"You'll need two tons of cement - minimum" sounds like a sort of shorthand way of speaking. For me, a full sentence would be "You'll need a minimum of two tons of cement", "You'll need at least two tons of cement" or "You'll need two tons of cement at least". I don't like "at the minimum" at all. I'm not disputing that it's grammatical and that it has a definition on the dictionary, I'm just not keen on it. Having said that, I do use it at the beginning of sentences sometimes. For example "At the absolute minimum, I will need twelve pairs of socks for my walking holiday".
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