Here is another idea by Mr.Wilson about why the DIY rock gets brittle.
"I think it was due to chloride or sulphide attack which is common with all cements. It could also be that the oyster shell aggregate dissolved and made it brittle. Crushed oyster shell dissolves like any other form of calcite, and the smaller the particles, the quicker is does so. I later switched to aragonite sand/gravel largely because it is clean and cheap if you hunt around."
Here is another product that Wilson mentions.
"I have two sample bags of BASF Emaco s88c T, marine cement ready for pick up at my supplier. It's a sulphide and chloride resistant cement so it shouldn't degrade in saltwater as most cement mixes do. The Emaco s88c T is not a fast setting cement so it gives me more time to sculpt it, but it may slump if it doesn't have enough body. Microsilica or silica fume can be used as a 10% addmix with cement to stabilize PH, improve sculptability, and quicken the cure time."
Here is the product.
http://www.thoroproducts.com/pdf_appl/appl_thorite.pdf
I want to know about the Thorite. It has my attention. It would be nice to know that the rock or the wall that you build wont attack itself.