Which property of mortar allows it to retain its original size and shape after compaction?

The correct answer is plasticity, which is the property of a material that allows it to deform under stress without breaking and to retain that deformation after the stress is removed. In the context of mortar, plasticity enables it to be worked into shape during the masonry process. When mortar is compacted and shaped, its plasticity ensures that it can hold that shape during the setting process and after it has hardened.

This characteristic is especially important in masonry work because it allows for adjustments and fine-tuning of the mortar application without the concern of it disintegrating or returning to its original state. Additionally, plasticity ensures a good bond between the masonry units, promoting connection and durability.

The other properties listed do not adequately describe this behavior. Viscosity refers to a fluid's resistance to flow, which isn't applicable after the mortar has been compacted. Elasticity refers to the ability of a material to return to its original shape after deforming, which applies differently in this context since the focus is on retaining the new shape rather than returning to an original one. Strength relates to the material's ability to withstand forces without breaking, but it does not directly address the aspect of retaining size and shape after compaction.