Analysis of Causes and Preventive Measures for Air Bubbles in Plastering Construction - Hydroxypropyl Methylcellulose

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Air bubbles are a common problem during plaster plaster application. They not only affect the aesthetics of the wall surface but can also reduce the adhesion between the coating and the substrate in severe cases. The formation of air bubbles is usually the result of multiple factors, which can be analyzed from the following aspects:

I. Material-related Reasons

1. Gypsum Powder Characteristics: During the processing of gypsum powder, if the particle size distribution is unreasonable and there is too much fine powder, it is easy to trap air and form bubbles when water is added and stirred.

2. Additive Issues:

 Excessive Air Entraining Agent: To improve workability, some manufacturers add air entraining agents. However, if added in excess, it will produce a large number of tiny, stable bubbles.

Insufficient Water Retention Agent: Water retention agents (such as cellulose ethers) ensure that the water in the gypsum slurry is not absorbed by the substrate too quickly, thus allowing sufficient time for air release. If the amount of water retention agent added is insufficient, water loss is too rapid, the slurry sets prematurely, and the bubbles are trapped inside.

Compatibility of Water-Reducing Agents or Retarders: Poor compatibility between various additives may produce unexpected chemical or physical reactions, leading to an increase in bubbles.

 II. Causes Related to Substrate Treatment (This is the most common cause)

1. Substrate Water Absorption Rate Too High or Too Low:

Too High: Substrates such as aerated concrete blocks and porous bricks absorb water extremely quickly, rapidly drawing moisture from the gypsum mortar. This causes premature skinning on the mortar surface, preventing the escape of internal air and moisture, thus forming air bubbles.

Too Low: Substrates such as smooth concrete shear walls and residual release agent from formwork do not absorb water. When air bubbles in the mortar come into contact with the substrate, they cannot be absorbed and broken down, instead accumulating upwards, forming hollow areas or surface bubbles at the interface between the gypsum and the substrate.

2. Uneven Substrate Drying: If the substrate is excessively dry in some areas, with visible water or moisture, the gypsum mortar will set at different rates and retain moisture differently in different areas, easily leading to air bubbles in the dry areas.

3.Substrate Defects: Substrate surfaces with numerous pores and cracks trap air. When gypsum mortar is applied, the air is squeezed out, forming air bubbles. The presence of dust, oil, release agents, and other isolating substances on the substrate surface affects the adhesion between the gypsum mortar and the substrate, making it easier for gas to accumulate at the interface.

III. Reasons Related to Construction Process

1. Improper Mixing Method:

Mixing Speed and Time Too Long: High-speed and prolonged mechanical mixing will introduce a large amount of air, dispersing it evenly in the slurry and forming numerous tiny bubbles.

Uneven Mixing: Manual mixing or insufficient mixing will result in additives (such as water-retaining agents) not fully dissolving and functioning, also easily leading to bubble formation.

2. Insufficient Setting Time After Mixing: If the mixed slurry is not allowed to "mature" for 2-3 minutes to allow additives to fully dissolve and some large bubbles to burst naturally before being applied to the wall, it will introduce more bubbles.

3. Improper Application Techniques:

Premature Finishing: When the slurry is still too soft, finishing at this time will push bubbles to deeper layers or cause them to burst on the surface, forming pinholes.

Delayed Finishing: When the slurry has begun to set and lost its plasticity, finishing at this time cannot close the surface bubbles.

Applying Too Thick a Layer at Once: If the plaster layer is too thick, the internal moisture and air have a long escape path, and the surface has already solidified, preventing internal air from escaping.

Improper timing of finishing: Uniform application direction: Always applying in one direction can easily trap air between the material and the substrate.

 IV. Environmental Factors

1. High Ambient Temperature: High temperatures accelerate the setting reaction and moisture evaporation of gypsum, shortening the workable time of the plaster and reducing ventilation time.

2. Windy Weather: Strong winds accelerate moisture evaporation from the wall surface, causing rapid skin formation, similar to the effect of high temperatures.

3. High Substrate Temperature: In summer, walls exposed to direct sunlight reach very high temperatures, drastically accelerating the dehydration and setting of the gypsum.

Summary and Preventive Measures

To avoid air bubbles, the following comprehensive measures can be taken:

1. Prepare the substrate: Moisten the substrate with water before construction, but avoid standing water. For substrates with high water absorption, it is recommended to use a wall primer or interface agent for sealing; this is the most crucial step.

Ensure the substrate is firm, flat, and free of oil and dust.

2. Standardize mixing: Use a low-speed electric mixer and mix according to the water-cement ratio specified in the product instructions.

After mixing for 2-3 minutes, let it stand for 2-3 minutes (maturation), then mix briefly before use.

3. Optimize construction: Apply in layers, with each layer not too thick (generally, a single layer thickness of no more than 1cm is recommended).

Use different directions for troweling, for example, first apply a thin vertical layer, then a thicker horizontal layer for leveling, which helps to expel air.

Finish at the right time: The best results are achieved when the surface is no longer wet and feels slightly firm but not sticky to the touch.

4. Control the environment: Avoid construction in high temperatures or strong winds. If construction is required, measures such as sunshade and wind protection should be taken.