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Drying Shrinkage and Deformation Mechanisms in NaOH-Activated Slag: Investigating Pore Structure, Microcracks Formation, and Autogenous Effects

par Sirotti, Marco
Président du jury Gerard, Pierre
Promoteur Staquet, Stéphanie
Publication Non publié, 2023-11-16

Thèse de doctorat

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Résumé :

The necessity to house the worldwide growing population is making the production of Portland cement (PC) grow alongside with its CO2 emissions in the atmosphere. In order to reduce said emissions, the scientific community is looking for viable and greener alternatives with comparable mechanical properties and good durability. Alkali-activated materials seem to represent a possible replacement to PC, even though their long-term durability is still not fully understood.One of the main issues for the durability of construction materials is the volume stability in drying conditions, that is how the volume of the material decreases when exposed to environmental conditions. The main consequence of the volume contraction is the development of microcracks which reduce the mechanical strength, allow for water vapour and CO2 to penetrate further inside the material, and may also lead to failure of the structure. In order to assess the long-term durability of a material is therefore crucial to understand how it shrinks according to the different external conditions.The present study focuses on the activation of Blast Furnace Slag (BFS) using different sodium hydroxide solutions and takes into account three main aspects of durability: the tendency to cracking in restrained conditions, the pore structure and hydraulic conductivity of the material, and the drying shrinkage in free conditions. The first experimental campaign on restrained shrinkage has been conducted using the ring test coupled with the assessment of the mechanical properties both in sealed and drying conditions; the goal is to understand when the formation of microcracks takes place and what is the maximum deformation the material can endure before failure. The second experimental campaign modelled the total porosity, the pore size distribution, and the hydraulic conductivity of the material based on the mass variation of the material exposed to different relative humidity levels. Finally, the third experimental campaign investigated the drying shrinkage at three different relative humidity (RH) levels, compared to the volume change in sealed conditions. Furthermore, it also considered the creep deformation according to the drying history of the samples, the effect of drying on flexural strength and E-modulus as an indicator of the presence of microcracks, and the carbonation depth in natural conditions.The results of the different test campaigns highlight the difference in behaviour between alkali-activated slag and PC. Specifically, increasing the amount of activating solution increases the porosity and worsens the mechanical properties, as it happens when increasing the amount of water in PC, but it refines the pore structure and reduces the strain and the cracking tendency in restrained conditions. Moreover, it increases the drying shrinkage and reduces the CO2 penetration inside the material. On the other hand, increasing the molarity of the activating solution makes the material resist better to cracking in restrained conditions, improves the mechanical properties and the resistance to carbonation, and refines the pore structure while reducing the total porosity. Finally, increasing the molarity does not lead to a clear increase or decrease in drying shrinkage as it strongly depends on the composition and the drying conditions.