Try this good simple basic recipe to make some of the cheapest porous cement reef rock around. It costs only about 9 cents a lb using this recipe! Using a five gallon bucket, add 5 cups of common easy to find type I/II Portland cement from Home Depot, a hardware store or building supply store. Add 25 cups of crushed oyster shell from a farm feed store. Add six cups of water and mix very thoroughly with a small hand held flower gardening digger. Use rubber gloves to protect your hands from chemical burning due to extended exposure to the high pH of the fresh cement – this is like soaking your hands in supersaturated kalkwasser. Tip the bucket on a 45 degree angle and rotate or roll it to help tumble and mix the cement well. Use the gardening hand digger to mix the cement as you tumble it in the bucket. If the mix is too dry add a little more water (slowly). Karen Holt tries for a final mix with the consistency of cottage cheese. Others go a bit wetter than this, but not too slushy. Many sand substitutes can also be used to make your cement if desired. More on this later. You can get crushed oyster shell at the farm feed store since it is used as a very common chicken feed supplement. And that, my friend, is why THIS cement reef rock recipe is so ‘cheep, cheep, cheep’!
Do not make the final product too runny or you will not be able to shape it into nice looking irregular shapes as well. An almost slushy mixture that will still stand up just a little and take some shape (just a bit) is just right for molding. In fact, James Wiseman advises that a wetter mix not only assures that the silicates in the cement mix get hydrated and bound, but a slightly wetter mix will produce more micro-pores or capillaries throughout the cement for better bacterial colonization inside the rocks. On the other hand, this can also weaken the cement just a bit, but usually not critically so. On the other hand, don’t make it too dry or the cement will have too many large airspace gaps between sand or crushed oyster shell bits and it will then be weaker than the mix that is too wet. Maximum hardness is mostly reached after about a month of curing. Now, using rubber gloves, scoop handfuls of it into a bed of dampened sand (or substitute) for molding and shaping your cement into reef rocks.
Beforehand, you need to prepare this waterproof box or container with sand, crushed coral, aragonite sand or crushed oyster shell. Large plastic garbage can lids work well too. Dig irregular and creative shaped molds in the slightly dampened sand. Karen Holt prefers using the crushed oyster shell cement with about a 1:4 ratio of cement to crushed oyster shell, and uses damp crushed oyster shell for the sand molding bed also. She makes some interesting and intricate shapes with the cement that she calls ‘lace Rock’.
Another alternative box for the sand molding bed that I have used is a large shallow Rubbermaid or Sterilite semi-clear plastic ‘under bed’ storage container with a lid. The lid can be used to cover the container to keep the cement rocks from drying out while they set up for two days. I found some of these cheap containers for only $4.50 at Wal-Mart. They are made by Sterilite and are 2′ long by 1.5′ wide and about six inches deep. Just right for making four larger rocks or eight smaller ones at once.
You can dig irregular holes in the damp sand molding bed to form ‘molds’ for making creative and or artistic concrete rocks. I like to make holes through some rocks and mound up little heaps of cement on them in irregular shapes so that they look more natural. This is a great way to make arches, rocks with caves and long flattish rocks for creating dramatic overhangs and more caves in your reef aquarium. I like to make small cement rocks to place the large flatter rocks on in my reef aquariums. This keeps more of the sand substrate surface exposed in my reef aquariums. You can easily get up to 90% sand exposure this way. In HANDY Reefs with plenums and sand, I sink these hand made cement support or pedestal rocks down to the top screen just over an inch below the top of the sand. In Berlin style reefs with an inch or two of sand, sink these base support rocks or pedestal rocks down into the sand to the bottom of the tank for more solid support of the rocks above.
Karen Holt made a beautiful hollow lacy thin cave rock for my wife by burrowing out a deep hole in the sand molding box (she used crushed oyster shell in place of sand). She fills the hole with a layer of cement by dropping or dribbling small clumpy amounts of the cement mixture onto the hollowed out molding bed so that the fresh cement clumps interconnect for the most part, but it isn’t just one solid layer. It ends up looking just a little holey and porous. She then fills in over that with barely damp crushed oyster shell to help it keep its shape as the cement sets up. When it sets up for about two days you can use a stick to brush the sand (or oyster shell) out and off of the new rock. The branching rocks that she makes are quite striking also. The shape variations can be numerous and fun to experiment with.