Jeroen Dils

Holistic approach of pre-existing flaws on the decay of two limestones

This study aims to understand the influence of the microfacies and the determination of pre-existing flaws on the weathering behavior of two types of limestones. Therefore, both Lede and Noyant limestones were independently weathered by strong acid tests and freeze-thaw cycles. In order to characterize the weathering patterns inside the stones, a combination of high resolution X-ray CT, SEM-EDS and thin section microscopy was used. The advantage of high resolution X-ray CT is its non-destructive character and the obtained 3D structural information. By using this technique, a time-lapse sequence of the weathering patterns was obtained for both gypsum crust formation as well as crack formation due to freezing and thawing. This way, a clear link could be made with the initial non-weathered state. Thin section microscopy and SEM-EDS provided additional chemical information. The focus of this study lies in the processes that occur in the bioclast fragments in the stone and the influence of the surrounding cement or matrix. The results show that weathering patterns vary for both limestones although the causes of weathering were similar. In case of the Noyant stone, the weathering by crystallizing gypsum was mainly restricted to the microporous matrix of the stone, while in case of the Lede stone, several foraminifera and shell fragments were preferentially recrystallized. In general, the underlying microstructure determines the weathering pattern of the stone.

Impact of Air Entrainment on the Microstructure and Mechanical Performance of High Performance Mortar

High performance self-compacting mortar has the ability to push out air bubbles under its own weight. Consequently, the resistance against freeze-thaw cycles with or without deicing salts can decrease due to the total air content reduction. In order to assure the necessary expansion zones1,2 air entraining agents (AEA) are commonly used to increase the amount of stable air bubbles. Depending on the mixture, the workability and rheology decrease or increase by the entrained air bubbles3. This will depend on the ratio between the surface tension and the shear stress applied during the test. If the latter can overcome the first, the bubbles will deform and increase the fluidity of the mixture. Besides the influence on the durability and the fresh concrete, air entraining agents also alter the pore structure and the mechanical performance of the mortar. The effect of AEA on these properties is the subject of this paper. The pore structure is examined on two different levels. On the one hand, mercury intrusion porosimetry is used to investigate the capillary porosity, ranging from 10 nm to 10 μm. On the other hand air void analysis and fluorescence microscopy is performed to evaluate the larger air bubbles ranging from 0.1 mm to 1 mm4. Both techniques showed an overlap in their measuring range. Consequently it was possible to compare the techniques critically. Similar as in literature, mercury intrusion porosimetry underestimates the amount of larger air bubbles in mortar, due to its measuring principle5. Furthermore, the bubbles with a diameter of 80 μm increase significantly by the addition of AEA. This confirms the average air bubble size often used in literature to explain the influence of AEA on the workability and rheology3. The influence of air entraining agent on the mechanical performance was tested by the compressive and bending tensile strength. In conclusion, a good balance is necessary between the air content necessary for a proper freeze-thaw resistance without changing the mechanical performance drastically.