The dissolution of calcium carbonates in water
Regarding the slurry statement, you can only make slurries with insoluble compounds or they would dissolve and your slurry would go away leaving you with a clear solution!  

Your attempt to alter the eggshell composition by extracting with the surfactant should have had little effect.  You didn't give any quantitative results from your ICP analysis, but I think one or both of two things happened:
1) There was residual Ca from the surfactant remaining on your eggshell
 and/or
2) You may have extracted the few percent of protein that makes up eggshells, leaving a more "concentrated" $\ce{CaCO3}$ residue as compared to the untreated shells.