I've heard a lot about integral water repellents for block. What are they and what is their purpose? Do I need to specify them in the mortar as well as the block?

Integral water repellents are usually polymeric liquid admixtures incorporated into concrete masonry units during manufacturing to reduce the wicking characteristics of the units. As such, they make masonry materials hydrophobic by limiting the amount of water that can pass through the masonry due to capillary suction (wicking).

They are particularly beneficial for use in colored architectural concrete masonry units where post-applied finishes such as paints and other weather-resistant materials are not desired.

They are also beneficial for use in masonry that is intended to receive weather-resistant finishes, as they significantly limit any water that does manage to get into the cores of the block from migrating to the interior of the building by capillary suction. Integral water repellants also are effective at reducing efflorescence, since water migration throughout the block is reduced.

When used to mitigate water penetration, it is essential that integral water repellent admixtures be incorporated into the mortar at the jobsite, as well as into the block and any other masonry wall component, such as precast lintels, to ensure adequate bond and similar reduced water capillary suction characteristics. A compatible water repellent admixture from the same manufacturer should be used in the mortar that was used in the block to ensure compatibility.

While these admixtures can limit the amount of water that can pass through units and mortar, they have little impact on moisture entering through relatively large cracks and voids in the wall. Therefore, even with the incorporation, proper detailing to prevent water entry and quality workmanship to ensure filled and adequate mortar joints is still essential.

Any water that does penetrate past the external surface of masonry with integral water repellents is more likely to quickly drain to the base of the wall because the units are not prone to retain it. So proper flashing and weep holes are critical to divert any water that finds its way into the masonry back to the outside.

Questions also arise about he effect of integral water repellents on mortar bond strength, due to the decreased water absorption characteristics of the units. Research has shown that bond strength is primarily influenced by the mechanical interlock of mortar to the small voids in the block.

Also, some manufacturers incorporate bond enhancers into their integral water repellent mortar admixture to help assure the necessary bond is achieved. You can request test data from the integral water repellent admixture manufacturers that will document the bond strength typically achieved with units and mortar containing their admixture system.

Regarding walls to be grouted, the hydrostatic pressure from the grout forces water into the surrounding masonry unit resulting in the proper bond and curing of the grout. Integral water repellents are not typically added to masonry grout.

This Q&A is from The Masonry Society