Application of Defoamers in Mortar
Release Time:
2021-08-11
Source:
Defoamer
Application of Defoamers in Mortar
With the rapid development of new building materials, special mortars have found widespread application in the construction industry, and an increasing number of architectural chemical admixtures are being incorporated into mortar products. In cement-based or gypsum-based formulations, to meet certain specialized requirements in construction, existing mortar products often require modification; for example, redispersible polymer powders and cellulose ethers are commonly added as chemical admixtures. During the water-mixing process, mechanical agitation introduces air into the fresh mortar, where the air becomes entrapped by the wet mortar to form bubbles. Redispersible polymer powders and cellulose ethers, which contain hydrophilic functional groups, can reduce the surface tension of the liquid and thereby promote foam aggregation. This results in the instantaneous formation of an adsorption film with hydrophobic groups oriented toward the bubble and hydrophilic groups facing the liquid phase. Under normal conditions, when the solution viscosity is relatively low, the formed bubbles rise due to buoyancy, coalesce at the liquid–air interface, and eventually escape into the ambient air. As the liquid film at the interface thins under the influence of gravity, a pressure gradient develops, causing the film thickness to decrease over time and ultimately leading to bubble rupture. However, in fresh mortars with high viscosity, on the one hand, the increased viscosity slows down the average rate of liquid drainage from the liquid film, making it less likely for the film to thin; on the other hand, the higher viscosity also reduces the diffusion rate of surfactant molecules, which is beneficial for foam stability. Consequently, a large number of bubbles introduced into the mortar remain trapped within the matrix. Uniform and stable microbubbles can enhance mortar performance—for instance, improving workability and enhancing freeze–thaw resistance and durability. Theoretically, an increase in bubble content can adversely affect the mortar’s physical properties by raising its porosity and reducing its compressive strength. Particularly for waterproof mortar applications, suppressing and eliminating foam has become essential.
Defoamers in mortar inhibit foam formation and destabilize existing foam. Their mechanism of action is as follows: defoamers penetrate the liquid film, reducing its viscosity and creating a new interface with low surface tension, which diminishes the film’s elasticity, accelerates liquid drainage, and ultimately leads to thinning and rupture of the film. Currently, the defoamers used in mortar products are formulated as complexes consisting of a primary active ingredient combined with one or more additives. The chemical composition of defoamers varies among suppliers, and their compatibility with different types of mortar systems also differs. The following provides a brief overview of the effects of defoamers on mortar performance.
