If you are dead broke and cement plates are the only way you can lift, go for it. And kudos to you for not letting poverty get in the way of getting strong. If you aren't broke and plan to stick with lifting (and even if you're not), I'd spring for real olympic plates, and even consider getting a pair of 45 lb bumper plates as well.
You can buy cheap, but perfectly usable CAP barbell 45 lb. plates from Walmart.com for under a dollar a pound with free delivery to your house. This is a good deal for something that will essentially never break or wear out. You won't have to carry, mix, and pour cement to make some kind of substitute that probably isn't going to hold up to deadlifting, snatches, and cleans. Also your home gym won't look like something from the Flintstones. If you lift for many years, the cost of having the real thing will amortize down to almost nothing. If you don't lift for many years, you can sell them for pretty close to what you paid.
What kind of surface are you going to be lifting on? If you are on a wood platform or rubber platform (as you should be), it wouldn't surprise me if concrete takes chunks out of it at some point. If it is something harder, you risk breaking your concrete plates. How accurate do you think you'll be able to make your plates? It isn't so important that your 45 lb plates weigh exactly 45 lbs, but it is important that they be close in weight from one side to the other. Lastly, if you are planning on lifting heavy, I think you will find that the extra width and possibly sloppy fit to the bar are going to be a problem. If you planning on deadlifting over 405 (which can happen very quickly if you are serious about your lifting), you'll have at least four of these hunks of concrete on each side of your bar, and that just doesn't sound like a great idea to me.
BTW, if you are interested in Stronglifts, you might also want to check out Starting Strength.
Whatever you do, good luck with your program! I gave up on gym memberships about 14 years ago to lift heavy at home. It was the best investment in my own health and sanity that I ever made.