Total voids in concrete encompass gel water volume, capillary pores, and entrapped air. Gel water (retained within the cement hydration products) and physically entrapped or adsorbed water are significant for the hydration process. For complete hydration, it's estimated that the space needed for the products of a cubic centimeter of cement doubles. Capillary pores constitute the unoccupied space within the hydrated cement paste, with their size largely influenced by the water-to-cement ratio and hydration degree.

During mixing, concrete traps a small amount of air, forming voids that can be significantly larger than capillary voids, potentially weakening the concrete's strength. These air voids vary in size, with entrapped ones reaching up to 3 millimeters, much larger than the finer capillary voids, which range from nanometers to a few micrometers in diameter. The distribution of these capillary voids is believed to be a more precise indicator for assessing hydrated cement paste characteristics than total porosity.

Air voids, usually spherical, contrast with the irregularly shaped capillary voids. The impact of these voids on concrete is considerable, affecting strength and permeability. To optimize the concrete's properties, understanding and controlling the volume and distribution of these voids are essential.