[Cob] cob and cement

[Jilly]

I thought I read somewhere that cement and cob don't go together, that the cement breaks down the cob? Is that right?

I made a gargoyle with a pipe in the middle, with the idea to attach it to one corner of the house, where the rain would spout of it. 
But if I make the cast, and use cement, can I attach it to the cob? Or was it just cement plaster or stucco that was the issue with partnering it with cob? 

jilly