This paper focuses on the microstructural investigation of ultra-high performance concrete (UHPC) mixtures featuring various replacement levels for blast furnace slag (BFS), in the aim of becoming more environmentally-friendly. Three levels of slag are applied (namely 30%, 50% and 80%) per unit volume of cement. The microstructural characteristics examined included X-ray diffraction (XRD) analysis, transmission electron microscopy (TEM), and mercury intrusion porosimetry (MIP). These characteristics are assessed along with the mechanical strengths at 3 and 90 days. Similarly, the effects of chemical and thermal activation methods on incorporating BFS into UHPC mixtures at high volumes were studied from a microstructural point of view, and a comparison is drawn between the non-activated and activated mixtures. The results of this investigation show that the beneficial physical impact of slag replacement lies at the 30% level, due to its filler property in stimulating hydration to quickly generate portlandite. For this reason, the portlandite and C-S-H peaks are quite similar to those in the reference mixture (i.e. 0% BFS) at 3 and 90 days, as demonstrated in the XRD analysis. Moreover, the pore size distribution could be refined, the microstructure densified (as indicated on the TEM images), and compressive strengths at both ages improved. In contrast, for slag replacement levels of 50% and 80%, the portlandite and C-S-H peaks are significantly lowered, as observed in the XRD analysis, thus causing a magnification of the capillary pores and a drop in compressive strength at 3 and 90 days. Chemical activation through a potassium hydroxide, with a dosage of 10.17 kg/m3, at the 80% BFS replacement level is able to accelerate hydration and portlandite consumption, as exhibited in the XRD analysis through the absence of portlandite peaks and limited quantities of C-S-H at 3 and 90 days. However, this step results in smaller pores and a higher compressive strength. As for the thermal activation, the portlandite in the blended mixtures with 50% and 80% BFS replacement decreases in quantity, as confirmed by XRD analysis, and the reaction is perfectly activated by means of the solubility acceleration of the alkalis; consequently, the pore structure is compacted, as revealed on the TEM image and the compressive strength rises considerably.