Portland cement-blast furnace slag (PC-BFS) cement-based materials exhibit good durability in shallow marine environments but their performance in the deep-sea is poorly understood. This study investigates the degradation of PC-BFS cement pastes after one-year exposure in a deep-sea field site under high hydraulic pressure (3515 m depth) and low temperature (2 °C) conditions. The results show that the main degradation mechanisms involve ettringite formation and Ca leaching, driven by low-temperature seawater ingress under pressure. SEM-EDS and XRD analyses revealed distinct microstructural changes such as Mg and Al enrichment and sulfate/chloride incorporation in BFS rims. Fluorescence imaging confirmed complete seawater infiltration, facilitating BFS dissolution and reprecipitation cycles. The findings highlight the vulnerability of PC-BFS systems to ettringite-induced damage due to their high alumina and low sulfate balance. These insights inform strategies to optimise cementitious materials for deep-sea applications under extreme environments.