An important input parameter for numerical models that simulate cracking of the concrete cover due to reinforcement corrosion is the Elastic modulus of corrosion product (Ecp). Despite its relevance, Ecp is subject of significant variations according to the values reported in the literature, which vary from less than 100 MPa up to 360 GPa. Furthermore, Ecp values proposed in most of the present literature are representative of the corrosion product generated by anodic accelerated corrosion or extracted from the steel/concrete interface (SCI), which might differ from that formed in real corroding structures. Therefore, this study aims to investigate the Elastic modulus of naturally-generated corrosion product present at the SCI through nano-indentation conducted on six reinforced concrete polished sections. The polished sections were obtained from six 20-year-old reinforced concrete prisms cast with different cement type (CEM I, CEM II/B-V, CEM III/B, CEM V/A), same water/binder ratio (0.55) and which were previously exposed to NaCl solution wet/dry cycles. This study revealed that the range of Ecp did not considerably vary between corrosion products formed in different concrete mixes. However, corrosion product was microscopically found to consist of overlapping bands with different Ecp, varying for up to around 70 GPa between each other. Through Environmental Scanning-Electron Microscopy (ESEM) and Energy Dispersive Xray Spectrometry (EDS) analysis of the indented locations, it was found that Ecp is highly dependent on the presence of interfacial cracks and inversely proportional to the concentration of Si and Ca, representative for corrosion product mixed with the surrounding concrete. Furthermore, higher concentration of Fe leads to higher Ecp. Based on this study, an average range of values for Ecp between 80-100 GPa can be suggested for use in numerical models for corrosion induced cracking, regardless of cement type of the structure under investigation.