Concrete Reabsorbs CO2
During the life of a concrete structure, the concrete carbonates and absorbs the CO2 released by calcination during the cement manufacturing process. Once concrete has returned to fine particles, full carbonation occurs, and all the CO2 released by calcination is reabsorbed. A recent study indicates that in countries with the most favorable recycling practices, it is realistic to assume that approximately 86% of the concrete is carbonated after 100 years. During this time, the concrete will absorb approximately 57% of the CO2 emitted during the original calcination. About 50% of the CO2 is absorbed within a short time after concrete is crushed during recycling operations. (Nordic Innovation Centre Project 03018).
I believe the greater long term impact on CO2 levels of from making concrete come NOT from the energy used to make the concrete itself, but rather the CO2 absorbing vegetation the concrete replaces.