sequel to old substance under flooring

Sorry I didn't give a larger picture of what I'm doing in the first post on this topic. This entire apartment has a concrete floor. We're expanding the kitchen, which was originally hexagonal mosaic set in about 1-1/4" of mortar (over the concrete, which is the same level throughout the place) Over that, they put down 12 X 12 ceramic tiles. We're going to match these tiles in the expanded area for one continuous floor, so that means going down to the concrete where the herringbone was. That means two layers of concrete board set in mortar and whatever else to level up, and that's what's got to stick to the black stuff. A scraper gets down to the level of the concrete easily, so I guess the question is, will the latex mortar stick to it? I'll experiment. The Bosch hammer would be a great acquisition, but pricey. I see at Menards there's a Bosch 1-1/8" hammer for around $279 but I wonder if it's up to the job, because I'll be getting other tile jobs here and removing old tile and concrete board can be draining. It's hard to stop talking about tools - just picked up a Rigid Fuego mini sawzall which fills a gap in the tool market that's been there ever since the Porter Cable Tiger Cub disappeared about 15 years ago. The Fuego seems to be a quality tool.
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If I were going over an old floor with two layers of concrete underlay sheeting, I wouldn't be that careful about what I was trying to adhere to as the first layer.
I would certainly clean off all loose and removable debris and coatings. Some of the latex mastics are quite good, but I would still go with a solvent based adhesive.
The trick to a good, solid substrate when you have two layers is to adhere the first layer to the concrete, and the second layer to the first with plenty of mastic, and a few Tapcon style anchors.
You can make an inch thick solid (near monolithic) subfloor by lapping your joints properly, adhering properly, and adding a few mechanical fastners. Installed in this manner, adherence to the concrete is almost a moot point.
As a matter of fact, in high movement areas of town, when an inch or so of substrate (board or thickset) is applied under tile, many here are starting to use a slip sheet method of installation over the concrete and don't attach the built up material to the concrete at all. This requires a solid, bondable substrate, but not solid attachment of same.
I ran into this method about 20 years ago in Houston when remodeling a house there, and the tile guy I liked put down a sheet of 6 mil, then built a 1 1/2" mud bed over the plastic. The floor never cracked or moved where you could see it. Two neighbors remodeling at the same time were informed that "it wasn't necessary" to put a slip sheet in, and their tile cracked within a year. The cracking was blamed on slab movement ( no... really?) and there was no way to hold the tile contractors directly to blame as he put in the tile as specified in their respective contracts.
I was last inside that house about 5 years ago, and no cracks in the floor, even after all these years. That method pretty well accepted around here now.
As far as tools go, if you see a large tool that would help you zip through a job, why not rent? We have a lot of rental places around here that cater to contractors, and they have all kinds of tools. We even have two Lowe's stores and two HDs that rent tools these days. And they carry all bits, abrasives, cutters, etc. for the machines right there in the store, and they don't gouge you when you need to buy them.
As always, just my 0.02.
Robert
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