Can quick lime be used in mortar instead of hydrated lime? I know that in older buildings that used lime in the mortar, quick lime was slaked by mixing it in a tub with water for several hours before it was used. What would happen if the lime wasn't slaked? Won't the hydration of quick lime create a binder similar to portland cement, thereby reducing the amount of cement needed in the mortar?

Quick lime should not be used in mortar. Lime is calcium oxide. When quick lime hydrates, it becomes calcium hydroxide. As it hydrates, it expands. Some nonshrink grout manufacturers make use of this property of quick lime to counteract shrinkage. This requires careful proportioning and grinding to control particle size. To control expansion, these particles may also be coated to delay hydration. If particle sizes are too large, the lime will continue to hydrate and expand long after the grout has set, causing the mortar to break apart. Similar problems can occur in mortar if quick lime is used.

Lime is added not to improve the strength of mortar but rather to improve its workability. Although very finely ground quick lime may hydrate before the mortar is set, it does not improve the plastic properties of the mortar when in the form of quick lime. Its use would also increase risks of chemical burns. The hydration of lime does not create a binder in the mortar as portland cement does. In fact, lime lowers the strength of mortar as its percentage of the cementitious component increases. Although carbonation of hydrated lime will increase mortar strength over time, this is a very slow process. Depending on the porosity of the mortar, carbonation may take well over 100 years to complete.