Oyster shells is supposed to be bad for phosphate. It is cheap but I don't think it is work it. I found puka shells and crushed coral at petsmart. I got my white Portland cement from a company called Arrow. It is a supply place for masonry and concrete workers. It took me a few phone calls, but doable. "Do you have White Portland Cement? No. Do you know who does? Thanks".
Maybe about four calls and you'll find it. Home Depot and Lowes here in Spokane did not have it.
One more thing, wear gloves, it is caustic while setting up.
There are several videos on YouTube you can what before trying it.
55 Gallon, DIY Live Rock, 2 Taotronics 130w LEDs, CPR HOB skimmer and refugium, 525 GPH SunSun 5-stage Canister with UV Sterilizer using Chemi-pure Elite, and Two Fishes GFO reactor and 3 800 GPH powerheads. <><