Study On Air Voids Formation Mechanism And Influencing Factors Of Air-entrained Concrete

Air entraining agent(AEA)is widely used in concrete,which can improve the workability in the early stage and improve the frost resistance in the late stage by introducing fine and evenly distributed bubbles into concrete.The reduction of surface tension due to AEA is usually considered as the basis of the air-entraining behavior.However,it is not satisfactory to analyze the air-entraining mechanism and evaluate the air-entraining ability by surface tension alone.Because the solid-liquid-gas three-phase structure of air-entraining mixes is complex and can be influenced by many factors,the mechanism of the air-entraining process has not been well revealed.Therefore,it is of great theoretical significance and application value to study the formation and development process of the air void structure(AVS),to analyze the interaction between air bubbles and solid particles such as cement,and to investigate the influence of materials such as AEA,fly ash and inorganic salt on air-entraining behavior.Air Void Analyzer(AVA)and optical microscopy were used to investigate the entrainment of bubbles during mixing and the development of AVS during resting.The air-entraining process of different types of AEA was analyzed by the population balance model(PBM)which can be characterized by the entrainment coefficient and the disentrainment coefficient.By comparing the evolution of the AVS of mortars mixed by ordinary tap water and de-aired water during the standing process,it was found that the air content first decreased and then increased.This process is affected by air dissolution and diffusion.By comparing the AVS of hardened mortar entrained by anionic,cationic and nonionic surfactants it can be found that,under the combined effect of air void entrainment and development,the nonionic surfactant introduces more tiny air voids,the anionic surfactant is second and the cationic surfactant introduces the coarsest air voids.The AVS are mainly influenced by surface tension,rheological properties and the interaction between bubbles and solid particles such as cement.The interaction between bubbles and solid particles can be evaluated by the foam index test,the effective zeta potential alteration and the bubble motion behavior in the cement paste.Foam index test is a simple and fast method to evaluate the dosage of AEA.In this study,the stability of the agitation process was improved by introducing a horizontal shaker,and the influence of manual operation was reduced.By studying the influence mechanism of water-cement ratio on foam index test,it is found that prolonged shaking duration can eliminate the influence of frequency or solid content which is unrelated to the air-entraining process.The optimal shaking duration is 120s.Using the optimized foam index test method,the test conditions were closer to the actual situation,and the test results were more stable and reliable.The model of the charged mosaic surface of the cement particle was established by the ATR-FTIR test of surfactant solution-cement system and the charged properties of the main hydration products C-S-H and AFt in the simulated pore solution.The analysis of the special features of IR spectra at3000-2800 cm^-1 revealed the mechanism of interaction between surfactants with different electrical properties and bubbles formed by them and cement particles.The air-entraining behavior of anionic,cationic and non-ionic surfactants after partially replacement of cement by fly ash was compared.It is found that the main effect of fly ash was the adsorption of surfactants,which increased the amount of surfactants.However,there were still differences in the AVS when the air content was the same.The interaction between cement or fly ash and surfactant/bubbles was evaluated by the zeta potential alteration.It is found that the interaction between anionic and cationic surfactants and solid particles is strong,and the amount of large air voids and small air voids was increased when the zeta potential alteration was greater.The overall interaction between the nonionic surfactant and solid particles was weak,only small air voids were affected by it,and the influence of rheological properties was more pronounced for large air voids.Fly ash increased the viscosity and thus the amount of large air voids.The effect of inorganic salt on the AVS of fresh mortar was compared.Na₂SO₄ was found to increase the air content,Na Cl slightly increased the air content,and Ca Cl₂ decreased the air content.For the same inorganic salt,an increase in salt concentration could increase the amount of large bubbles and decrease the amount of small bubbles in fresh mortar.Based on the bubble rising drag formula in the liquid phase,the calculation formula for the interaction coefficient of bubbles and cement particles in cement paste mixed with surfactants was proposed.And the effect of inorganic salts such as Na₂SO₄,Na Cl and Ca Cl₂ on the interaction coefficient was calculated.It was found that with the increase of interaction coefficient,the interaction between bubbles and cement particles increased,and the air content of 0-200?m and bubble specific surface area in fresh mortar decreased linearly,while the air content of>1000?m and the spacing factor increased linearly.