Influence of capillary properties and evaporation on salt weathering of sedimentary rocks

International audienceThe importance of capillary imbibition and evaporation processes in the decay of stone through salt crystallization is estimated by different experiments adapted from the European standard EN 12370. The various tests consist in slightly modifying the salt (sodium sulphate) supply in the porous network of stones as well as the amount of evaporation and in checking the weathering evolution of the stone. The results show that ionic diffusion and dissolution of pre-existent salts are not efficient enough to imply supersaturation, hence to visually alter the stone. An exterior supply of salt is required. The influence of evaporation is clearly shown when evaporation process could not occur through a side of the sample. In this case, isolated sides are never subjected to weathering, even to simple salt efflorescence

Etude multiéchelle de la dégradation des roches par la cristallisation de sels dans les réseaux poreux

Crystallization of soluble salts in porous networks is a major source of decay for natural stones. The objective of this thesis is to specify this salt decay mechanism in the particular case of sodium sulphate crystallization. The basis of this work is to follow the evolution of samples of nine sedimentary stones of various origins (two biodetritic limestones, one detritic limestone, one lacustrine limestone, three quartz-rich sandstones and two layered shaly sandstones) during accelerated ageing tests. First, the mechanism of sodium sulfate crystallization is studied thanks to thermal monitoring of the samples. Different crystallization sequences are proposed to explain the decay during the cycles. Then a new way of quantifying salt decay in the long term is proposed, taking into account the hydromechanical properties of the stones. The importance of the microstructures on decay (porous network tortuosity or connectivity, intragrain cracks) is also assessed thanks to a mercury porosimetry study. Finally, the influence of the environmental conditions on decay is also evaluated: evaporation and temperature directs both the location and amplitude of salt decay.La cristallisation de sels solubles dans les réseaux poreux est une source majeure de détérioration pour les pierres naturelles. L'objectif de cette thèse est de préciser ce mécanisme de détérioration par le sel dans le cas du sulfate de sodium. Le point de départ de cette étude est de suivre l'évolution d'échantillons de neuf roches sédimentaires d'origines différentes (deux calcaires biodétritiques, un calcaire détritique, un calcaire lacustre, trois grès riches en quartz et deux grès argileux lités) au cours des tests de dégradation accélérée. Tout d'abord, le mécanisme de cristallisation du sulfate de sodium est étudié grâce à un suivi thermique des échantillons. Différentes séquences de cristallisation sont proposées pour expliquer la détérioration au cours des cycles. Ensuite une nouvelle manière de quantifier l'altération à long terme par le sel est proposée, tenant compte des propriétés hydromécaniques des roches. L'importance des microstructures sur l'altération (connectivité et tortuosité du réseau poreux, fractures intra- grains) est aussi déterminée par une étude par porosimétrie au mercure. Enfin, l'influence des conditions environnementales sur l'altération est aussi évaluée : l'évaporation et la température influencent tous les deux à la fois la localisation mais aussi l'amplitude des dégâts causés par le sel

Experimental percolation of supercritical CO2 through a caprock

-Leakage of CO2 into the atmosphere is the most crucial concern for geological storage of anthropogenic CO2. Leakage routes could develop through existing wells and pipelines, but also by natural migration of CO2 rich pore-fluid through the caprock and in fault zones. Therefore, a thorough geological characterization of the prospective formation identifying seal capacity, integrity and possible migration pathways must be performed prior to injection of CO2. A caprock is a low permeable confining layer trapping CO2 stored in a reservoir rock. Although the caprock acts as a seal, its lower boundary will be in contact with CO2 saturated pore water or even pure CO2. Chemical interaction between the pore fluid and the caprock may change its material properties. The aim of this study is to increase our understanding of the interaction between CO2 and caprock, focusing on the microstructural properties of the rock. A flow through cell is used to flood a shale core (40 mm long and 38 mm diameter) with supercritical CO2 at a temperature of 35 ∘C and at pressures above 7.5 MPa. A pressure gradient is applied across the sample to obtain a breakthrough of CO2 in the core. During flooding both axial and radial strain of the core are measured together with the acoustic velocities in the axial direction. The measurements are performed both for the brine saturated core and at various stages during flooding with CO2. The experimental results suggest flow of CO2 along defined pathways within the shale. These pathways are likely to be controlled by the increasing pore pressure at the bottom of the sample which allows reopening of the cracks in the lower part of the sample, allowing CO2 to enter the shale. Flow of CO2 into the cracks in the lower part of the sample is followed by percolation of CO2 in the upper part of the sample where the effective pressure is higher and crack reopening is less likely to occur

Evaporation et cristallisation de sels solubles dans un réseau poreux modèle

L’objectif de ce travail expérimental est d’observer à l’échelle du pore la cristallisation de sels dans un réseau poreux au cours de l’évaporation. Des tests effectués en laboratoire sur un réseau poreux modèle avec de l’eau pure puis de la saumure montrent que le sulfate de sodium a peu d'effet sur la vitesse d'évaporation, que le chlorure de sodium la ralentit fortement après quelques heures alors que la sulfate de magnésium l'inhibe presque entièrement dès le début

Altération par les sels des pierres de construction: Influence des propriétés capillaires et des conditions d'évaporation

9 pages, compte rendu de conférenceNational audienceL'importance de l'imbibition capillaire et des processus d'évaporation dans l'altération des pierres de construction sous l'action de la cristallisation des sels solubles est estimée par différents modes opératoires adaptés de la norme européenne EN 12370. Les différents tests consistent à modifier les conditions de l'apport en sel (sulfate de sodium) dans le réseau poreux des pierres ainsi que les possibilités d'évaporation, et de suivre l'évolution de la détérioration. Les résultats montrent que la dissolution de sels en quantité insuffisante dans le réseau poreux n'altère pas la pierre. Un apport extérieur de sels est nécessaire. L'influence de l'évaporation est aussi clairement démontrée par la localisation des dégâts dans les zones non-isolée

Recording of the thermal evolution of limestones undergoing experimental accelerated ageing tests

International audienceSodium sulfates are widely regarded as the most destructive salts for porous stone, concrete and brick. Thenardite (Na2SO4), mirabilite (Na2SO4, 10H2O) and heptahydrate (Na2SO4, 7H2O) are the common phases that occur under surface conditions. The heptahydrate phase has been largely neglected in most modern work about salt weathering. However, several recent publications suggest that it could play an important role in geochemical and planetary processes. Therefore its role in the Na2SO4-H2O system should be clarified.We present here results of accelerated ageing experiments performed on samples of a micritic limestone under two different ambient temperatures: 20°C (i.e. below the upper limit of metastability of the heptahydrate), and 30°C (i.e. above this limit and also below the upper limit of stability of mirabilite). Thermocouples were placed into the samples in order to follow the evolution of the temperature of the stones during the weathering tests. This method allows the recording of exo- and endothermic reactions that are linked to physicochemical processes, such as crystallization, wetting, evaporative cooling, etc.The experiments show significant differences in the pattern and degree of damage. In the experiment at 20°C, the loss of material is noticeable, and occurs as crumbling and scaling. In the experiment at 30°C, we observe just efflorescence with no visible loss of material. The recorded thermal evolution is also different for the two experiments, which provides some clues as to the different physicochemical processes occurring in each situation, depending on the ambient temperature