Abstract: There is provided a two part hardenable composition comprising a powder first part and a liquid second part. The parts are operable to form a glass ionomer cement which hardens to a solid mass upon mixing of the parts together. The composition comprises an inorganic glass and/or salt in the powder first part, an aqueous carrier in the liquid second part and an acid-functional polymer in the powder first part and/or the liquid second part. The composition also comprises an at least partially water-miscible additive in the first and/or second parts. The additive is operable to extend the working time of the glass ionomer cement upon mixing of the first and or second parts. The composition is operable to form a glass ionomer cement which, once hardened to a solid mass, has a plane strain fracture toughness of at least 0.7 MPa·m1/2 as measured at 24 hours from mixing. Also provided is a method of producing a glass ionomer cement.