Preventing leakage of CO2 along wellbore cement sheaths is a key factor for ensuring success of geologic carbon storage (GCS) operations. Here, we examine a potential alternative cementing material, engineered cementitious composites (ECC), for use in GCS wellbore cementing applications. ECC is a novel fiber-reinforced cementitious composite that exhibits strain hardening and improved tensile ductility in comparison to conventional cement. Improved ductility may prevent wellbore damage caused by CO2 injection pressures and casing expansion/contraction associated with thermal swings. Earlier work examining physical alterations of ECC exposed to CO2 at 10 MPa and 50°C found that damage to ECC was limited to microcracks with apertures less than 60 µm after several weeks of reaction. However, microstructural analysis revealed densification of the fiber/matrix interphase due to calcite precipitation, which could alter the engineered bonding properties between the matrix and fibers. Such alteration following carbonation could negatively impact the long-term ductility of ECC used for GCS wellbore cementing applications. This presentation will discuss recent results from static batch studies investigating the impact of CO2-acidified water on tensile ductility of ECC. Several ECC and ordinary Portland cement (OPC) coupons were exposed to CO2-saturated water under temperature and pressure conditions of 50oC and 10 MPa, respectively, and samples were retrieved after 2, 7, 14, and 28 days. Four-point flexural test and micro-CT analysis were carried out to investigate the impact of carbonation on the ductility and microstructural properties of ECC. Replicate experiments were also conducted under the same conditions but with a N2 headspace to isolate impacts associated with CO­2 exposure. While the samples exposed to N2 continued to exhibit a multiple microcracking behavior with no observable change in tensile ductility, the ductility of the composite exposed to CO2-acidified water showed an increase in the ultimate flexural strength and significant decrease in ductility as the duration of reaction increased. OPC coupons exhibited brittle behaviors under all test conditions. This suggests that the densification of the fiber matrix interface after exposure to CO2 can compromise ECC’s overall ductility.