Mineralogical Evolution and Expansion of Cement Pastes in a Sulfate-Confined Environment

Abstract

The geologic disposal of radioactive waste could lead to confined conditions in which cementitious materials impose a moderate alkaline pH, in which the rocks supply sulfate ions at rather low concentrations. In this context, the purpose of this work was to study the degradation of cement pastes under such conditions using a non-renewed 30 mmol/L Na2SO4 solution without pH regulation. Samples were investigated through laboratory testing and numerical modeling. XRD, SEM–EDS, and micro-indentation acquisitions were performed to follow the evolution of chemical, mineralogical, and mechanical properties during the weak external sulfate attack. Reactive transport modeling was performed with the HYTEC code. Based on these results, the Young’s moduli of the degraded zones were estimated using analytical homogenization. Decalcification occurred despite the high pH value of the solution and significantly affected the mechanical properties. Macroscopic swelling and cracking were caused by the formation of expansive sulfate minerals after 60 days, despite the low sulfate content of the tank solution. The modeling supported the discussion on the evolution of the mineral fronts (ettringite, portlandite, and gypsum).

A coupled hydro-chemo-mechanical approach to model the degradation and appearance of cracks in cementitious materials subject to carbonation

“…HYTEC models the evolution of the mineralogy, allowing to evaluate the evolution of the poroelastic properties of the material. These are computed through the use of analytical homogenization methods 5,8,[80][81][82] . For this, the cement system is represented as an evoloving C-S-H phase with several spherical inclusions representing portlandite, capillary porosity and calcite.…”

“…This volume fraction includes the different Ca/Si ratio as well as the C-S-H intrinsic porosity 82 . The homogenized chemical free strain ε ξ can then be computed using an expansion coefficient α ξ describing the macroscopic effect of the C-S-H phase shrinkage occurring locally in the form:…”

Experimental Study on the Physical and Mechanical Properties of Cemented Gangue Backfill under Acid Mine Water Erosion

“…However, as SO 4 2− ions diffuse/or absorb in the interconnected pores reacting with existing hydration products, the equilibrium state of the pore solution with the minerals in the CGB is disrupted, and monocarboaluminate reacts with sulfate to form ettringite in the presence of calcium hydroxide and water [30][31][32]. The formation of ettringite leads to a local decrease in porosity [33], which results in a denser internal structure of the CGB and a corresponding increase in mass, leading to a short-term increase in the strength of the CGB. This explains the decrease in the acoustic emission rate in the early part of the immersion.…”

A fully coupled Hydraulic Mechanical Chemical approach applied to cementitious material damage due to carbonation

“…Hytec models the evolution of the mineralogy, allowing to evaluate both the evolution of the poro-elastic properties of the material and the carbonation shrinkage Equation (2). The poroelastic properties are computed through the use of the Mori-Tanaka scheme 5,8,[83][84][85] . For this, the cement system is represented as an evolving matrix phase (the C-S-H phase with its gel porosity) with several spherical inclusions representing portlandite, capillary porosity and calcite.…”

“…As mentioned above, the cement paste poroelastic properties required in Equation (1) are estimated using a micromechanics method, where the matrix is modelled through an equivalent C-S-H phase (comprising C-S-H solid phase and gel pores) 13,85 . In order to capture the decalcification effect of the C-S-H resulting from chemical degradation on the matrix properties, the evolution of the Young modulus of the binding C-S-H phase with the Ca/Si ratio follows the relation 7 :…”