Mechano-Chemical Interactions at Cement-Geomaterial Interfaces in Repository and Borehole Scenarios

Joseph Mohagheghi, Thomas Dewers, Ed Matteo, Jason Heath, Carlos F. Jové-Colón

Coupled chemical and mechanical interactions can lead to leakage pathway development at cement-geomaterial interfaces, negatively impacting integrity of engineered cement barriers. We examine interactions, potential coupling, and pathways to failure in an observational, experimental and modeling study involving cement-clay, cement-salt, and cement-granite interfaces. A sample shotcrete-bentonite interface from the FEBEX heater test at the Grimsel Test Site in Switzerland, and a salt-cement interface from a mined-wellbore within a potash mine in southern New Mexico, are analyzed with micro-CT, multi-beam scanning electron microscopy, and electron X-ray dispersive spectroscopy at cm-to-nm length scales. We examine changes in alteration as manifested by pore structural changes and nanoindentation properties as a function of distance from the interface (bulk and shear modulus). A parallel effort examines interface evolution in controlled triaxial benchtop experiments which monitor cement-granite interface degradation in situ by acoustic and chemical methods, and post-experiment using nanoindentation. Initial results display evolving acoustic properties with time suggesting a mechanism for detecting interface alteration. Experimental results are being used in validating a coupled reactive-transport-mechanics model combining the ALBANY mechanics finite element code, the KAYENTA elasto-plastic constitutive model, and the PFLOTRAN reactive transport finite difference code. Future plans are to use the validated coupled chemo-mechanical model specifically in analyzing the spatio-temporal evolution of the shotcrete-bentonite and cement-salt samples, and in general as a predictive tool for assessment of cement barrier integrity.

Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia LLC, a wholly owned subsidiary of Honeywell International Inc. for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. SAND2018-8386 A