The specifications for the project I am working on require prehydrated mortar. Is a mortar that uses prehydrated lime considered a prehydrated mortar?
No. A prehydrated mortar is a mortar that is mixed 1 to 1 1/2 hours before it is used. Prehydrated mortar is used for tuckpointing to reduce the amount of shrinkage. Mortar that shrinks less creates tighter joints. Letting the mortar sit awhile can reduce shrinkage because a lot of water is used in the chemical reactions that occur in the first 1 to 1 1/2 hours. Using prehydrated lime doesn't make mortar a prehydrated mortar. Regardless of their use, all portland cement-lime mortars should be made with prehydrated lime. Technical Note 7F, published by the Brick Institute of America (BIA), describes how to make a prehydrated mortar: Thoroughly mix all dry ingredients, then add only enough clean water to produce a damp, workable consistency that will keep its shape when formed into a ball. Let the mortar stand in this dampened condition for 1 to 1 1/2 hours.