【Tech Insight】Why Does Your Mortar Always Crack or Fall Off? — Complete Guide to Mortar Strength Grades and Applications

 

1. Why Mortar Strength Matters So Much

In masonry construction, mortar is not just the “gray paste” between bricks —
it is the key to the wall’s stability, durability, and waterproofing.

If the strength grade is chosen incorrectly:

• Too weak → The wall becomes loose, cracks, or delaminates easily.

• Too strong → The plaster layer becomes harder than the substrate, leading to hollowing and peeling.

👉 Correct strength selection prevents rework, improves structural integrity, and extends the building’s lifespan.

2. Mortar Strength Grade System Explained

Mortar strength is represented by “M” or “Mb”, followed by a number that indicates its 28-day compressive strength (MPa).
For example: M5 = 5 MPa compressive strength.

Currently, there are two main grading systems in China:

1️⃣ Traditional Masonry Mortar (site-mixed)

Grades: M2.5, M5, M7.5, M10, M15, M20
👉 Used mostly in on-site mixed mortars for conventional projects.

2️⃣ Premixed Mortar (dry-mix or wet-mix)

Grades: Mb5, Mb7.5, Mb10, Mb15, Mb20, Mb25, Mb30
👉 Equivalent to the “M” system but applied in commercial mortar production.

Conversion Reference:
M5 ≈ Mb5  M10 ≈ Mb15  M20 ≈ Mb25

3. Typical Applications by Mortar Strength Grade

Strength Grade	Common Application	Typical Use	Features
M2.5 / Mb5	Light partition walls, temporary masonry	Non-load-bearing indoor walls, crack-resistant coating in insulation systems	Easy to apply, economical
M5 / Mb7.5	General masonry, interior plastering	Load-bearing and non-load-bearing walls in residential buildings, leveling layer	Most commonly used grade
M7.5 / Mb10	Multi-story masonry, exterior walls	Chimneys, small tanks, humid or rainy areas	Better compressive and weather resistance
M10 / Mb15	Underground or high-load walls	Foundations, basements, workshops, garages	Stronger compressive and moisture resistance
M15 / Mb20 and above	Structural reinforcement, waterproof layers, floor mortar	Pools, water tanks, sewage systems, high-rise core walls	High strength, wear-resistant, dense and durable

4. Key Selection Principles

✅ Strength Matching

• For masonry mortar, match the strength of the block.

  • High-strength concrete blocks → use M10 or above.

• For plaster mortar, do not exceed the substrate’s strength, otherwise cracks or hollows may occur.

✅ Environmental Adaptation

• Damp or underground areas: use ≥M10 / Mb15 (cement or mixed mortar).

• Dry indoor walls: M5 / Mb7.5 is sufficient, easier to apply and more cost-effective.

✅ Design and Code Compliance

Always follow the strength grades specified in the GB 50003 – Masonry Structure Design Code and project design documents.

5. Easy Memory Formula

🧠 M5 / M7.5: for general wall masonry and interior plaster.
🧱 M10 or above: for foundations, basements, and damp environments.
🚧 M2.5: for temporary partitions or basic leveling.

 

6. Conclusion: The Right Mortar, the Stronger Structure

Choosing the right mortar strength grade ensures better bonding, crack resistance, and long-term performance of your building.

At ARICH CELLULOSE, we provide a full range of construction-grade additives — HPMC, RDP, PCE, Defoamer, and Gypsum Retarder — designed to help you achieve:

• ✅ Stable water retention and controlled strength development

• ✅ Improved workability and adhesion

• ✅ Durable, weather-resistant, and crack-free mortar performance