He used oystercrete, which is basically concrete using oyster shells instead of gravel and argonite sand instead of regular sand. He also used portland white cement instead of regular grey cement. He posted youtube videos showing how he did it. Using sand/oyster shells to form moulds you can make pretty much anything you can imagine as far as caves, towers, and whatnot.
I think he decided a slightly different recipe was better then the one he posted on youtube, more of something I think, and when it's dry you need to soak the rocks in water to leach out the limestone from the concrete or your PH will go through the roof. That can take a month or even several months. He put them in the ocean for almost a month and they were still leaching and driving up the PH for another month or two while soaking in the aquarium with just water.
It's definately not easy or fast, but it's cheaper and you can make whatever shape you need. In the end it seems to be very porous, with high calcium content, and in theory that should make great "live rock" for a reef.
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