We're the manufacturers of Fast 2K Fence Post Backfill. I'm so sorry to hear about your clothesline pole installation. Fast 2K has been successfully used for the installation of fence and gate posts for quite some time by many fence contractors and home owners.
I'm not sure the size of your pole and hole, but if you had to put more than a few inches of dirt at the top of cured Fast 2K, that may explain the issues you had. Tamped soil at the top of the hole may allow the pole to move easily.
Fast 2K is a modified polyurethane composite. One bag of Fast 2K replaces approx. 100-120lb of concrete mix/cement. The insulation foam that I believe you've referred to to explain how Fast 2K looks like has big holes if you cut the foam after it's cured. Fast 2K is closed cell and structural product especially designed for the installation of fence and gate posts.
If you have any questions, please email us at fast2K@chemque.com
or 1-800-268-6111 or yet visit the website www.Fast2K.com where you can watch a demo video. Best regards.