you mean over years the concrete will deteriorate to nothing?
No it won't, well it should not anyway, but that is one of the issues with "cheaperized" forum recipes. The issue of phosphates is not an issue at all, unless you use a "cheaperized" forum recipe.
If you want to know the real recipe, it is as follows:
1 bag sea-flor grade ARAGONITE "sand."
1 bag Aruba shell ARAGONITE "gravel."
1 bag RIVERSIDE low alkalinity white cement.
2 - 3 cubic feet of plastic sawdust (go sweep up for an acrylic fabricator.)
a bucket of water (5 gallons) or two if you want to clean stuff up.
The sand and cement mixed dry, and then water added to just get it to an oatmeal consistancy (the only part that has not been "forumized.)"
The Aruba shell is used for the mold.
The correct mixture is (using measure devices of previous poster):
5 cups ARAGONITE SAND.
1 cup cement.
1 cup plastic "sawdust."
Produces a predictable 6 - 8 week cure time, unlike the "forumized" recipes whose real cure times
can well exceed 1 year to never.
Notice the differences in the ratio of sand to cement, and "oyster shell," to cement; a ratio of rock salt that is way off the wall. The "right" brand of cement is Riverside, not Portland, with a very low alkalinity (the only issue there should ever be with DIY rock.) There be differences, but I am not going to do a dissertation on it. Sorry to say, home depot and big box stores don't have it.
Basically, the cement/oyster shell/rock salt "rock" is a block of concrete chock full of phospate laden chicken feed oyster shell, (depending on where you get it) but it is also very very alkaline which is a bad thing, (unless you are culturing cichlids) and rock salt. Who would want that in their tank in the first place is beyond me. Playsand has oil, and heck knows what else is in it, having been stored in piles outdoors, and oiled to keep it from blowing away....sometimes I think folks just don't put enough thought into what they do and suggest others do, but at the same time I know it is sincere and well meant.
Incidentally, you are not making live rock. You can't make live rock; you can make base rock, throw it in the ocean and it may become live in a couple years, but that won't happen in a closed system: bacteria and some algae, don't make a rock "live." Well there are things you can do if you have the patience to culture the rock, and have good seed stock.