Functional analysis and synergistic mechanism of RDP in dry-mixed mortar

In the field of modern building materials science, redispersible polymer powder (RDP), as a spray-dried product of polymer systems such as ethylene-vinyl acetate (EVA), vinyl acetate-ethylene (VAE) and ethylene-vinyl acetate-vinyl chloride terpolymer (EVA-based terpolymer), has become a core functional additive for high-performance dry-mixed mortar. This article will systematically explain the key role of RDP in cement-based materials from three dimensions: molecular interface engineering, hydration dynamics regulation and composite system strengthening.

I. Preparation process and structural characteristics of RDP

1. Industrial production process

- Preparation of basic colloid by emulsion polymerization (solid content 40-60%)

- Addition of polyvinyl alcohol (PVA) protective colloid (addition amount 3-8%)

- Control of spray drying process parameters (inlet temperature 160-200℃, outlet temperature 60-80℃)

- Finished product characteristics: particle size distribution D50=80-120μm, glass transition temperature Tg=-5℃ to +20℃ adjustable

2. Microstructure characterization

- Core-shell structure design: hydrophobic polymer core (EVA/VAE) + hydrophilic PVA shell

- Specific surface area (BET method): 1.5-3.0 m²/g

- Redispersion characteristics: tested according to EN 1348 standard, redispersion rate>95%

II. Mechanism of action of RDP in cement-based system

1. Regulation of hydration process

- Delaying effect: polymer particles adsorbed on the surface of C3A, reducing the early hydration rate (calorimetric determination, 24h hydration heat decreased by 15-25%)

- Film formation mechanism: continuous polymer film formed at the solid-liquid interface (thickness 50-200nm, confirmed by SEM observation)

2. Interface strengthening mechanism

- Chemical bonding: carboxyl (-COOH) forms coordination bonds with Ca²+ (FTIR confirmed 1590cm⁻¹ characteristic peak)

- Mechanical interlocking: polymer film penetrates into aggregate pores (depth > 10μm, measured by mercury intrusion)

- Stress transfer: elastic modulus gradient transition (polymer film 2-5GPa vs cement stone 20-30GPa)

III. Quantitative analysis of key performance improvement

| Performance indicators | Blank mortar | RDP modified mortar (addition amount 2%) | Improvement range | Test standard |

|------------------|-------------------|-------------------------|-----------|-----------------|

| Flexural strength (28d) | 6.5 MPa | 9.2 MPa | +41.5% | ASTM C348 |

| Bond strength | 0.8 MPa | 1.5 MPa | +87.5% | EN 1542 |

| Elastic modulus | 25 GPa | 18 GPa | -28% | ASTM C469 |

| Drying shrinkage | 0.35% | 0.22% | -37% | ISO 1920-8 |

| Carbonation depth (180d) | 8.2 mm | 4.7 mm | -42.7% | GB/T 50082 |

IV. Formula design principles and engineering applications

1. System adaptability selection

- Tile adhesive: Use VAE RDP with Tg=+5℃ (to improve flexibility and open time)

- Self-leveling mortar: EVA-based RDP (particle size D50 < 100μm, improved fluidity)

- External wall insulation system: ternary copolymer RDP is recommended (weathering temperature range -30℃～+80℃)

2. Compounding technology solution

- Synergistic with cellulose ether: HPMC (0.02%) + RDP (1.5%) to achieve a balance between viscosity and adhesion

- Hydrophobic system construction: RDP (2%) + calcium stearate (0.5%) to make water absorption < 5% (EN 13480)

- Early strength formula: sulphoaluminate cement (30%) + RDP (1.2%) to achieve 24h strength > 15MPa

V. Development trend of cutting-edge technology

1. Functional modification

- Silicone grafted RDP: contact angle > 110°, achieving super hydrophobic properties

- Nano-SiO2 hybrid RDP: wear resistance increased by 300% (Taber method test)

2. Green preparation technology

- Bio-based VAE: using plant-derived vinyl acetate monomer (carbon footprint reduced by 40%)

- Low-temperature spray drying process: energy consumption reduced by 35% (inlet temperature reduced to 140°C)

Conclusion

RDP, through its unique redispersion characteristics and interface regulation capabilities, enables dry-mixed mortar to achieve a leapfrog development from a rigid brittle system to a flexible composite material. It is recommended to use the life cycle assessment (LCA) method in engineering applications, combined with the durability requirements of different climate zones (such as focusing on carbonization prevention in hot and humid areas, and focusing on freeze-thaw cycles in cold areas), to optimize the type and dosage ratio of RDP. The latest research shows that the development of intelligent responsive RDP (temperature-sensitive/humidity-sensitive) will provide a new technical path for self-repairing mortar systems.