Acrylic should hold up, I wonder if it was polycarbonate.
BTW, the driveway marker that I used as the stirring rod was attacked by the kalk. I hadn't run the stirrer for a week or so and the orange color from the driveway marker was bleeding into the kalk! I rebuilt using a PVC rod. Kalk is also hard on silicone.
Nylon and PVC are both good to use with kalkwasser.