This paper presents results from a comprehensive experimental campaign designed to investigate the synergistic effects of SCMs, specifically, ground granulated blast furnace slag (BFS) and silica fume (SF) on the hydration and compressive strength of ultra-high performance cement pastes (UHPCP). Experiments were performed on 65 mixtures with extremely low w/b ratios (0.16, 0.20) and different SF and BFS replacements under different curing conditions. New insights into the synergistic effects of the SCMs in terms of evolution of portlandite and bound water content, hydration kinetics of UHPCP under standard and elevated curing conditions, and compressive strength are presented. It is demonstrated that effectively controlling the w/b ratio, curing conditions, and importantly the addition of an optimal SF content will favourably influence the hydration and compressive strength of UHPCP. However, the use of high BFS replacement may induce an adverse effect on the hydration and compressive strength of UHPCP. A linear correlation between the bound water content and compressive strength as well as portlandite content is revealed, which serve as valuable indicators for material design.