The corrosion of carbon steel in alkaline anaerobic conditions and high temperature (80°C) was studied through mock-up tests representative of imperfect contact between steel and cementitious material. The evolution of the whole system, including cement mineralogy, reservoir solution chemistry, corrosion product and rate, was followed over one year. SEM-EDX and XRD analyses indicated that the corrosion product layer was mostly constituted of (Fe,Al)Si-Hydrogarnet ((Fe,Al)Si-HG) and that its densification progressively inhibited corrosion. Thermodynamic and reactive transport modelling helped to better constrain the chemical and temperature conditions of (Fe,Al)Si-HG formation.