Scientific Mechanism of RDP in Enhancing Mortar Compressive Strength

Redispersible polymer powder (RDP) is a key polymer modifier in dry-mix mortar systems. While enhancing flexibility, adhesion, and water resistance, it also significantly improves compressive strength.

1. Chemical Properties and Mechanism of Action of RDP

RDP is mainly produced by spray drying of ethylene-vinyl acetate copolymer (VAE), acrylate copolymers, etc. Upon contact with water, it can be redispersed into polymer emulsion particles, uniformly distributed in the cement matrix. With cement hydration and system drying, it forms a continuous polymer film. This film intertwines with cement hydration products, forming a "flexible skeleton," thereby optimizing the mechanical properties of the mortar.

2. Microstructure Densification and Enhanced Compressive Strength

RDP significantly reduces the porosity of the system and improves pore connectivity by filling the pores and capillary channels between cement hydration products, forming a denser mortar microstructure. This densification effect results in a more uniform stress distribution, thereby improving compressive strength.

Furthermore, the polymer film formed by RDP possesses excellent flexibility, capable of absorbing and dispersing localized stress under load, inhibiting the propagation of microcracks, delaying material failure, and improving compressive strength.

3. Interfacial Bonding Strengthening Mechanism

RDP not only improves the structure through physical filling but also forms weak chemical interactions such as hydrogen bonds with hydration products (e.g., C-S-H gel), enhancing the bonding force at the polymer-inorganic interface. This mechanism facilitates effective stress transfer at the interface and prevents interfacial delamination.

Simultaneously, RDP improves the wettability of cement paste to aggregates, strengthens the structural quality of the interfacial transition zone (ITZ), and enhances the overall synergistic load-bearing capacity of the mortar under compressive stress.

4. Contribution of Polymer Film Formation to Mechanical Properties

The polymer film formed by RDP acts as a "flexible bridging" within the mortar, strengthening the bond between cement hydration products and aggregates, thereby improving the overall structural integrity. This film structure can also be inserted between C-S-H gel layers, improving its brittleness and imparting a certain degree of plastic deformation capability to the mortar, thus increasing the compressive failure threshold.

During the hardening process, the RDP membrane can alleviate internal stress caused by moisture evaporation and temperature changes, reduce the generation of microcracks, and ensure structural integrity.

5. Optimization and Balance of RDP Dosage

There is an optimal dosage range for RDP to improve compressive strength. Typically, it is 2%–4% of the cement mass. Excessive addition will increase the polymer phase ratio, reduce the system stiffness, and thus be detrimental to compressive strength development. Therefore, flexibility and strength must be balanced in the formulation design.

Summary

RDP improves the compressive strength of mortar through the following mechanisms:

Filling pores and forming a continuous polymer film, optimizing the microstructure density;

Absorbing and dispersing stress, inhibiting microcrack propagation;

Strengthening the bonding performance of the polymer-cement and cement-aggregate interfaces;

Forming a flexible bridging network, enhancing the overall structural integrity and deformation capacity.

Through the synergistic effect of the organic-inorganic phases, RDP achieves a balance of rigidity and flexibility in mortar at the microscopic level, significantly improving compressive strength and durability while ensuring workability.