Blog

A waterproof mortar contains silicone hydrophobic powder, and water droplets even bead on the surface.

But during a water-absorption test, the result is still higher than expected.

Does this mean the hydrophobic agent is not working?

Not necessarily.

Surface water repellency and overall water absorption are related, but they are not exactly the same. A mortar may repel water at the surface while still containing pores and capillary channels that allow water to enter over time.

Water can enter hardened mortar through connected pores.

If the mortar has high porosity, even a good hydrophobic agent may not completely compensate for the internal structure.

High porosity may result from:

• excessive mixing water;
• poor aggregate grading;
• excessive entrapped air;
• insufficient compaction;
• unsuitable powder-to-aggregate balance.

This is why waterproof mortar should not be designed around the hydrophobic additive alone.

The complete pore structure also matters.

Increasing mixing water makes mortar easier to spread, but after drying, the excess water leaves behind additional pores. These pores can increase water uptake.

Therefore, when comparing hydrophobic additives, keep the water dosage consistent. If Sample A uses less water than Sample B, differences in absorption cannot automatically be attributed to the hydrophobic powder.

A polycarboxylate superplasticizer may be useful in some mortar systems where lower water demand is required while maintaining workable consistency.

Too little hydrophobic powder may provide insufficient protection.

But simply increasing the dosage is not always the best solution either.

The additive must:

• disperse uniformly in the dry mix;
• remain compatible with cement and other additives;
• develop sufficient hydrophobicity after curing;
• avoid negatively affecting strength or workability.

SidleyChem’s silicone hydrophobic powder is designed for applications including waterproof mortar, joint filler, decorative mortar, and external insulation mortar.

The correct dosage should always be confirmed in the complete local formulation rather than selected independently.

A common mistake is to judge waterproof performance only from a water-drop test. Strong beading looks impressive, but it mainly shows surface hydrophobicity. For a more useful evaluation, also measure capillary water absorption over a defined period. A mortar may show good surface beading but still absorb water through internal pores, cracks, edges, or poorly compacted areas. Therefore, compare:

• water-drop behavior;
• water absorption over time;
• specimen weight gain;
• surface and internal defects.

Redispersible polymer powder can also influence the water resistance and internal structure of polymer-modified mortar.

Depending on the grade, RDP can improve adhesion, flexibility, and resistance to water penetration.

However, hydrophobic powder and RDP perform different functions.

RDP should not simply be used as a substitute for a hydrophobic agent, and a hydrophobic agent cannot correct a weak or poorly designed binder system.

The two need to be evaluated as part of the complete mortar formulation.

Waterproof performance should not be evaluated too early.

If the specimen has not cured sufficiently, cement hydration and polymer film development may still be incomplete.

High temperature, rapid drying, or poor curing can also create microcracks and increase water penetration.

When comparing samples, keep curing time, temperature, specimen size, and testing conditions consistent.

High water absorption in waterproof mortar does not always mean the hydrophobic powder has failed.

The real cause may be:

• excessive porosity;
• too much mixing water;
• insufficient hydrophobic agent;
• poor dispersion;
• unsuitable RDP or binder balance;
• insufficient curing.

The best waterproof mortar combines low capillary absorption with good strength, adhesion, workability, and durability.

Do not judge the formulation only by whether water beads on the surface.