Poro-city is an investigation into 3D printing lightweight structures comprised of materials with variable compositions. Taking inspiration from trabecula bones, Poro-city integrates computational form-finding strategies with biologically inspired fabrication. Concrete independently has a high compression strength but what limits its structural capability is the heavy weight of the material. By combining concrete with Styrofoam balls, the resulting material has the added advantage of being lighter yet still structural. Depending on the added number of Styrofoam balls, the resulting material will have a wide range of variable densities, and therefore a correlating structural performance level. This gradation ranges from an extremely lightweight, nonstructural material, to an extremely heavy and structural material. With the help of computational optimization softwares, Poro-city will be able to identify the level of structural requirements in the design and therefore, the correlating composition of the variable material required. By printing functionally graded concrete with variable compositions combined with optimization softwares, Poro-city will allow for not only the production of structures that are lighter, stronger, and more freeform, but also structures that can be optimized in terms of its strength, weight and material usage.