Reactive Molecular Dynamics Simulations to Understand Mechanical Response of Thaumasite under Temperature and Strain Rate Effects

Understanding the structural, thermal, and mechanical properties of thaumasite is of great interest to the cement industry, mainly because it is the phase responsible for the aging and deterioration of civil infrastructures made of cementitious materials attacked by external sources of sulfate. Despite the importance, effects of temperature and strain rate on the mechanical response of thaumasite had remained unexplored prior to the current study, in which the mechanical properties of thaumasite are fully characterized using the reactive molecular dynamics (RMD) method. With employing a first-principles based reactive force field, the RMD simulations enable the description of bond dissociation and formation under realistic conditions. From the stress–strain curves of thaumasite generated in the x, y, and z directions, the tensile strength, Young’s modulus, and fracture strain are determined for the three orthogonal directions. During the course of each simulation, the chemical bonds undergoing tensile deformations are monitored to reveal the bonds responsible for the mechanical strength of thaumasite. The temperature increase is found to accelerate the bond breaking rate and consequently the degradation of mechanical properties of thaumasite, while the strain rate only leads to a slight enhancement of them for the ranges considered in this study

Expansion mechanisms in sulphated rocks and soils

Geological formations containing sulphates are commonly associated with the development of severe expansions when they are involved in tunnel excavation. The intensity of the observed expansive behaviour in these materials is greater than in other expansive soils and rocks free of sulphates. Swelling in sulphated formations can also occur in the rock mass although no tunnel is excavated, and also within fills made of compacted material from excavations in sulphated rocks. The functionality and stability of engineering works are affected in the majority of the cases. The prediction of strains and swelling pressures in sulphated formations is a difficult task. On that direction, the Thesis analyses and describes the mechanisms and conditions leading to expansions in sulphated rocks through the detailed investigation of three exceptional cases of damage induced by expansions involving sulphated formations in Spain. Lilla tunnel, the first case-history analysed, was excavated in Tertiary anhydritic claystone. Expansions developed led to severe tunnel floor heaving and to very high pressures against tunnel lining. The second case, a bridge founded on piles within an anhydritic claystone, Pont de Candí bridge, experienced an unexpected and sustained heave of the central pillars as a consequence of the development of swelling strains below the deep pile foundations. The third case, the compacted access embankments to another bridge, Pallaressos embankments, experienced a very significant volumetric expansion. As a result, the bridge was axially compressed and damaged. Mechanisms leading to swelling in tunnels in sulphated rocks have been described in the literature; however, an alternative interpretation is proposed. In the first two cases the development of swelling phenomena is explained by the precipitation of gypsum crystals in rock discontinuities. The presence of anhydrite plays a central role in the phenomenon because its dissolution leads to supersaturation conditions of groundwater in calcium sulphate with respect to gypsum, which will result into gypsum precipitation. The process of precipitation of gypsum in discontinuities is thought to act as local jacks inducing swelling strains. The maximum pressure exerted by crystal growth has been estimated under a thermodynamical point of view. Expansions in the third case analysed are a result of massive growth of ettringite and thaumasite minerals in embankments reinforced by Portland cement due to sulphate attack to cementitious materials. Mineralogical analysis and the simulation of the chemical reactions involved in sulphate attack have shown that expansions in this type of embankments would proceed for a long time because of the availability of the necessary components for ettringite and thaumasite formation. A finite element model of embankment swelling developed has shown that great loads can be generated due to swelling against rigid structures restraining the expansion in some direction. A coupled Hydro Mechanical and Chemical model formulated in a porous media has been developed to simulate volumetric expansions explained by gypsum precipitation. The model is consistent with field and laboratory observations and describes the kinetics of precipitation and dissolution of gypsum and anhydrite, the solute transport and the development of strains induced by crystallization. The model has been applied to interpret and simulate the heave experienced by Pont de Candí viaduct. Modelling capabilities were checked against the long term history of viaduct heave and also, against the foundation response when the vertical load from a surface embankment was added to counteract swelling.Las formaciones geológicas que contienen sulfatos están asociadas tradicionalmente al desarrollo de expansiones severas cuando son atravesadas por túneles. La intensidad del comportamiento expansivo observado en estos materiales es mayor que en otras rocas y suelos expansivos libres de sulfatos. El hinchamiento asociado a formaciones sulfatadas también puede ocurrir en el macizo rocoso aunque no se excave un túnel, y también en rellenos construidos con material compactado procedente de excavaciones en rocas sulfatadas. En la mayoría de casos la funcionalidad y estabilidad de las obras de ingeniería se ven afectadas. La predicción de deformaciones y presiones de hinchamiento en formaciones sulfatadas es una tarea difícil. En este sentido, la Tesis analiza y describe los mecanismos y las condiciones que conducen a expansiones en rocas sulfatadas, a través de la investigación detallada de tres casos excepcionales de daño debido a expansiones que involucran formaciones sulfatadas en España. El túnel de Lilla, el primer caso analizado, se excavó en arcillita anhidrítica. El desarrollo de expansiones condujo a un levantamiento severo de la solera y a presiones contra el revestimiento muy altas. El segundo caso, un puente cimentado en pilotes en una arcillita anhidrítica, el viaducto de Pont de Candí, experimentó un levantamiento inesperado de los pilares centrales debido al desarrollo de hinchamientos por debajo de las cimentaciones profundas. En el tercer caso, los terraplenes de acceso a otro puente, los terraplenes de Pallaressos, experimentaron una expansión volumétrica muy significativa y el puente resultó comprimido axialmente y dañado. Los mecanismos que conducen al hinchamiento en túneles en rocas sulfatadas se han descrito en la literatura; sin embargo, se propone una interpretación alternativa. La precipitación de cristales de yeso en discontinuidades de la roca explica el desarrollo de fenómenos de expansión en los dos primeros casos. La presencia de anhidrita juega un papel central en el fenómeno expansivo porque su disolución conduce a condiciones de sobresaturación del agua del macizo en sulfato cálcico respecto el yeso, que resulta en la precipitación de yeso. Se cree que el proceso de precipitación de yeso en discontinuidades actúa como gatos locales generando deformaciones. La presión máxima ejercida por crecimiento de cristales se ha estimado desde un punto de vista termodinámico. Las expansiones en el tercer caso son el resultado del crecimiento masivo de los minerales de etringita y thaumasita en terraplenes estabilizados con cemento Portland debido al ataque sulfático a los materiales cementantes. Ensayos mineralógicos y la simulación de las reacciones químicas involucradas en el ataque sulfático han mostrado que las expansiones en este tipo de terraplenes continuarían durante un periodo de tiempo largo, debido a la disponibilidad de los componentes necesarios para la formación de etringita y thaumasita. Un modelo de elementos finitos del hinchamiento del terraplén ha mostrado que estas expansiones pueden generar presiones muy altas contra estructuras rígidas. Se ha desarrollado un modelo acoplado Hidro-Mecánico y Químico formulado en medio poroso para simular expansiones volumétricas explicadas por la precipitación de yeso. El modelo es consistente con observaciones de campo y laboratorio y describe la cinética de la precipitación y disolución, el transporte de soluto y el desarrollo de deformaciones inducidas por cristalización. El modelo se ha aplicado para interpretar y simular el levantamiento experimentado por el viaducto. La capacidad de modelación se ha verificado frente a las medidas de levantamiento del viaducto en el tiempo y también, frente a la respuesta de las cimentaciones a la construcción de un terraplén en superficie para contrarrestar el hinchamient

Maqarin Phase IV report

Cement and concrete are extensively used in the construction of repositories for low- and intermediate-level radioactive wastes (L/ILW). In underground silos, like those in Sweden and Finland for example, much of the waste is conditioned with concrete (e.g. spent ionexchange resins) and is packed in concrete containers (e.g. Figure 1.1)1. Even in many high-level radioactive waste (HLW) repositories, cement and concrete may be widely used. Instances include the paving of tunnels, shotcreting of tunnel walls, and injection and grouting of fractures. Consequently, in many repository designs, cement-based materials are expected to dominate the repository. In the Swiss L/ILW concept, for example, current designs envisage the use of up to 1.5 million tonnes of cement, approximately 85-90% by weight of the total repository. This will ensure the long-term maintenance of hyperalkaline conditions, predicted to suppress the solubility of key radionuclides in the waste (e.g. Hodgkinson and Robinson, 1987) and to enhance their sorption on the cement (e.g. Stumpf et al., 2004)