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Geological carbon sequestration is a promising option to slow global warming but leakage of stored CO2 through fractured wellbore cement can compromise storage security. Novel cementing materials that can withstand chemical attack, prevent fracturing, and potentially self-heal when damaged will improve long-term storage security of sequestered CO2. Here, a novel engineered cementitious composite (ECC) was exposed to water in equilibrium with CO2 at 10 MPa under static batch and flow-through conditions at 50 degrees C. The extent of alteration after 44 days of reaction under static conditions was determined using optical microscopy and backscatter electron spectroscopy. A single carbonated front was observed in the altered zone in contrast to previous studies on Portland cement in which multiple zones were identified. The rate of alteration was diffusion-controlled with a 50-year projected depth of alteration of 72 mm. A general increase in microhardness of the altered and unaltered zones suggests that the mechanical integrity of the composite will not be compromised following exposure to CO2-rich water. While microcracks developed at the rim of the core after greater durations of exposure, the crack widths were generally less than 60 mu m and prior studies have demonstrated self-healing of ECC microcracks of this size. Permeability decrease was observed in saw-cut samples under flow-through conditions and was attributed to a combination of dissolution-driven fracture closure and calcite precipitation.