How Does Redispersible Polymer Powder Effect the Flexibility of Mortar?

The admixture has good effects on improving the performance of construction dry-mixed mortar. Redispersible polymer powder is made of a special polymer emulsion after spray drying. The dried polymer powder is some spherical particles of 80~100mm gathered together. These particles are soluble in water and form a stable dispersion slightly larger than the original emulsion particles, which form a film after dehydration and drying.

Different modification measures make the redispersible latex powder have different properties such as water resistance, alkali resistance, weather resistance and flexibility. Redispersible polymer powder used in mortar can improve the impact resistance, durability, wear resistance, ease of construction, bonding strength and cohesion, weather resistance, freeze-thaw resistance, water repellency, bending strength and flexural strength of the mortar. As long as the cement-based material with latex powder is in contact with water, the hydration reaction will begin, and the calcium hydroxide solution will soon become saturated and crystals will be precipitated. At the same time, ettringite crystals and calcium silicate hydrate gels will be formed. The solid particles are deposited on the gel and unhydrated cement particles. As the hydration reaction progresses, the hydration products increase, and the polymer particles gradually gather in the capillary pores, forming a densely packed layer on the surface of the gel and on the unhydrated cement particles. The aggregated polymer particles gradually fill the pores.

Redispersible latex powder can improve the bending strength and adhesion strength of mortar because it can form a polymer film on the surface of mortar particles. There are pores on the surface of the film, and the surface of the pores is filled with mortar, which reduces the stress concentration. And under the action of external force, it will produce relaxation without breaking. In addition, the mortar forms a rigid skeleton after the cement is hydrated, and the polymer in the skeleton has the function of a movable joint, which is similar to the tissue of the human body. The membrane formed by the polymer can be compared to joints and ligaments, so as to ensure the elasticity and flexibility of the rigid skeleton.

In the polymer-modified cement mortar system, the continuous and complete polymer film is interwoven with cement paste and sand particles, making the whole mortar finer and denser, and at the same time making the whole an elastic network by filling capillaries and cavities. Therefore, the polymer film can effectively transmit pressure and elastic tension. The polymer film can bridge the shrinkage cracks at the polymer-mortar interface, heal the shrinkage cracks, and improve the sealing and cohesive strength of the mortar. The presence of highly flexible and highly elastic polymer domains improves the flexibility and elasticity of the mortar, providing cohesion and dynamic behavior to the rigid skeleton. When an external force is applied, the microcrack propagation process is delayed due to the improved flexibility and elasticity until higher stresses are reached. The interwoven polymer domains also act as a barrier to the coalescence of microcracks into penetrating cracks. Therefore, the redispersible polymer powder increases the failure stress and failure strain of the material.

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