As far as I know, aragonite is unlikely to fusing in clumps, but possible causes could be adding kalk too much or too fast and/or unstable levels of pH (specially dropping very low at night).
An stable pH in ranges 8.3-8.5 is recommended (I have to say I couldn't be able to mantain levels over 8.2)
Could be bacteria too--they form biofilms which will bind grains of sand together. The films are surprisingly robust too--not just slime, but actual hard films like glue. Just a thought--may not be the reason.