Zirconium oxide (ZrO₂) has been nominated as a radiopacifying agent for use in MTA-like Portland cement-based root-filling materials. This research examines the impact of 20 wt% ZrO₂ nanoparticles in the size range 50 to 75 nm on the early hydration chemistry of white Portland cement. Nano-ZrO₂ was found to accelerate the degree of hydration by 26% within the first 24 h by presenting efficient nucleation sites for the precipitation and growth of the early C-S-H gel products. The presence of nano-ZrO₂ was also found to divert the fate of the aluminium-bearing reaction products by lowering the ettringite to monosulphate ratio, reducing the size of the ettringite crystals and by increasing the Al:Si ratio of the C-S-H gel phase. The chemical and microstructural changes conferred upon the cement matrix by the nano-ZrO₂ particles had a positive impact on in vitro biocompatibility with respect to MG63 osteosarcoma cells (via MTT assay).