I should post, for anybody that read all those other pages for DIY base rock, for the best concrete/mortar that will last in saltwater: (saltwater is corrosive to concrete)
1: Use the least amount of water that you can that will allow workability and still stick together. The water-to-cement ratio should not exceed a 0.40 ratio.
2: Make sure the sand is moist that you use to mold it around so it doesn't suck the water from the concrete mix, changing the water to cement ratio.
3: Regardless of the temperature (still above freezing, preferred 70degF), keep the concrete moist but not wet for roughly 7 days (if you can stand to wait that long) before removing from "mold", then start curing in fresh water(for 28 days best). "Drying" concrete makes for poor/weak concrete!
4: If you have access to air entrainment admixture, use about 6%, (it makes it so that you can use less water, and stronger.) not typicaly available though.
5: Use type II portland cement, or if available, use type V cement. Type II is much more typical and available. Quickcrete would be fine if it is with a type II or type I/II with very little water added, probably too much aggregate though for this application.
For this application a type II portland cement with any sort of aggregates and/or sand/crushed coral and a little water will yield best results. By letting it cure in moisture before removing it from the mold it will be less likely to deteriorate after you finally get your tank up and growing, it would be a shame if it started flaking off once it's established.
You'll have to play with the mix ratios depending on what materials you are using, but the drier the better with it still sticking together. Once you get cement wet, the chemical reaction starts and it is setting up, so work quickly, don't re-wet, and don't re-mix if at all possible.
That's probably more than anybody needed or wanted to know about cement and concrete, but now you know in case you ever wondered. If I missed something or wasn't clear about something, feel free to message me with questions, I'll try to help if possible.
-Bob
