Organics generally does not form network with inorganics because of their poor compatibility. Here, we report organic/inorganic dual network (DN) formed by epoxy and high alumina cement. In the presence of water, the cement quickly forms inorganic network via hydration followed by the gradual curing of epoxy yielding the organic network interpenetrating with the former. Such DN has improved mechanical properties either at early or final stage in comparison with the cement or epoxy after curing. It also exhibits high bonding strength with concrete so that it can be used to repair concrete cracks and blocking water leakage. The novel organic/inorganic DN is expected to find applications in constructional and geotechnical engineering. POLYM. COMPOS., 39:E2490–E2496, 2018. © 2018 Society of Plastics Engineers