Cementing metals from solutions using copper, here we are discussing metal ions in solution, a metal ion which if upon the gain of electrons will become a metal atom, if enough of these metal atoms come together we will be able to see them as a metal powder which will gain enough mass to precipitate from solution by gravity.
When we have metal ions in solution and we add a metal atom (or a solid group of metal atoms like a mass or metal bar of pure copper) the copper metal which has all of the electrons in its outer shell) will donate its electron to any metal ion in solution which is lower in reactivity than the copper atom (see your reactivity of metals chart for more details).
So upon a gain of an electron the metal ion in solution becomes a metal with a full shell of electrons.
the copper atom loses electrons and becomes an ion in solution.
Cementing and plating, are actually two different things, although the reaction is the same, or similar.
The name or term cementing comes from cementing silver out of solution where the silver metal powder resembles cement (wet concrete), plating the metal atoms plate the copper).
Note we can also copper observes a corrosion of the copper from salts, carbonates or hydroxides ... which can discolor the copper bar that may or may not be actual metal atoms ...
In "cementing" or displacing a metal in solution with another metal, like cementing silver from solution, some free acid can assist the reaction, it will help in keeping the copper atoms exposed to the silver ions in solution, reactions and bubbles can remove silver from the surface of the copper exposing more copper atoms to more of the silver ions in solution (the silver basically does not plate the copper blocking the copper atom from the silver ions in solution, in some way the bubbles act as we do when we shake the silver from the bass bar, the acid itself can help to take electrons from the copper.
Now let us look at the pH of a solution if we raise the pH we can get closer to hydroxides, this change of pH can even bring metals out of solution as salts, or compounds of metals (depending on what has lowered the pH).
Say we have metal ions in solution just a big soup of poisonous metal ions in a toxic solution (leaching ore produces many different species of the metal ions and toxic compounds depending on its makeup, say we actually did have ions of metals more noble than copper in solution, and we lower the pH, now we can have other salts or compounds of metal in solution, some of which may be coming out of solution by just the change in pH...
I do not see anything "cemented" in your experiment of cementing metals, all I see in the pictures is some cut copper with discoloration, which could be a coating of metal or salts or compounds...
Where are the "cemented" materials (metal powders or salts), these powders will not be coating your copper they will be the powders which come out of solution (basically fall loose from your copper buss bar, and fall to the bottom of the reaction vessel) these powders can be tested to find out what metal or compound, or salt is coming out of solution.
Now you have a coating on the copper, you can brush or scrap off this coating and test it.
Or continue your experiment with different pH until you actually get some cemented metal powders or salts, and test them.
My best guess says, that if you acidify the cement (from the pH 4 solution) much of the salts will go back into solution, as the acid takes electrons from the salts...
Leaching ore with a chloride solution, I surely hope you completely understand the dangers of your experiments, I can see you are struggling with the chemistry, which without an understanding of, in the case of leaching ore can surely lead to an early grave and no gold.
