Which mortar ingredient decreases the shrinkage of mortar while setting and drying?

The ingredient that decreases the shrinkage of mortar while setting and drying is lime. Lime is added to mortar mixes primarily to enhance workability and reduce shrinkage. When lime is used, it helps to bind the aggregates together and increases the plasticity of the mix, allowing it to retain moisture more effectively during curing. This moisture retention leads to less shrinkage as the mortar dries, resulting in a stronger and more durable final product.

While sand is essential for the structure and bulk of the mortar, it does not specifically address shrinkage the way lime does. Cement, on the other hand, is crucial for the initial strength and bonding of the mix, but it can contribute to shrinkage as it dries and hardens. Gypsum is typically used in certain types of plaster and may influence setting times but is not a major component in standard mortar compositions for the purpose of reducing shrinkage. Thus, lime stands out as the most effective ingredient in minimizing shrinkage during the setting and drying process of mortar.