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Hydrophobic Vs. Hydrophilic

Water-activated chemical grouts can possess two distinctly different properties. They are either hydrophilic or hydrophobic. Hydrophilic grout will absorb the water it finds in the concrete or soil. Hydrophobic grout will repel it and push it away.

Prime Resins has been manufacturing both hydrophilic and hydrophobic grouts for years. To help you understand the difference between these two types of grouts we have compiled this list of frequently asked questions. Understanding the properties of each will allow you to select the right grout for your application.

1) When should you use a hydrophobic grout, and when should you use a hydrophilic grout?

To know which type of grout is best for your application, you need to understand the differences between them. Start with the following table. It describes the properties of both types of grouts and shows a few of their recommended applications.





Reacts upon contact with water

Absorbs water while curing

Repels water while curing

High expansion (greater than 2000%)

Low Expansion (less than 700%)

Cures to flexible gel or foam

Cures to rigid foam

Bonds tightly to damp concrete/masonry

Set time controlled by catalyst

Low viscosity




Sealing leaks in joints or cracks

Repairing failed water stops

Filling small voids

Filling large voids

Stabilizing soil


As you can see, hydrophilic grouts are best for sealing leaks and repairing water stops, while hydrophobic grouts are best for filling voids and stabilizing soil.

2) Why is a hydrophilic grout best for sealing leaks in joints or cracks?

Hydrophilic grouts have two properties that make them best for sealing leaks.   They get a tenacious bond with concrete by chasing and absorbing the water in the crack and in all of the micro-fractures that branch off the main crack.

As hydrophilic grouts cure, they form a tight flexible seal that will expand and contract as pressure or temperature changes cause the crack or joint to open or close.


3) Why doesn’t a hydrophobic grout work just as well to seal a leak in a joint or crack?

There are two reasons for this--hydrophobic grouts repel water and they cure to a rigid foam.

While contact with water starts the reaction, these grouts actually push water away as they react. The surrounding water isn’t absorbed into the grout, but instead is forced through the crack and deep into the micro-fractures. Since the micro-fractures are filled with water, the hydrophobic grout can’t flow in and establish a tenacious bond with the surrounding surfaces.

After a hydrophobic grout cures, it becomes a rigid foam. Because it is rigid, it is unable to re-expand to its original size if it is compressed. When temperature change or other pressures cause the crack to close, the foam will be compressed. Then as the pressure changes and the crack reopens, the poorly bonded foam will break away from the surrounding surfaces and the leak may return.


4) Why are hydrophobic grouts recommended for stabilizing soils and filling voids?

Hydrophobic grouts have three properties that make them best for these jobs—low viscosity, controlled setup time, and high expansion. The low viscosity allows the grout to travel easily through loose soil.

The set time can be adjusted from 30 seconds to 5.5 minutes by changing the percentage of catalyst added to the resin. Faster set times are used when filling voids. Slower set times are used when stabilizing large areas to give the grout more time to migrate through the soil.

As the grout expands and sets, the loose soil and sand becomes trapped in the rigid foam. The highly expansive foam pushes out in all directions compressing the surrounding soil and driving out moisture. The result is a solid waterproof ball or curtain of grout and soil.

5) Are there situations where a hydrophobic grout could be used to stop a leak?

There are a few. The best example is stopping a gushing leak in a below-grade structure. Obviously, large amounts of water are moving quickly through a gushing leak. Most likely, this fast moving water is eroding the surrounding soil and carrying small particles of it through the leak. This creates large voids outside the structure. Until the leak is stopped, the surrounding soil will continue to settle and collapse into the voids. Usually a hydrophobic grout injected into these voids will quickly stop the fast moving water and stabilize the soil.

The grout can be injected either by drilling through the wall of the structure and pumping the grout to the outside, or by driving injection pipes down from the surface and then pumping grout through the pipes into the voids. This will often stop the leak completely.

If the structure is not subject to thermal expansion or contraction or other types of structural movement, no additional repairs may be necessary. However, if the hydrophobic grout injection only slows the leak, or if the structure will be subject to movement, it is advisable to finish the repair by injecting a hydrophilic grout into the crack or joint.

6) Which Prime Resins grouts are hydrophilic and which are hydrophobic?

Prime-Flex 900LV, 900LVSF, and 970 Hydro Gel are hydrophilic grouts used to stop leaks through cracks and to repair failed water stops. Prime-Flex 900LVSF has the added advantage of being solvent-free and UL certified to comply with ANSI / NSF for contact with potable water.

Prime-Flex 910 and 920 are hydrophobic grouts used for stabilizing soil and filling voids.

If you have questions about what type of grout (hydrophobic or hydrophilic) is the best solution to your problem, call Prime Resins’ technical support line at 800-321-7212.