Grout is a particularly fluid form of concrete used to fill gaps. Grout is generally a mixture of water, cement, and sand, and is employed in pressure grouting, embedding rebar in masonry walls, connect-ing sections of pre-cast concrete, filling voids, and sealing joints such as those between tiles. It is often color tinted when it will remain visible, and sometimes includes fine gravel when being used to fill large spaces such as the cores of concrete blocks). Unlike other structural pastes such as plaster or joint compound, correctly mixed and applied grout forms a waterproof seal. Although both grout and its close relative mortar are applied as a thick emulsion and harden over time, grout is distin-guished by its high viscosity and lack of lime (added to mortar for pliability); grout is thin so it flows readily into gaps, while mortar is thick enough to support not only its own weight, but also that of masonry placed above it.