Multiphysics Evaluation of Low CO2 and Low PH Cementitious Matrices Exposed to Mg-bearing Environments PDF Download

Author: Charlotte Dewitte
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Languages : en
Pages : 0

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Book Description
IRSN is in charge of assessing the safety of the design of the French deep geological repository project for radioactive waste (Cigéo). One of the major safety issues concerns the closure systems. The cementitious materials used in civil engineering structures (underground geological disposal of radioactive waste, dams, etc.) are designed to withstand various stresses and strains for several decades or even hundreds of years. However, in contact with soft or sea water, the leaching of the cement matrix is accompanied by an enrichment in magnesium leading to the formation of brucite (filling the porosity and forming a protective layer) and/or magnesium silicate hydrates (M-S-H), the properties of which are little studied. With the emergence of new binders, the mineralogical composition of hydrated concrete changes and the mechanisms of alteration in the presence of magnesium are different from those encountered in ordinary portlandite-rich cements. The formation of M-S-H is favoured over that of brucite. The objectives of this thesis are: (i) to understand the reaction mechanisms of magnesium attack and M-S-H formation in low-calcium cementitious matrices, (ii) to study the influence of M-S-H formation on microstructural and mechanical properties and (iii) to propose microstructural and mechanical data of M-S-H that can be integrated in chemo-mechanical models.Firstly, the influence of the composition of the cementitious paste on the phases formed during magnesium attack was studied. The presence of portlandite implies the formation of brucite. Incomplete hydration of low C/S binders, even in the absence of portlandite, can also lead to the formation of brucite. M-S-H is formed within the paste and corresponds to a deep magnesium enrichment. Secondly, a characterisation of M-S-H was carried out on M-S-H pastes in order to acquire data on the intrinsic physical and mechanical properties of M-S-H and thus to be able, via small-scale chemo-mechanical modelling, to understand the mechanisms of damaged properties under these attacks. Finally, a multi-physical study, at the mesoscopic scale, of the damaged properties of low-calcium matrices by waters containing magnesium was carried out. Two low C/S cementitious pastes (model paste based on colloidal silica and real paste based on silica fume and slag - T3) were exposed to solutions with different Mg concentrations (5 and 50 mM). The chemical and mineralogical characterisation showed that a deep decalcification takes place in parallel with a strong magnesium enrichment of the paste, corresponding to the formation of M-S-H. Micro-structural and mechanical characterisations (by indentation), carried out to observe the evolution along this degradation front, showed an increase in porosity and a strong loss of local elastic properties despite the formation of M-S-H. Mechanical homogenisation, based on previously determined M-S-H paste data, confirms that degraded M-S-H have low elastic properties. Increasing the Mg concentration does not change the mineralogy and chemistry of the degraded zone but only the degraded depth. The results on the real paste (T3) are similar to those on the model paste, validating the results on the latter. In addition, the model paste was also studied in pure leaching to compare the impact of leaching with that of magnesium attack on the low C/S binders. A study using a reactive transport code was carried out to better understand the differences in kinetics and phenomenology. It confirms that the presence of magnesium accelerates the degradation.