On the German Wikipedia page about warhammers it says their weight reached from 3 to 14 Kg! Having done some really heavy sports in my past, I was truly surprised by this last number! I searched extensively. First of all I found one more reference to the same number on Mittelalter Wikia. There they claim the so called Luzerner Hämmer (hammers of Lucern) had a long haft and weighed 14 Kg.
I also tried some more general searches. The Bec de Corbin reproduction here weighs 3.75 lbs. This article about pole axes lists examples up to 7 pounds 11 ounces (which is just below 3.5Kg). This reproduction weighs just 2 pounds 4 ounces.