Any idea what this is?

Hi. Not sure if anyone can help. Just removed old flooring in my house (built 1904) and wondering what this layer of grey slate like material over the top of the old quarry tiles is. I’m assuming it’s some kind of adhesive for a subsequent set of tiles to sit on but hoping it doesn’t contain asbestos. Any ideas?

 

It’s a self levelling compound. It’s cement based

 

Great, thanks for that