I guess the first question since, this is a forum full of tufa nuts - Why would you buy concrete? Rather make it out of something that will not be so likely to crack. Sorry had to ask, they probably look cool.
I think that what ever you use to seal them , you will have to do it every year. Esp in CT. They shape of the container is important. If it is open, like a bowl it will be less likely to crack due to frost heave. I would look on line for a high quality concrete sealer. Products like Thompson s water seal are ok, but maybe not as long lasting as high quality sealer. Wait till next year, most sealers need warm temps ..

Good luck

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They do not need to be sealed. They will need to have excellent drainage tho. They should weather for a few years before painting. If you wanted to paint it now I would wash them in a mild acid. You can just paint it with an acrylic paint. I painted a concrete bird bath with a high gloss acrylic 15 years ago. It needs repainting but the paint job did very well. I painted the entire piece. Its may be best to use an interior/exterior high gloss acrylic paint, because it have enough vehicle to penetrate and seal it. I would not use silicone products because they evaporates in a month or two. You could paint it to look like aged copper. That would be the best idea if you like that look. I have been a commercial painting contractor and have painted lots of concrete. When you paint it it must be painted on the inside as well as out to have a consistent perm rate. Acid does not mean vinegar, don't worry about it, just don't drop it.

I would strongly consider staining vs painting but thats just me, just look up stained concrete, they come in almost any color.