Impact of air entrainment on the microstructure and mechanical performance of high performance mortar

摘要

At the Magnel Laboratory for Concrete Research an intensive vacuum mixer which can regulate the air pressure is available. As such the amount of entrapped air in cementitious materials can be varied. The effect of the reduced air content due to vacuum mixing on the rheology and workability was already investigated in previous work. Furthermore, the previous work investigated the influence of entrained air on the rheological properties. The impact of vacuum mixing on the compressive strength and the microstructure of (ultra)high performance mortar is documented elsewhere. However, the impact of air entrainment on high performance mortar has not yet been published. Therefore, this paper will focus on the evolution of the pore structure of air-entrained high performance mortar by using mercury intrusion porosimetry, fluorescence microscopy and air void analysis. This data will enable to verify the pore diameters, often used to explain the evolution of the rheology by the ratio of shear stresses and the surface tension. Furthermore it explains the evolution of the density, the compressive strength and the bending tensile strength. The air entrainment was varied between 0 % abd 2.5 % wt.cement. As a consequence the air content was systematically increased. In case of the air void analyser, the amount of air cavities was increased from 1 % to 14 %. From the cumulative air void fraction it was noticed that pores with a diameter of 80 µm were dominant in the mortar. From data of the mercury intrusion porosimetry the amount of capillary pores was increased from 7.4 % and 22.2 %. The critical diameter at lower percentage of air entrainment was 40 nm, a more continuous curve was obtained for the highest percentages. Furthermore, the amount of pores situated between 10 µm and 100 µm were limited or not existing. In conclusion, this paper highlights the underestimation of the lareger air pores by mercury intrusion porosimetry. Besides this, the decrease in compressive strength and bending tensile strength can be explained by the changes in the pore structure. Finally, it was checked whether the increase in plastic viscosity due to air entrainment was caused by the air bubbles or by the polymer itself.