A gate valve (10; 130; 140) comprising: a valve housing (16; 148) including a passageway arranged along a passage axis (20; 142) from a first axial side to an opposed second axial side of the valve housing (16; 148); and an expandable gate member (12; 150) movable within the housing (16; 148) from a retracted position displaced from and not blocking the passageway to a blocking position disposed in the passageway, comprising a sealing member (56; 152) capable in an expanded state of the gate member (12; 150) of sealing a sealing surface (18; 14) disposed on the first axial side, an opposed member (88; 154) capable in the expanded state of the gate member (12; 150) of abutting an abutting surface (14; 18) facing the sealing surface (18; 14) and disposed on the second axial side, a pneumatic cavity having movable expandable sides (66, 84) disposed inside the gate member (12; 150) such that positive gaseous pressure applied to the pneumatic cavity activates the gate member (12; 150) to a compressed state, a compression spring disposed inside the pneumatic cavity and biasing the gate member (12; 150) to an expanded state, and first and second annular bellows (110, 112) forming walls of the pneumatic cavity, wherein in a non-expanded state of the expandable gate member (12; 150) the sealing member (56; 152) does not seal the sealing surface (18; 14) and the opposed member (88; 154) does not abut the abutting surface (14; 18). a valve housing (16; 148) including a passageway arranged along a passage axis (20; 142) from a first axial side to an opposed second axial side of the valve housing (16; 148); and an expandable gate member (12; 150) movable within the housing (16; 148) from a retracted position displaced from and not blocking the passageway to a blocking position disposed in the passageway, comprising a sealing member (56; 152) capable in an expanded state of the gate member (12; 150) of sealing a sealing surface (18; 14) disposed on the first axial side, an opposed member (88; 154) capable in the expanded state of the gate member (12; 150) of abutting an abutting surface (14; 18) facing the sealing surface (18; 14) and disposed on the second axial side, a pneumatic cavity having movable expandable sides (66, 84) disposed inside the gate member (12; 150) such that positive gaseous pressure applied to the pneumatic cavity activates the gate member (12; 150) to a compressed state, a compression spring disposed inside the pneumatic cavity and biasing the gate member (12; 150) to an expanded state, and first and second annular bellows (110, 112) forming walls of the pneumatic cavity, wherein in a non-expanded state of the expandable gate member (12; 150) the sealing member (56; 152) does not seal the sealing surface (18; 14) and the opposed member (88; 154) does not abut the abutting surface (14; 18). a sealing member (56; 152) capable in an expanded state of the gate member (12; 150) of sealing a sealing surface (18; 14) disposed on the first axial side, an opposed member (88; 154) capable in the expanded state of the gate member (12; 150) of abutting an abutting surface (14; 18) facing the sealing surface (18; 14) and disposed on the second axial side, a pneumatic cavity having movable expandable sides (66, 84) disposed inside the gate member (12; 150) such that positive gaseous pressure applied to the pneumatic cavity activates the gate member (12; 150) to a compressed state, a compression spring disposed inside the pneumatic cavity and biasing the gate member (12; 150) to an expanded state, and first and second annular bellows (110, 112) forming walls of the pneumatic cavity, wherein in a non-expanded state of the expandable gate member (12; 150) the sealing member (56; 152) does not seal the sealing surface (18; 14) and the opposed member (88; 154) does not abut the abutting surface (14; 18).