Freezing point depression of ZnCl2

Dec 2016
I did an experiment on how different concentrations of ZnCl2 affect the freezing point of demi water. My results were that till a concentration of about 10 mol dm-3 the freezing point decreased (more or less in a linear manner). After that at concentrations of 15mol dm-3 the freezing point started to increase again (approach 0 degrees Celsius again).

Attached in the file is a graph of the freezing point depression vs. the concentration of ZnCl2 in demi water.
X-axis: concentration of ZnCl2 in mol dm-3
Y-axis: freezing point depression in Kelvin

Does anybody know why the freezing point depression initially decreases, and then past a certain concentration increases again?

Thanks in advance!