Views: 21 Author: Site Editor Publish Time: 2025-06-26 Origin: Site
Choosing the right compressive strength for plaster mortar (typically rated M5 to M20) is a critical balancing act. Contrary to intuition, higher strength isn't always better. Performance, durability, and cost-effectiveness depend on selecting the strength grade that best matches the specific application requirements. Let's examine the key considerations and data.
Experimental data reveals significant trade-offs as mortar strength increases:
| Strength Grade | 28-Day Compressive Strength | Shrinkage Increase | Cracking Risk | Bond Strength (Peak) |
|---|---|---|---|---|
| High (≥ M15) | 15-20 MPa (High) | +0.15 - 0.30 mm/m | ↑ 40% | Lower than M10-M15 |
| Medium (M10-M15) | Moderate | Moderate | Moderate | 1.2 - 1.5 MPa (Peak) |
| Low (M5) | ~5 MPa (Lower) | Minimal | Low | Lower than M10-M15 |
High-Strength Drawbacks: While offering the highest compressive load resistance (15-20 MPa), M15+ mortars exhibit significantly increased shrinkage (0.15-0.3 mm/m) and a 40% higher risk of cracking. Crucially, their bond strength to substrates is often lower than medium-strength mortars.
The Bond Strength Peak: The optimal zone for adhesion (bond strength) consistently lies within the M10-M15 range, achieving peak values of 1.2-1.5 MPa.
Substrate Type:
Concrete Substrates: Require higher bond strength. Recommendation: M10-M15.
Masonry Substrates (Brick, Block): Benefit from better deformation compatibility. Recommendation: M5-M7.5.
Environmental Conditions:
High Humidity/External Walls: Increase strength by 1-2 grades (e.g., from M7.5 to M10) for enhanced durability.
Large Temperature Fluctuations (>30°C variation): Decrease strength by 1 grade (e.g., from M15 to M10) to reduce shrinkage cracking risk.
Finishing System Compatibility:
Tile Cladding: Use mortar ≤ M10. Higher strength mortars are too rigid and prone to cause hollow spots (bond failure) under tile movement.
Paint Finishes: Use mortar ≥ M7.5 to ensure sufficient surface hardness and prevent paint damage or unevenness.
While strength is vital, advanced formulations address its inherent trade-offs:
Polymer-Modified Mortars (e.g., with RDP):
Benefit: Dramatically improves adhesion (+60% at M7.5 strength) and flexibility without needing excessively high compressive strength.
Result: Enables the use of more crack-resistant medium-strength mortars (M7.5-M10) even where high bond is required.
Fiber-Reinforced Mortars:
Benefit: Increases crack resistance by 3-5 times by controlling plastic shrinkage and distributing stresses.
Result: Allows the use of higher-strength mortars (M15+) where absolutely needed, while mitigating their inherent higher cracking risk.
Gypsum-Based Plasters:
Benefit: Offers inherent humidity regulation (moisture buffering) and typically lower shrinkage than cement-based mortars.
Result: Excellent for interior applications, providing good strength (often similar to M5-M10 cement) with superior crack resistance and indoor climate benefits.
The optimal plaster mortar strength isn't a single number. It's the grade (usually within M5 to M15) that provides sufficient bond strength and durability for the specific substrate and environment, while minimizing shrinkage and cracking risk, and complementing the chosen finish. Leveraging polymer modifications, fibers, or gypsum chemistry allows formulators and applicators to achieve this balance more effectively than ever before, ensuring durable, high-quality plasterwork.
