I had one tube clump up this year. It won't go thru the nozzle. (It was used once before and it was just fine.) I just mixed it in a small container and used it. It hardened in the same amount of time and it did not fail. But at the time, I wondered the same things (e.g. what caused it to clump and was the strength compromised.) I ended up tossing the rest of it in that tube(s). Mine cam from the same supplier.