The Role of Defoamers in Cement-Based Materials
Release Time:
2022-02-08
Source:
A large number of poorly formed air bubbles are the most significant factor reducing the workability, mechanical properties, and durability of cement-based materials, primarily manifesting in the following three aspects.
(1) When cement slurry foams excessively, bubbles accumulate at the pump outlet, forming an air cushion that causes the pump to run dry and halts construction.
(2) A large number of air bubbles increase the volume of the cement paste; however, these bubbles are prone to rupture, leading to volumetric shrinkage and deformation.
(3) A large number of air bubbles can reduce the density of the cement paste, thereby degrading the mechanical properties and durability of the hardened cement paste.
Air bubbles typically exist in cement paste in two forms. In the first form, when the buoyant force acting on the bubbles exceeds the resistance offered by the cement paste to the escape of the gaseous phase, the bubbles rise and coalesce at the liquid–gas interface, forming a continuous network of visible bubbles. Such bubbles readily give rise to honeycombing and a rough, pitted surface, resulting in defects in the apparent quality of cement-based materials. In the second form, when the cement paste exerts sufficient resistance to the escape of the gaseous phase, the bubbles are dispersed discontinuously throughout the paste. These bubbles can significantly reduce the density of cement-based materials and adversely affect their mechanical properties and durability.
Air-entraining agents are admixtures that help reduce entrapped or generated macroscopic air bubbles in concrete and mortar during mixing and placement. These agents consist of substances that lower the surface tension of the liquid phase and function by inhibiting bubble formation and rupturing the bubble film. Air-entraining agents represent the most direct and effective means of eliminating harmful air voids in cement-based materials. By using air-entraining agents to eliminate detrimental macroscopic air bubbles, the pore structure can be optimized, resulting in hardened cement-based materials that exhibit a smooth surface, absence of honeycombing and roughness, good homogeneity, and superior mechanical properties and durability.
