Not exactly sure why your original mix failed but yes, you're trying to make concrete, (mortar is a no gravel finer mix for laying brick, block and stone, plus rendering finishes etc. in layers usually no more than 1/2" or 13mm thick)
It's possible that it was too thin? On a really solid base you might just get away with 2" (50mm) but really 3" (75mm) is a 'minimum', with 4" (100mm) being the 'norm'.
Right then concrete. A 'standard' mix if there is such a thing, is 1:2:4 which is one cement, 2 sharp/washed sand and 4 gravel/crushed stone. This is fine for general concreting jobs like pathways, floor slabs etc. often in thicknesses from around 3" up to 6" (75mm to 150mm).
Thicker concrete for things like foundations and larger mass concrete is usually a 'weaker' mix of 1:3:6 which is one cement, 3 sharp/washed sand and 6 gravel/crushed stone.
In all concrete just enough water should be added until the mix folds nicely off the paddles/tines inside the mixer. If the paddles run straight through the mix it's too wet, if it crumbles it's borderline too dry. Dry concrete will dry very strong but it needs careful placing and compacting/agitating to get rid of any air in the mix. Technically as someone else said earlier, there only needs to be enough water in the mix to hydrate all the cement particles but in practice bung enough in until you get a nice workable product that doesn't slop but doesn't heap up in crumbly piles either.
These two mixes cover the vast majority of concreting works in domestic construction (in the UK at least).