Waterproofing Mini-Lesson | Knowledge Popularization: What's the Culprit Behind Concrete Seepage? Learn About Capillary pores and Gel Pores

Concrete is the main base material for all kinds of buildings. It is made by mixing cement, sand, stone, water, and, when necessary, admixtures and mineral admixtures in appropriate proportions, and then mixing, molding, curing and hardening. Therefore, it has a certain degree of density. The cement hydration products (such as CSH gel) fill the gaps between the aggregates, which theoretically seems to prevent water penetration.

However, during the drying and hardening process of ordinary concrete, due to water evaporation and volume changes caused by cement hydration, tiny capillary pores and gel pores inevitably form. These pores constitute pathways for the penetration of water and other harmful substances.

Pore diameters in concrete (roughly classified):

• Among them, capillaries smaller than 0.1 μm play a major role in strength and permeability.

• Capillaries larger than 1 μm are the main channels for the migration of water and ions.

• Gel pores : < 0.002 μm, mainly existing inside cement hydration products. Water molecules can enter, but the penetration is extremely slow.

• Capillary pores : 0.01 μm to 10 μm. These are the key pores affecting the permeability and strength of concrete.

• Macroscopic defects such as cracks and air bubbles: > 10 μm, up to the millimeter level.

Microcracks and pores at the 5µm scale

The numerous tiny capillaries and pores inherent in concrete, along with uneven compaction during construction leading to honeycomb and voids, allow moisture to naturally permeate the structure and be transported into the interior.

Capillary pores: the main channels for moisture penetration

Experiments have shown that pores with a diameter of less than 25 nm and closed pores have little impact on the impermeability of concrete. Therefore, in the field of waterproofing and seepage prevention research, we usually focus more on capillary pores.

Simply put: capillaries are the "main problem," serving as the main channels for water and harmful ions (such as chloride and sulfate ions) to penetrate into the interior of concrete.

Under pressure or capillary action, liquid water can migrate relatively freely in interconnected capillaries. Even if the size (pore diameter) of the capillaries is small and their connectivity (torsional complexity) is complex, ion migration can still occur in concrete under actual building conditions. Chloride ions, which cause steel corrosion, and sulfate ions, which cause concrete erosion, can dissolve in water and be transported deep into the concrete through these channels. Simultaneously, concrete inevitably faces freeze-thaw damage —water seeping into the capillaries expands in volume when it freezes, generating internal stress, and repeated cycles lead to concrete spalling and cracking. Furthermore, there is alkali-aggregate reaction —water can act as a medium, transporting alkali to reactive aggregates through the channels, causing the concrete to expand and crack, ultimately leading to cracking and damage extending from the inside out.

Therefore, the core strategies for waterproofing and seepage prevention, the selected materials, and the system solutions are closely related to the capillary pores of concrete.

1. Penetrating crystalline waterproofing material

The idea is that the active substances in the material penetrate into the capillaries with water, react with cement hydration products to form water-insoluble needle-like crystals, which continuously fill and block the capillaries and microcracks.

2. Surface sealing and hydrophobic materials

The approach involves using silicone water-repellent agents, polymer waterproof coatings, and waterproof mortars to form an impermeable or water-repellent film on the concrete surface, blocking the entry of water into the capillary system. As a "closing-off" strategy, this serves as the first line of defense against water entering the concrete, but it does not address existing internal defects.

Gel pores: sponge-like pores that adsorb water molecules

In waterproofing and leak sealing, gel pores can be considered a "secondary problem." Due to their small size, water has great difficulty flowing through them, making them virtually impermeable. Their seepage prevention performance can only be indirectly affected by influencing the overall microstructure's density and tendency to shrink and crack.

However, the pores of gel have a large internal surface area, which strongly adsorbs water molecules . This water does not participate in the flow and penetration, but it does affect the humidity state of the concrete. Typically, the diameter of a water vapor molecule (H₂O) is approximately 0.0004 μm (0.4 nanometers, or 4 × 10⁻⊃1;⁰ meters). Therefore, when dealing with damp environments such as basements, inorganic penetrating waterproofing agents (which address liquid water) alone are insufficient.

Furthermore, when the environment is dry, adsorbed water escapes from the gel pores, causing the CSH gel layer to shrink, which in turn triggers overall concrete drying shrinkage. If this shrinkage is constrained, microcracks will form. Once these microcracks extend into the capillary network or interconnect, they create new and more severe seepage channels. This is the most significant indirect negative impact of gel pores on seepage prevention.

The concrete matrix can be improved at the microscopic level by adjusting the formula during the design and production process . Using mineral admixtures such as silica fume can generate more fine CSH gel by utilizing the "volcanic ash effect" or "micro-aggregate effect," refining and segmenting capillary pores, and optimizing the matrix where the gel pores are located; or the water-cement ratio can be reduced to decrease the initial water consumption at the production end, thereby reducing the total amount of water evaporation and loss in the future.

From the user's perspective, preventing water seepage caused by gel pores requires more attention to moisture-proofing strategies than simply waterproofing.

For large-scale projects such as underground engineering and tunnels, we can install reliable waterproof membranes and insulation boards on the outside of the structure. The insulation layer can reduce temperature changes, thereby indirectly reducing humidity changes, while the waterproof layer with polymer membrane as the base material can completely isolate external liquid water.

In home settings, we need to take a more systematic approach. A complete waterproof and moisture-proof system should include :

1， Waterproofing layer (such as penetrating crystallization) : solves the problem of liquid water (including capillary water) intrusion and protects the structure.

2， Vapor barrier/moisture barrier (interior wall) : A continuous moisture barrier film or moisture barrier coating is installed on a dry base surface to actively prevent water vapor from diffusing into the room.

3， Environmental control (dehumidifier) : Actively reduces indoor air humidity (water vapor partial pressure), which is the ultimate way to prevent condensation and mold.

In conclusion, there's no need to worry too much about problems caused by capillary pores or gel pores. As long as the concrete's "quality" is poor, Canlon can provide you with professional solutions .