The document investigates the use of granular slag, a waste product from iron production, as a substitute for natural fine aggregates in cement mortar for masonry and plastering. The study demonstrates that replacing natural sand with granular slag can enhance mortar flow properties and compressive strength, with optimal results observed at 50-75% slag replacement. The findings indicate that utilizing granular slag not only provides a sustainable alternative to depleting natural resources but also promotes environmentally friendly construction practices.