Grout

Grout is a particularly fluid form of concrete used to fill gaps. It is used in pressure grouting, construction to embed rebars in masonry walls, connect sections of pre-cast concrete, fill voids, and seal joints such as those between tiles. Grout is generally a mixture of water, cement, sand, often color tint, and sometimes fine gravel (if it is 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 are applied as a thick emulsion and harden over time, grout is distinguished from its close relative mortar by its viscosity; 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 on top of it.

Grout varieties include tiling grout, flooring grout, resin grout, non-shrink grout, structural grout and thixotropic grout.

Tiling grout is often used to fill the spaces between tiles or mosaics, and to secure tile to its base. Although ungrouted mosaics do exist, most have grout between the tesserae. Tiling grout is also cement-based, and comes in sanded as well as unsanded varieties. The sanded variety contains finely ground silica sand; unsanded is finer and produces a non-gritty final surface. They are often enhanced with polymers and/ or latex.

Structural grout is often used in reinforced masonry to fill voids in masonry housing reinforcing steel, securing the steel in place and bonding it to the masonry. Non-shrink grout is used beneath metal bearing plates to ensure a consistent bearing surface between the plate and its substrate.

Portland cement is the most common cementing agent in grout, but thermoset polymer matrix grouts based on thermosets such as urethanes and epoxies are also popular.

...
Wikipedia