A water-soluble insert for use in joining pipes, made of a material comprising a salt selected from the group consisting of KCl, Na Cl, and mixtures thereof; and MgO. Upon addition of a small amount of water to the material, the MgO reacts with hygroscopic impurities in the chloride salt to provide a material with superior properties. The insert is cast from the material, in general by being placed on a form and compressed. Also disclosed is the use of the insert in joining the ends of two pipes, especially pipes made from thermoplastic. The insert is placed in the matching pipe ends, which are then welded. After the join is complete, water is flowed through the pipe, dissolving the insert.