[Aquattro]

Are you using a fully saturated solution of kalk? That is 2 tsp/gal. Are you adding vinegar to the kalk solution? During the first three months of a sandbed, excess calcium can react with the aragonite, almost using it as a seed for deposition. This is why you often hear people complain about Ca levels during the early months. Since, and I'm assuming, you don't have a tank full of calcium hungry SPS yet, you have excess calcium going into the system. Something to keep in mind is you don't need to use a fully saturated CaOH mix. I would probably determine how much alkalinity your system consumes and add accordingly. I'll see if I can find an email from Craig Bingman on determining Ca/alk requirements. I would probably cut the kalk dosage in half(unless you can verify you need it) and make sure you don't get large pH swings. This may affect the solubility of the sand and at least be a partial cause of the clumping.
Is it becoming solid all the way thru, or just the surface?