Q It is generally desirable to have a low water-cement ratio in concrete to increase its strength. Wouldn't the same hold true for mortar?

It would seem that adding a super plasticizer or other water reducing admixtures to mortar would improve its strength. Why is this not done?

A Adding water-reducing admixtures to mortar can retard hydration and adversely affect the bond to the masonry units. Concrete uses forms that absorb very little water. Mortar, on the other hand, is placed against absorptive brick or concrete masonry surfaces. Also, concrete walls, slabs, and beams are generally very thick, compared to mortar joints.

If water-reducing admixtures were used, moisture needed for hydration would be drawn out of these thin joints placed between two absorptive surfaces. As a result, there may not be sufficient water available for proper hydration.

The bond of the mortar also could be affected by the introduction of a water-reducing admixture. Mortar in masonry walls needs to be workable to develop bond and mechanical keying to the masonry units. If the mortar has little excess water, the top surface of the joint, onto which the subsequent units are placed, may be too dry and stiff to develop a proper bond.

For these reasons, water-reducing admixtures generally are not recommended for masonry mortars. Instead, it is important to add water to mortars during construction to maintain proper flow by re-tempering them as soon as they lose moisture by evaporation.

A consultant at Wiss, Janney, Elstner Associates Inc., Northbrook, Ill.