The demand for accurate characterization of concrete made from slag-based CEM II cement is becoming increasingly important, because the construction sector shifts towards eco-efficient materials to reduce the environmental impact of cement production. In the present contribution, the basic creep behavior of two distinctively different mature slag-based CEM II concretes is identified from macroscopic creep tests, and traced back to mixture-invariant characteristics of microscopic hydrates, by means of an adaption of an experimentally validated three-step micro-viscoelastic model for CEM I-concretes. The model is complemented by a Powers-Acker-type hydration model for CEM II and an isochoric creep function for hydrates made from slag-based CEM II. The creep function accounts for (i) short- to long-term creep by means of a piecewise-defined function including a power law for the short-term portion and a logarithmic law for the long-term portion; (ii) the decrease of hydrate stiffness moduli with increasing temperature; and (iii) the increase of the hydrate shear creep modulus with decreasing relative humidity. Aging effects resulting from ongoing hydration are considered by means of a strain rate-based viscoelastic analysis. Downscaling of mechanical properties from the concrete level to the scale of the hydrates evidences a shear creep compliance of CEM II hydrates that is twice as large as the one of ordinary Portland cement hydrates, for both analyzed CEM II concretes. Thus, hydrates made from slag-based CEM II exhibit a much more pronounced creep activity than CEM I hydrates. The enhanced creep activity can be explained by pozzolanic hydration reactions of slag, where non-creeping portlandite is consumed, leading to the production of additional, creep-active, calcium silicate hydrates. It is concluded, that slag-based CEM II concretes are especially suitable for structures under displacement-controlled conditions, e.g. for segmental tunnel linings, because the more pronounced creep behavior implies a faster stress relaxation.