Mortar Admixture is a mix of sand, cement, and water that is used to bind together a concrete mixture. It can be used in its own right as a mortar for masonry or as a liquid for slurry. Mortar Admixture is a mix of sand, cement, and water that is used to bind together a concrete mixture. It can be used in its own right as a mortar for masonry or as a liquid for slurry.