Expansion and stresses induced by crystallization in cement-based materials in presence of sulfates

Abstract : In-pore crystallization can lead to expansion of a variety of porous media, including concrete, stone, or soils. For instance, sulfate attacks of cement-based materials can lead to crystallizations of gypsum or ettringite, which may cause damage and limit the durability of concrete structures. A better understanding of how crystallization induces deformation of cementitious materials is a prerequisite to designing efficient ways of mitigating the detrimental effects of salt crystallization. In this thesis, we aim at understanding how crystallization leads to expansion, for cement-based materials in the specific case of the presence of sulfate ions, which is relevant for sulfate attacks. The main originality of the study was to perform experiments with granular materials compacted into oedometric or isochoric cells. The tested samples were manufactured by grinding C3S pastes, regular Portland cement pastes, or mixtures of phases of which those pastes are made (e.g., monosulfoaluminate AFm), and then compacting them within the cell into 2-cm-high cylindrical specimens. In the cells, the highly permeable compacted samples could be flushed with sodium sulfate solutions in less than 1 hour. In an oedometric cell, the sample is prevented from expanding radially, but is allowed to expand axially: we measured how injections of solutions induced an axial expansion. In an isochoric cell, the sample is prevented from expanding both radially and axially: we measured how injections of solutions induced the development of axial and radial stresses. A salient feature of the isochoric cells we developed is that all solution flushed throughout the sample could be recovered: thus, from the measurements of concentrations and volumes of input and output solutions, the amount of sulfate remaining in the sample over the experiments could be determined. In parallel to the deformation/stress measurements, we also performed the mineralogical and microstructural characterizations of the samples before and after testing by using a variety of techniques, including X-ray fluorescence, thermogravimetric analysis, X-ray diffraction, aluminum nuclear magnetic resonance and scanning electron microscopy with X-ray analysis. The evolutions of the output concentrations and of the mineralogy over the injection process could be well predicted with the geochemical modeling software CHESS. Experimental results of the campaign, in conjunction with results from mineralogical and microstructural characterizations, made it possible to reveal what the main parameters are that govern expansion. Thanks to the original protocol we developed, expansion or development of stresses started immediately after the injection of solution, stabilized after a few days to a few dozen days, and crystallization occurred homogeneously throughout the height of the sample. One interesting conclusion is that, even when ettringite crystallizes in macropores, i.e., outside of the C-S-H gel porosity, ettringite can lead to an expansion. Also, we showed that gypsum crystallization contributes to expansion. In isochoric testing, we showed that both crystallization of ettringite and of gypsum can induce stresses, and that the magnitude of those stresses is linearly related to the volume of those crystals formed. The conclusions drawn from this experimental study make it possible to better understand the physical processes through which crystallization induces expansion or stresses in porous solids, and thus to orient the modeling of sulfate attacks in cement-based materials
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Nam Nghia Bui. Expansion and stresses induced by crystallization in cement-based materials in presence of sulfates. Materials. Université Paris-Est, 2016. English. ⟨NNT : 2016PESC1096⟩. ⟨tel-01459137⟩

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