One World Trade Center is the main building of the new World Trade Center in lower Manhattan. Construction began in April 2006. When it is completed in 2013, it will be the tallest building in the U.S. The building's structural columns are comprised of steel and concrete. The concrete performance for these columns is divided into four phases: 14,000, 12,000, 10,000, and 8600 psi from the lower portion of the building to the top, respectively. The 14,000 and 12,000 psi phases were extremely challenging. Approximately 38,000 cubic yards of these concrete mixes were used in the columns through the first 40 floors. Two high-strength column mixes met and exceeded the project requirements by replacing portland cement with reclaimed fly ash and/or slag to produce a sustainable alternative to traditional concrete. The admixture supplier's Green Sense Concrete uses supplementary cementitious materials, non-cementitious fillers, or both, with Glenium high-range water-reducing admixtures to meet or exceed performance targets. This combined technology resulted in significant environmental savings including more than 30,000 gallons of fresh water, 8 million kWh of energy, 12 million pounds of CO2 emissions, and nearly 750,000 pounds of fossil fuel.