Substituting cement by supplementary cementitious material (SCM) is the most-practiced approach of reducing CO2 footprint of concrete. However, in case of UHPC, for which ultra-high performance in terms of both strength and durability is of utmost importance, high-volume reduction of cement is often difficult or highly challenging. In this study, the possibility of high-volume incorporation of ground granulated blast-furnace slag (GGBS) in UHPC with very-low-C3A Portland cement (having 1.1% tricalcium aluminate or C3A) as primary binder has been explored. Benefits of using very-low-C3A cement as primary binder in UHPC, over cement with a moderate C3A content (9.3%), in terms of strength and workability have also been investigated. Results of the UHPC mixes incorporating GGBS suggest that 60% replacement of very-low-C3A cement by GGBS reduces the 28-day compressive strength of UHPC by 16.1% with respect to the strength of UHPC without GGBS. Nevertheless, ultra-high strength (>150 MPa) can still be achieved up to 60% replacement of cement by GGBS, without the need for any special curing or fibres. The water absorption and initial rate of absorption of UHPC, and the corrosion risk of rebar embedded in UHPC reduce with the increase of very-low-C3A cement replacement by GGBS. Up to 60% replacement of cement by GGBS, UHPC exhibits carbonation resistance similar to that of the UHPC without GGBS.