Concrete is made by mixing coarse sand or gravel with water and cement. The porosity of the concrete depends largely on the size of the particles of sand or gravel used, as well as the gradation of the particles. Gradation refers to how uniform the particles are in relation to one another, and this is directly related to the concrete's porosity. Concrete that is more porous will contain particles of varying sizes, whereas less porous, compacted concrete will contain particles that are more uniform in size.