Blog

Gypsum mortar is widely used for interior wall finishing because of its smooth surface, fast setting behavior, and excellent workability. However, shrinkage during drying is one of the most common issues that can lead to surface cracks and reduced durability.

Understanding why shrinkage occurs and how to control it is essential for producing high-quality gypsum-based products and achieving stable construction performance.

Shrinkage in gypsum mortar mainly occurs during the drying and hardening process. As water evaporates from the material, the internal volume decreases, generating stress within the hardened structure.

Several factors can accelerate or intensify shrinkage:

• Excess water in the mix
• Rapid moisture loss from the surface
• High absorbency of the substrate
• Poor formulation design
• Extreme environmental conditions such as high temperature or strong wind

When these factors combine, the internal stress exceeds the material’s tensile strength, leading to visible cracks.

One of the most effective ways to reduce shrinkage is through proper use of functional additives.

1. HPMC (Cellulose Ether)

HPMC improves water retention and slows down evaporation. By keeping moisture inside the system for longer, it allows more uniform hydration and reduces internal stress development.

2. Gypsum Retarder

A gypsum retarder helps control the setting speed. When the setting process is too fast, uneven hydration occurs, which increases shrinkage risk. Retarders help ensure a more stable and uniform structure formation.

3. Starch Ether

Starch ether improves consistency and reduces sagging. It also contributes to better internal structure stability, which helps reduce deformation during drying.

Shrinkage control is not achieved by a single additive alone. A balanced formulation is required.

Manufacturers should carefully control:

• Gypsum purity and fineness
• Water-to-powder ratio
• Type and dosage of additives
• Compatibility between HPMC and retarders

Overuse of water or improper additive selection can actually increase shrinkage instead of reducing it.

Even a well-designed mortar can crack if construction practices are poor.

To minimize shrinkage-related cracking:

• Avoid applying mortar under direct sunlight or strong wind
• Pre-treat highly absorbent substrates
• Ensure uniform mixing on site
• Avoid applying overly thick layers in one pass
• Allow proper curing time

These practices help maintain moisture balance during the critical setting phase.

Gypsum mortar shrinkage is a natural physical process, but excessive shrinkage can be effectively controlled. By combining high-quality additives such as HPMC, gypsum retarders, and starch ether with optimized formulation design and proper construction methods, manufacturers and contractors can significantly reduce cracking risks.

A well-balanced system ensures better dimensional stability, improved surface quality, and longer-lasting gypsum plaster performance.