Does Absorption – Porosity of the Surface Affect Bonding?

In construction and finishing works, surface porosity plays a critical role in determining how well a product bonds to a substrate. Whether you are applying tile adhesives, self-leveling compounds, waterproofing membranes, or coatings, the absorption characteristics of the surface directly influence performance and long-term durability.

What is Porosity? 

Porosity refers to the proportion of open pores or voids within a material. These pores can be:

• Capillary pores – formed during hydration or solidification of cementitious materials.
• Air voids – trapped during mixing.

Effect of Porosity on Bonding

Highly porous substrates: Examples: old concrete, screeds, plaster, AAC blocks.

• Absorb mixing water from cementitious products too quickly.
• Adhesive does not get enough water for proper hydration → weak bond.

Low-porosity or non-porous substrates: Examples: glazed tiles, dense stone, metals.

• Provide little to no mechanical key for bonding.
• Adhesive sits on the surface without anchoring → debonding risk.

Moderate porosity substrates: Examples: cement boards, fair-faced concrete.

• Provide a good balance for mechanical interlock and hydration.
• Often ideal for strong adhesion.

How Absorption / Porosity Affects Bonding

1. Sealing the surface – Reduces excessive suction on highly porous substrates so the product does not lose water too quickly.
2. Mechanical bond improvement – Primers create a tacky or micro-rough layer for better grip.
3. Chemical bonding – Some primers contain resins or reactive agents to bond chemically with both the substrate and the applied product.
4. Uniform surface condition – Primers minimize dust, laitance, or micro-particles that block adhesion.

High vs. Low Porosity in Practice

• High Porosity → High Water Absorption, Product’s mixing water is drawn into the substrate → weak hydration → poor bonding.
• Low Porosity → Very Low Water Absorption, Adhesives can “float” without proper grip unless primed or roughened.

Example:

• Concrete screed with 8% absorption will pull water from cement-based adhesive in minutes.
• Glazed tile with <0.5% absorption won’t allow adhesive penetration without surface roughening or a primer.

Porosity, Strength, and Durability

In cement-based systems (concrete, mortar, screed):

• Cement + Water → Hydration → Binder formation.
• Higher strength requires lower porosity.
• Lower porosity generally means higher durability.

The hydration process links strength, porosity, and durability — improving one influences the others.

Standards for Measuring Porosity / Water Absorption

1. Cement, Concrete & Mortar

• ASTM C642 – Density, absorption, and voids in hardened concrete.
• BS 1881-122 – Water absorption in hardened concrete.
• IS 3085 – Permeability of cement mortar and concrete.

2. Ceramic Tiles & Fired Clay

• ASTM C373 – Porosity, water absorption, bulk density of ceramic tiles.
• ISO 10545-3 / EN ISO 10545-3 – Porosity of ceramic tiles.
• ASTM C20 – Porosity and density of refractory brick.

3. Natural Stone

• ASTM C97 – Absorption and bulk specific gravity of dimension stone.
• EN 1936 – Density and porosity of natural stone.
• IS 1124 – Porosity of building stones.

4. Refractories

• ASTM C830 – Porosity and density of refractory shapes by vacuum.
• ASTM C20 – Boiling water method for refractories.

5. Plastics & Polymers

• ASTM D570 – Water absorption of plastics.
• ISO 62 – Water absorption of plastics.

Simple Site-Level Porosity Test (Qualitative) as per ASTM F3191

1. Clean the surface.
2. Place a drop of water with a pipette.
3. Wait 1 minute.
4. Wipe the droplet.
5. Observe:
• Absorbs quickly → High porosity.
• Remains rounded → Low porosity.
• Moderate absorption → Ideal bonding potential.

Recommendations

• High porosity → Use primers to seal the substrate and reduce suction.
• Low porosity → Use bonding primers to create a mechanical/chemical key.
• Always ensure the surface is clean, sound, and free of contaminants.

Absorption and porosity directly affect adhesion. Understanding your substrate and applying the correct primer or preparation method ensures a strong, durable, and reliable bond — preventing costly failures in tile, stone, or coating installations.

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