Self level compound, I don’t know what I’m doing

[snapfish]

I need to self level an area around 4 m². The subfloor is made up of three different things, see photo, terracotta tiles, concrete and some other material. I’ve phoned Weber, Mapei and ardex tech help. And settled on using Ardex NA, as with the other two I would need two different types of primer, one for the none porous tiles and the other for the porous concrete.
My question is, I’m not sure what the other material is, is there way to tell if it’s cement based or this calcium sulphate screed I keep reading about.
Ardex say it needs to come up, but the tech document online says it can just primed with p51, so it’s Doesn’t touch the new self-levelling compound as it will react with it.
I can scratch it with a chisel, and when I soak it in water it doesn’t seem to break down. Am I worrying about it too much and just should get on with the job.

 

[CBW]

Looks like tiles, concrete and existing self levelling compound/screed.

 

[mrgobby]

My money on another levelling compound and/or concrete patched in repair rather than calcium sulphate. Personally I'd prime and carry on, Ardex will cover themselves with the least risky option and just based on a photo you can understand that.

 

[snapfish]

Ardex Recommends p51, but I can only get it in 5 litres. Can I use Weber pr360 instead? Or should I pay the extra and go with the Arden stuff?

 

[mrgobby]

Remainder probably sell on gumtree!