Do you know about crack-resistant mortar? -RDP/VAE/PP

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I. What is it?

Crack-resistant mortar is a functional mortar used to prevent or reduce cracking on substrate surfaces. It is typically composed of cement, quartz sand, polymer powder, crack-resistant fibers (such as polypropylene fibers), and various additives.

You can think of it as a "high-performance, resilient" version of cement mortar. It not only possesses the strength of traditional cement, but more importantly, it boasts excellent flexibility and bonding strength, effectively resisting stress caused by temperature changes, shrinkage, etc., thereby inhibiting the generation and development of cracks.

II. Core Functions and Roles

1. Crack Resistance: This is its primary function. It absorbs and disperses stress, preventing cracking caused by stress concentration.

2. High-Strength Adhesion: It can firmly bond to various substrates (such as concrete, blocks, and insulation boards), preventing hollow areas and detachment.

3. Waterproofing and Seepage Prevention: The formed coating is dense and has excellent waterproof performance, protecting the substrate from moisture erosion.

4. Weather Resistance and Durability: It can withstand the long-term test of harsh weather conditions such as wind, sun, rain, and freeze-thaw cycles.

5. Load-Bearing Protection: In thin-plaster exterior wall insulation systems, it wraps around alkali-resistant mesh fabric, together forming the system's protective layer, providing a solid base for the finishing layer (such as paint or stone paint).

III. Main Application Scenarios

1. External Wall Insulation Systems: This is its most classic application. It's used for applying a thin layer of plaster to the surface of insulation boards (such as EPS boards, XPS boards, and rock wool boards), typically with alkali-resistant fiberglass mesh embedded to form a crack-resistant protective layer.

2. Interior and Exterior Wall Plastering and Leveling: Used for plastering concrete, aerated concrete blocks, and other base layers, replacing traditional cement mortar and fundamentally reducing the risk of wall cracking and hollow areas.

3. Repair Projects: Used to fill wall cracks, pits, and other defects.

4. Other Crack-Resistant Areas: Such as basements, tunnels, and other locations requiring crack resistance.

IV. Why is it crack-resistant? Working Principle

Its crack-resistant ability mainly comes from two key substances in its formula:

1. High-molecular polymer powder:

This is the soul of the crack-resistant mortar. It dissolves in water in the mortar and forms a flexible high-molecular film after drying.

This film, like a "fishing net," tightly interweaves the cement paste and aggregate, greatly improving the mortar's flexibility and bonding strength. When the base layer deforms, it can "digest" stress through its own micro-deformation, rather than brittle fracture.

2. Crack-resistant fibers:

These are usually short-cut polypropylene fibers. They are evenly distributed in the mortar, forming a randomized support system, like "plant roots," effectively inhibiting micro-cracks caused by drying shrinkage and thermal expansion and contraction in the plastic stage and early hardening stage of the mortar.

The polymer provides "toughness," and the fibers inhibit "micro-cracks." The synergistic effect of both achieves a crack-resistant effect greater than the sum of its parts (1+1>2).

V. Construction Steps and Key Points

Proper construction is crucial to ensuring the performance of the crack-resistant mortar. The following are the main construction steps and details requiring special attention:

1. Substrate Preparation: First, remove dust, oil, and debris from the wall surface to ensure a clean substrate. The substrate should be flat and firm. For substrates with high water absorption or excessively dry surfaces, apply a primer or spray a small amount of water to moisten the surface before construction.

2. Material Preparation: Strictly follow the water-cement ratio specified in the product instructions (e.g., a common weight ratio is approximately 1:0.21-0.23, but refer to the product instructions for specific details). First, add water to the mixing container, then add the dry powder of the crack-resistant mortar. Use an electric drill with a mixing head to mix thoroughly for at least 5 minutes, which can be extended to at least 7 minutes in winter, until a uniform, viscous paste is formed. The mixed slurry must be used within 1.5 to 2 hours; adding water or re-mixing is strictly prohibited. 

3. Application: Crack-resistant mortar is typically used in conjunction with alkali-resistant fiberglass mesh or galvanized steel wire mesh to enhance the crack-resistant protective layer. A common practice is layered application:

First, apply a first coat of crack-resistant mortar, approximately 2mm thick.

Immediately press in the alkali-resistant fiberglass mesh, ensuring it is flat and wrinkle-free, overlapping sequentially. The overlap width is typically 30-50mm (80-100mm for outer edges).

Then, apply a second coat of crack-resistant mortar, covering the mesh completely without visible mesh marks. The total thickness of the crack-resistant mortar layer is generally between 3-8mm.

4. Curing: After application, the crack-resistant mortar layer does not require watering, but should be kept in a well-ventilated, dry environment. During application and storage, the temperature should be above 5℃. Subsequent application procedures can proceed after approximately 7 days of curing.