Indonesia's infrastructure growth is increasing rapidly in recent years. One of the main components in infrastructure development is concrete which uses cement as the binder. Higher infrastructure development will also increase the demand for cement. However, cement industry is known as a significant source of CO2 emissions with total of 8% contribution to the global CO2 emissions. If cement remains the main component in the manufacture of concrete, this number of emissions will continue to increase year after year. Preventive action needs to be done by conducting research to find alternative other than cement. This research is aimed to observe the feasibility of geopolymer concrete to be proposed as that alternative, since it does not need cement for the application. The geopolymer concrete was made from Geofast nickel slag as the main component following standard for Concrete Mix Design for Normal Concrete. The result shows that geopolymer concretes which tested after 28 days have higher compressive strength and flexural strength compared to those which tested after 14 days. The strength of all testing obtained from geopolymer concrete in this study is also higher than the Portland cement. Thus, this geopolymer concrete holds promising potential to be used as alternative for Portland cement. Nevertheless, the use of normal concrete mix proportion for geopolymer should be studied further.