As a landscape gardener in recent years, I use cement all the time. It's massively alkaline to the point of being dangerous if handled by the skin for too long. You can get chemical burns. But I wouldn't worry too much, I'm covered in it most days. You'd want to mix it with sand or it'll be brittle, but that's for construction use, strength isn't an issue if just used in a Malawi tank. Add sand also as its around half the price of cement. £2.50 for a 20kg bag of sand vs £4.65 for a bag of cement. Normally you'd want a ratio of 5 bags of sand to 1 bag of cement for mortar mix, or 3 bags of gravel, 2 bags of sand and 1 bag of cement, that'll make you concrete, which is stronger still.
Concrete takes around 28 days to fully cure, but never stops curing after that, it simply increases in hardness over time/years. Depending on your mixture ratios and how much water you add, you'll have a working time with it of between 1 and 3 hours, then it'll start firming up. It sets from the inside out, so once it starts to firm up, you won't be able to push your finger into it more than half a fingernail's depth, but the surface will still be workable. If you wet the surface at that point, you can smooth it if you wanted a smooth surface.
Get Blue Circle general purpose cement, don't get any fast set, you need to be pretty experienced to use that as you don't get much working time. Postcrete for instance goes firm in seconds and is solid in a couple of minutes.
If you want any other info let me know.