Impact of Surfactant and Calcium Sulfate Type on Air-Entraining Effectiveness in Concrete

Abstract

The paper presents the evaluation of the influence of calcium sulfate on the air void microstructure in concrete and its action mechanism depending on the character of the air-entraining agent. Gypsum dehydration has been previously proven to negatively influence the air void structure of air-entrained concrete. Ettringite, nucleating from tricalcium aluminate and calcium sulfate, influences the adsorption and mode of action of anionic-based polycarboxylate ether admixtures. The authors suspected the admixture’s air-entraining mechanism was also affected by these characteristics. Gypsum dehydration was confirmed to influence the air void structure. In the case of the anionic surfactant, the content of air bubbles smaller than 300 µm was lower compared to cement with gypsum and hemihydrate. On the other hand, the content of air voids with a diameter up to 60 µm, which are the most favorable, was higher. The results obtained led to the conclusion that the mechanism of air entrainment was twofold, and in most cases occurred through the lowering of surface tension and/or through the adsorption of surfactant on cement grains. The adsorptive mechanism was proved to be more effective in terms of the total air content and the structure of the air void system. The results and conclusions of the study provide guidelines to determine the proper surfactant type to reduce the risk of improper air entrainment of concrete, and emphasize the importance of gypsum dehydration of cement in the process of air entrainment.

Computational Simulations of Adsorption Behavior of Anionic Surfactants at the Portlandite–Water Interface under Sulfate and Calcium Ions

“…The synthetic betaine surfactant, although capable of air entrainment, is incompatible with cement solutions because it precipitates when interacting with calcium or sulfate ions . Other researchers , studied the influence of gypsum on the properties of the air void. Due to gypsum dehydration, the amount of Ca 2+ and SO 4 2– releases to pore solution and the performance of air entraining agents are influenced.…”

“…Therefore, gypsum dehydration should be taken into consideration and must be scrupulously avoided. In another reference, 40 Sypek proved the adsorptive mechanism of the anionic surfactant on the ettringite surface and that the released sulfate and calcium ions by gypsum dehydration alter the hydration of C3A and the formation of ettringite, rather than affecting the surfactant's mode of action directly. However, in the cement system, just considering C3A and its hydration products, ettringite, is not comprehensive.…”

Preparation and Properties of Different Polyether-Type Defoamers for Concrete

“…However, there are still many problems in the practical engineering applications of concrete, such as resistance to chloride diffusion, slump loss, early-age plastic shrinkage and cracking, expansion of the U-type expansive agent, and hydration heat regulation of mass concrete. These problems are mainly because of air bubbles with large sizes and in excessive amounts, which are harmful and formed in the process of fresh concrete mixing [ 10 , 11 , 12 ]. The reduction of these harmful bubbles is thus highly desirable to improve the performance of concrete [ 13 , 14 , 15 , 16 ].…”