The phrase "cement a union" can be replaced with various synonyms depending on the context. One could say "strengthen a relationship" or "solidify a bond" to refer to the process of making a connection more robust. Alternatively, "tie the knot" or "get hitched" could be used when discussing the formalization of a romantic relationship into marriage. In a business setting, "forge a partnership" or "establish a collaboration" accurately captures the intent behind cementing a union. Ultimately, the most effective synonym will depend on the specific situation and the desired connotations, but any of the aforementioned phrases could be used as an alternative to "cement a union".