Yes, Prof. Joseph Davidovits put the word GEOPOLYMER (its english spelling) in the Public Domain in 1979. It means that everybody can freely use this word in scientific papers, for commercial or non-commercial purpose without any authorization.
No-one can register this word alone as a trademark in any country, it will not be valid. However, it is possible to associate the word geopolymer with other words to form a trademark. More information here.
But, please, keep in mind that the word geopolymer has a precise meaning, a clear definition . Using it means you agree with this science and definition.