer pour in place slab idea. my question is do you guys think it will be ok to have my hot gasses flowing against the hardie backer board? and dos anyone have any ideas on how i can get a secure seal between hardie backer board and cinderblock?

Sorry to hear about your pinky! I have one pinky that is permanently fatter than the other from a similar incident. Majesty hard to fit in my nose...

What mortar are you using? Fireclay and sand?
I would try affixing expanded metal mesh to the underside of the concrete board with screws, skim coating that with refractory mortar and pressing this down into a mortar bed  on  top of the cinder blocks.
Mind you, the board at not have the strength to support being sat on,so you may still want to pour a slab atop of it.

Alternatively,build a form and pour in place.
The sides should rise a little above the top of the cinder blocks.
The bottom should fit inside of the blocks,held a little below the top of the block walls with vertical lengths of 2x4 .
Pour a slab reinforced with fibers and/or metal mesh.
There should be an access point for inspection and cleaning, use this to remove the 2 X4's.
Leave the bottom of the form in place, to be burned out ,or perhaps use the cement board.
The cement board might crumble in the heat,  but that could be removed via the inspection/cleaning port.

I have tested hardi-backer board; with a propane torch, it can be made to char on exposed edges and surfaces, but without flames hitting it, it should be quite durable. I would consider it a fine bottom form for a poured-in-place concrete slab if you want a concrete surface. Hold it back a couple of inches from the edges so that you only see concrete if you want a neater effect.

Just picked up some supplies and going back at it.  Pouring the slab over the baddie backer was the plan. I'm going to try a skin coat of Portlant cement to try to get it to seal. If that doesn't work I'm going to use a heat resistant caulk and just run a bead along the outside edge. I have it set up so there is an overhang to give the bench a bit more width along with ascetic appeal. My main concern was having gasses flow along the hardy backer.

As far as clean out goes I'm making my bell (which is essentially the whole build) easily removable for inspection/cleanings as I will be able to also look into the channels from this point.

For a better idea of what I'm talking about, the build is acctually a batch box model with a pyramid bell, I have a thread started showing its progress and 3D sketch here https://permies.com/forums/posts/list/60340#515032

Thanks for the help I'll keep yah guys updated!

P.S. The pinkys gonna make it!