Franz J. Ulm

Mechanical properties of calcium-leached cement pastes: Triaxial stress states and the influence of the pore pressures

Application of concrete in nuclear waste containments requires knowledge of its mechanical behavior when subjected to calcium leaching. In order to address real-life situations, multiaxial stress states of leached material must be considered. This paper reports results from a series of triaxial tests of calcium-leached cement paste obtained from accelerated leaching tests that operate on an acceleration rate of 300, compared with natural calcium leaching. Along with the global strength loss due to chemical decohesion, an important loss of frictional performance is reported. Environmental scanning electron microscope (ESEM) pictures of both leached and unleached material are presented, and they indicate that this loss of frictional performance can be associated with a highly eroded microstructure perforated by the leaching process. In addition, the frictional behavior of leached cement pastes is found to be strongly dependent on the drainage conditions of the material and thus, on the interstitial pore pressure. Through a poromechanical analysis, it is shown that this high pore pressure sensitivity of leached cement paste can be attributed to the low skeleton-to-fluid bulk modulus ratio, Ks/Kf, of the degraded material, which, together with the increase in porosity, leads to the high compressibility of calcium-leached materials. This low Ks/Kf ratio is the consequence of an intrinsic chemical damage of the solid skeleton, which occurs during calcium leaching.

Fracture Mechanisms in Organic-Rich Shales: Role of Kerogen

In this work we study role of kerogen in the fracture properties of organic-rich shales and, in particular, in the ductility of shales. The presence of kerogen and clays in shale is known to increase the ductility. We propose here a multiscale approach to develop a fine understanding of shale ductility from the molecular scale. We develop and validate a methodology at the molecular scale that can capture the toughness and ductility of a material. We apply this methodology successfully to a silica polymorph and to a kerogen analog, and we confirm the significant ductility of kerogen. Interestingly the silica-kerogen interface exhibits a similar ductility, which is central for the properties of the heterogeneous shale. Finally, we consider a tentative upscaling considering the pull out phenomenon as a likely mechanism of fracture of the shale.

Cement As a Waste Form for Nuclear Fission Products: The Case of (90)Sr and Its Daughters.

One of the main challenges faced by the nuclear industry is the long-term confinement of nuclear waste. Because it is inexpensive and easy to manufacture, cement is the material of choice to store large volumes of radioactive materials, in particular the low-level medium-lived fission products. It is therefore of utmost importance to assess the chemical and structural stability of cement containing radioactive species. Here, we use ab initio calculations based on density functional theory (DFT) to study the effects of (90)Sr insertion and decay in C-S-H (calcium-silicate-hydrate) in order to test the ability of cement to trap and hold this radioactive fission product and to investigate the consequences of its β-decay on the cement paste structure. We show that (90)Sr is stable when it substitutes the Ca(2+) ions in C-S-H, and so is its daughter nucleus (90)Y after β-decay. Interestingly, (90)Zr, daughter of (90)Y and final product in the decay sequence, is found to be unstable compared to the bulk phase of the element at zero K but stable when compared to the solvated ion in water. Therefore, cement appears as a suitable waste form for (90)Sr storage.