Can quicklime be used in mortar instead of hydrated lime? In older buildings that used lime in mortar, quicklime was slaked by mixing the lime in a tub with water for several hours before it was used. What would happen if the lime wasn't slaked? Wouldn't the hydration of quicklime also work as a binder similar to portland cement, thereby reducing the amount of cement required in the mortar?
Quicklime should not be used in mortar. Lime is calcium oxide. When quicklime hydrates, it becomes calcium hydroxide. As it hydrates, it expands. Some nonshrink grout manufacturers make use of this property of quicklime to counteract shrinkage. This requires careful proportioning and grinding to control particle size. To control the 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 mortars if quicklime is used.
Lime is not added to improve the strength of the mortar but rather to improve its workability. Although very finely ground quicklime may hydrate before the mortar is set, it does not improve the plastic properties of the mortar when in the form of quicklime. Its use would also increase risks of chemical burns. The hydration of lime does not act as a binder for the mortar. Lime will lower the strength of the mortar as its percentage of the cementitious components 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.