Why is redispersible latex powder added to dry-mix mortar?
Release Time:
2015-08-24
Source:
Why is redispersible latex powder added to dry-mix mortar?
Dry-mix mortar is composed of Redispersible Latex Powder It is prepared by physically blending inorganic binders—such as cement, hydrated lime, gypsum, and clay—with various aggregates, fillers, and other additives, including hydroxypropyl methylcellulose, polysaccharide ethers (starch ethers), and cellulose fibers. When dry-mix mortar is mixed with water, the hydrophilic protective colloids and mechanical shear forces facilitate the rapid dispersion of latex powder particles into the water, ensuring that the redispersible latex powder can form a complete film. The specific composition of the latex powder influences the mortar’s rheological properties and various application performance characteristics, such as its affinity for water upon redispersion, the resulting viscosity after dispersion, its impact on air content and bubble distribution in the mortar, and its interactions with other admixtures. These factors enable different types of latex powder to impart distinct effects, including enhanced flowability, increased thixotropy, and elevated viscosity.
It is generally believed that redispersible latex powder improves the workability of fresh mortar: when dispersed, the latex powder—particularly the protective colloid—exhibits strong affinity for water, increasing the slurry’s viscosity and enhancing the internal cohesion of the placing mortar. After the fresh mortar containing a latex-powder dispersion has been placed and shaped, as the substrate absorbs moisture, hydration reactions consume water, and evaporation into the air proceeds, the moisture content gradually decreases, the resin particles draw closer to one another, the interfacial boundaries become increasingly blurred, and the resins progressively coalesce to form a continuous film. The polymer film-forming process occurs in three stages. In the first stage, polymer particles in the initial emulsion move freely via Brownian motion. As water evaporates, particle mobility is progressively restricted, and the interfacial tension between water and air causes the particles to align and aggregate. In the second stage, once the particles begin to come into contact, network-like moisture is removed by capillary evaporation; the high capillary tension acting on the particle surfaces deforms the latex spheres, promoting their fusion, while the remaining moisture fills the pores, resulting in the approximate formation of a film. In the third and final stage, polymer molecular diffusion (sometimes referred to as self-adhesion) leads to the development of a truly continuous film. During film formation, isolated, mobile latex particles consolidate into a new thin-film phase that exhibits high tensile strength. Clearly, for redispersible latex powder to be able to form a film within hardened mortar, the minimum film-forming temperature (MFT) must be lower than the mortar’s curing temperature.
With the final formation of the polymer film, a composite system is established within the hardened mortar, consisting of an inorganic–organic binder network: a brittle, rigid skeleton formed by hydraulic materials, and a flexible network created by redispersible latex powder that forms films in the voids and on solid surfaces. The polymer resin film generated by the latex powder enhances tensile strength and increases cohesive forces. Owing to the polymer’s flexibility, the material exhibits far greater deformability than the rigid structure of cement stone, thereby improving the mortar’s deformation performance and significantly enhancing its stress-dissipating capacity, which in turn boosts the mortar’s crack-resistance. As the dosage of redispersible latex powder increases, the overall system shifts toward more plastic behavior. At high latex-powder contents, the polymer phase in the cured mortar gradually surpasses the inorganic hydration-product phase, leading to a fundamental transformation: the mortar becomes an elastomer, while the cement hydration products assume the role of a “filler.” Modification with redispersible latex powder improves the mortar’s tensile strength, elasticity, flexibility, and impermeability. Incorporation of redispersible latex powder facilitates the formation of a polymer (latex) film that becomes part of the pore walls, thereby sealing the mortar’s highly porous structure. The latex film possesses a self-stretching mechanism that can exert tensile forces at its bond interface with the mortar matrix. Through these internal forces, the mortar is maintained as a coherent whole, thus increasing its cohesive strength. The presence of highly flexible and elastic polymers further enhances the mortar’s flexibility and elasticity. The mechanisms underlying the increases in yield stress and ultimate strength are as follows: when external loads are applied, improved flexibility and elasticity delay the initiation of microcracks, requiring higher stresses before such cracks develop; moreover, the interwoven polymer regions impede the coalescence of microcracks into through-cracks. Consequently, redispersible latex powder elevates both the material’s ultimate stress and ultimate strain.
The polymer film in polymer-modified mortar plays a critically important role in the hardened mortar. The redispersible latex powder dispersed at the interface not only forms a film but also performs another key function: it enhances adhesion to the materials it contacts. In the microstructure of the interfacial zone between polymer-modified tile adhesive mortar and ceramic tiles, the polymer film creates bridging bonds between the vitrified ceramic tiles—characterized by extremely low water absorption—and the cement-based mortar matrix. The contact zone between these two dissimilar materials is a particularly high-risk area where shrinkage cracks tend to form, leading to a loss of bond strength. Therefore, the ability of the latex film to heal such shrinkage cracks is of great significance for tile adhesives.
