a gas spring for a sliding refractory gate valve , comprising a container , a ram which is extendable relative to the container , a bellows connected at one end to the ram and at the other end to a part of the container , a gas within the container outside the bellows and acting to compress the bellows and extend the ram , wherein the ram has a blind internal bore and a mechanical compression spring is at least partially disposed in the bore so as to be interposed between the ram and the container and also acting to extend the ram .