Shane Graber:

One thing I've also heard but no one's been able to really explain it well is the potential to cement a sandbed. I've read thru the archives on reef-l, reefkeepers-l, FishNet, and talked to Shimek and Randy Holmes-Farley about it and no one really knows *why* sandbeds cement together. When I'm talking cementing I'm talking about actual cementing and not the bacterial biofilm "cement" that Shimek talks about. The sandbed chunks won't fall apart w/ bleach or by boiling so that rules out a bacterial biofilm. The best guess I have on it is that if your pH climbs too high (greater than 8.4-8.6?) you can run the potential of causing your sandbed to cement together over a period of time. It doesn't happen overnight. :) The condition seems to be more prone to happening with new sandbeds that don't have the bacterial biofilm on the sand particles already. However mine happened on an established sandbed. FWIW, I've read about people having this problem with kalkwasser, calcium reactors, and 2 part additives but the latter two were on new sandbeds and not established sandbeds.

Mike Kirda:

You drop aragonite into saltwater, calcite formation begins almost immediately. Why? Simple- you are dropping a crystal into a saturated solution. Energy flow forces it to form.

Why does kalkwasser dosing seem to augment the formation of clumping? Also simple - it elevates pH, which also augments calcite precipitation.

Let's go through what happens when you set up a tank... You add the washed aragonite, then add newly mixed up salt water, right? Very, very bad move- You are building a brick. All that dissolved calcium and alkalinity immediately begins falling out of solution. The sand grains that touch begin to cement together with calcite. This is a double whammy- not only can critters not navigate this brick, but when bacteria films finally do form, calcite does not dissolve as easily as aragonite, therefore, the sand does not dissolve.

So, what can you do about it?

First of all, don't add the aragonite to freshly mixed salt water. It is essentially sterile, and has basically ZERO bacteria in it. How is a bacterial biofilm going to form in that?

You first need to soak your aragonite in water that has bacteria in it. Use water change water from an existing tank. This is chock full of bacteria. Soak the aragonite in it for a few weeks if possible. Add flake food or apple juice or SOMETHING to support bacterial growth. You need to get that biofilm established. Don't do water changes- maintain salinity, but don't add new salt water. You want to minimize any calcite formation.

Once you go to add the sand to the tank, make sure that you age as much salt water as possible beforehand. You can do the same to it- add something to get some bacterial life in it.

Once you add the sand and water, let it settle in for a while. Add a good half inch of real live sand on top of the 'dead' aragonite. Feed the tank DT's or cryopreserved pastes. Add your sandbed critters. Keep feeding. Let the sandbed come to life for a month or two. Also, keep the water motion relatively low so sandbed is not blowing around...

Then go ahead and start calcium supplementation, add live rock, etc.

This is the only way you can be sure that sand clumping does not occur. In my experience, if you follow this advice, your sandbed absolutely will not clump. Only time/place it might is if the water blows away the veneer of live sand, exposing some 'dead' aragonite to the water above. Even then, it will be minimal.