Concrete wont harden

[jwr]

I had a left over half bag of ready-mix concrete. It's about 2 years old. It was still powdery but had a few small chunks in it, which easily broke up with my fingers. I filled a 8" round hole in my sidewalk where there used to be a wooden post. The weather has been in the 80s and it should have dried quickly. I mixed it so it was like thick paste (the way it should be). 24 hours later it was all sand-like and was not hard. I wet it down. (I know it should be kept wet) and the whole surface washed off. Now it's all sand looking. Does old cement go bad? It seems this stuff has.

JWR

[dpb]

You answered your own question by response (and by experiment it seems)... :)

Cement is hygroscopic and will absorb water from the atmosphere when exposed and gradually "cure". As you may know, concrete actually undergoes chemical reaction as it sets (saying it "dries" is a misnomer). This happens slowly from the moisture in the air and if it has gone far enough, there's just not enough left to make a decent mix when used as you've found out. The moral is if the job is of any significance at all as to the strength of the concrete, don't use up old material.

[Goedjn]

Yes, old cement goes bad. To fix it, you have to put in in a big can with only a tiny hole in the top, and heat it till it glows. For about a day.

[jwr]

I fixed it. The stuff in the hole never got hard. It was like compact sand. I dug it out of the hole, mixed a coffee can of portland cement to the same stuff and reused it. It's hardening now. I was lucky that a friend had a few pounds of portland cement left from a job and he was going to toss it. This is probably a 50-50 mix, so it should get very hard now.