Let M be a class of (mono) morphisms in a category A. To study mathematical notions, such as injectivity, tensor products and flatness, one needs to have some categorical and algebraic in-formation about the pair (A, M). In this paper we take A to be the category Act-S of S-acts, for a semigroup S, and Msd to be the class of STRONGLYs-dense monomorphisms and study the categorical properties, such as limits and colimits, of this class.