I got this idea from a long thread on Reef Central, so I figured I'd try it myself and then post some pictures. I've done two batches now and it seems like its going to work out really well. I've done just some normal lumpy-shaped rocks, and I also used PVC sections to make rock columns that will eventually be pilings in my brackish tank.
This is pretty much what you need - portland cement and rock salt. I used Morton's Solar Salt, its nice chunks of probably 3-7mm, and was 4 bucks for a 40lb sack. The portland cement was 10 bucks for 90 pounds.
This is the leftovers from the first batch - I had a lot of little pieces that either fell off the edges, or I dropped them, and they cured. It looked kind of like gravel. Instead of throwing it away, I just decided to mix it in with the next batch.
Closeups of the ingredients: Cement
Rock salt
I tried a little sand in this batch, not sure how much of a difference it made:
Some crushed coral for texture, mostly because I have about 20 lbs of it sitting around and I want to get rid of it:
I take all this and mix it up in a bucket. Using my highly technical and calibrated measuring device :headshake2: I use about 1 cup cement, 3 cups rock salt, half a cup sand and about a cup of cc. The important part is mixing the cement and rock salt in a ratio of 1:2 to 1:4, this is what gives it its porosity. Any other aggregate you add in, doesn't really matter, you just don't want to add too much or the rock will be too dense. So, dump it all in a bucket and mix it up.
Now, you can add water. It takes a surprisingly little amount of water to get the consistency you need. I added way too much my first try and ended up adding a bunch more cement and salt afterward. For this batch, I probably used maybe 12-16 fl oz of water. When its mixed, it should almost hold its shape. It won't be like clay or play doh, but it shouldn't be real runny. Oh yeah and if you have any cuts, the combination of cement and salt feels wonderful. Maybe wear gloves.
I had already started making the pilings, so this batch is going to make them more or less round columns. I started by putting pvc down on the ground and piling the mix on top of it. Then I chipped the sharp edges off, and this batch will finish the other side. You can see on a few of them I got a little overzealous on my chipping...
So, yeah. There's no real science to it. Just pile it up in whatever shape you want. Make caves, ridges, use layers of mix and plain dry rock salt to leave holes, etc.
This is a smaller piece thats dry.
The next step is curing the cement. I left it on the floor for about a week to make sure the cement was nice and dry. Then you dump it in a bucket full of water and let it soak for 6-8 weeks,changing the water often. I change it every night. The water both dissolves the rock salt, leaving lots of pores, but also leaches out the alkaline stuff in the cement. For a while, the rock will drive the water up to about pH 11-12. After a while it should come down to 7-8. After about 4 or 5 weeks, throw it in a bucket of clean water and let it sit for 3 days, then test the pH. If it doesn't spike up, its ready to go into the tank.
This is pretty much what you need - portland cement and rock salt. I used Morton's Solar Salt, its nice chunks of probably 3-7mm, and was 4 bucks for a 40lb sack. The portland cement was 10 bucks for 90 pounds.
This is the leftovers from the first batch - I had a lot of little pieces that either fell off the edges, or I dropped them, and they cured. It looked kind of like gravel. Instead of throwing it away, I just decided to mix it in with the next batch.
Closeups of the ingredients: Cement
Rock salt
I tried a little sand in this batch, not sure how much of a difference it made:
Some crushed coral for texture, mostly because I have about 20 lbs of it sitting around and I want to get rid of it:
I take all this and mix it up in a bucket. Using my highly technical and calibrated measuring device :headshake2: I use about 1 cup cement, 3 cups rock salt, half a cup sand and about a cup of cc. The important part is mixing the cement and rock salt in a ratio of 1:2 to 1:4, this is what gives it its porosity. Any other aggregate you add in, doesn't really matter, you just don't want to add too much or the rock will be too dense. So, dump it all in a bucket and mix it up.
Now, you can add water. It takes a surprisingly little amount of water to get the consistency you need. I added way too much my first try and ended up adding a bunch more cement and salt afterward. For this batch, I probably used maybe 12-16 fl oz of water. When its mixed, it should almost hold its shape. It won't be like clay or play doh, but it shouldn't be real runny. Oh yeah and if you have any cuts, the combination of cement and salt feels wonderful. Maybe wear gloves.
I had already started making the pilings, so this batch is going to make them more or less round columns. I started by putting pvc down on the ground and piling the mix on top of it. Then I chipped the sharp edges off, and this batch will finish the other side. You can see on a few of them I got a little overzealous on my chipping...
So, yeah. There's no real science to it. Just pile it up in whatever shape you want. Make caves, ridges, use layers of mix and plain dry rock salt to leave holes, etc.
This is a smaller piece thats dry.
The next step is curing the cement. I left it on the floor for about a week to make sure the cement was nice and dry. Then you dump it in a bucket full of water and let it soak for 6-8 weeks,changing the water often. I change it every night. The water both dissolves the rock salt, leaving lots of pores, but also leaches out the alkaline stuff in the cement. For a while, the rock will drive the water up to about pH 11-12. After a while it should come down to 7-8. After about 4 or 5 weeks, throw it in a bucket of clean water and let it sit for 3 days, then test the pH. If it doesn't spike up, its ready to go into the tank.
