Sousa, Lurdes2015-04-072015-04-0719950927-2852, 1572-9095http://hdl.handle.net/10400.19/2753This paper deals with the problem of the existence of solid hulls for concrete categories. Sufficient conditions are given for the existence of a solid hull of a concrete category. For concrete categories over Set with a small finally dense subcategory, we show that the existence of solid hulls is equivalent to Weak Vopenka's Principle.engconcrete categoryfinally dense extensionMacNeille completionsolid hullreflective hullorthogonal closurejournal article