Ash Content Testing in Dry Mix Mortar and Putty: Importance and Application
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1. The Nature of Cracking: Shrinkage and Internal Stress
In exterior wall systems, polymer-modified mortars and putties act as leveling and protective layers, directly affecting durability. However, cracking, debonding, and surface defects remain common issues.
From a material perspective, cracking occurs when shrinkage-induced stress exceeds the material’s strength. Rapid or excessive shrinkage during hydration leads to surface cracking.
The risk increases when:
👉 Water retention is insufficient
👉 Hydration is incomplete
👉 Formulations prioritize fast drying or early strength
2. Main Causes of Cracking: Internal and External Factors
2.1 Internal Factors (Materials & Formulation)
Cement inherently undergoes shrinkage, and different types behave differently:
👉 Ordinary Portland Cement: better stability
👉 Sulphoaluminate cement / white cement: higher shrinkage
Excessive use of film-forming additives (such as PVA or latex powders) may:
❌ Block pore structure
❌ Limit hydration
❌ Create a rigid surface layer
This leads to internal stress accumulation and cracking.
2.2 External Factors (Application & Environment)
External conditions also play a major role:
👉 Excessive application thickness
👉 Poor substrate condition
👉 High temperature, dry or windy environment
👉 Structural movement or settlement
These factors accelerate moisture loss or introduce additional stress. Cracks are dynamic and may change with temperature and humidity, making repair difficult.
3. How to Prevent Cracking: Key Control Measures
3.1 Material & Formulation Control
👉 Use stable cement systems (preferably OPC)
👉 Control shrinkage instead of only increasing strength
👉 Improve water retention for proper hydration
👉 Use polymers appropriately, avoid overuse
3.2 Application Control
👉 Apply in thin layers:
- Rendering mortar ≤ 2.5 mm
- Exterior putty ≤ 1 mm
👉 Use multi-layer application instead of thick coating
3.3 Structural Design Optimization
👉 Introduce expansion joints
👉 Allow stress release in the system
👉 Optimize façade insulation design
Conclusion
Cracking in exterior putty is not caused by a single factor but by the interaction of materials, application, and environmental conditions. A systematic approach is essential to effectively reduce cracking risks and ensure long-term performance.
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