Sulfate de magnésium | EPFL Graph Search

Le sulfate de magnésium est un composé chimique minéral anhydre de formule MgSO communément appelé sel anhydre d'Epsom. En réalité, c'est l'heptahydrate de sulfate de magnésium des chimistes ou l'epsomite des minéralogistes qui était et est encore le plus souvent récolté et purifié, puis vendu le plus souvent en masse cristalline, sous le nom commercial et traditionnel de « sel d'Epsom », « sel anglais », « sel de Sedlitz ». Ce sulfate naturel de magnésium hydraté était aussi dénommé « sel amer » pour son goût ou sa saveur amère, repérable en bouche après ingestion. Caractéristiques physico-chimiques Ce sel anhydre, matière poudreuse de masse molaire , incolore à légèrement coloré, principalement de maille orthorhombique, possède une densité assez élevée, avoisinant 2,66. Il est soluble dans l'eau, mais précipite uniquement en heptahydrate de sulfate de magnésium. Ce corps chimique hydraté, de densité beaucoup plus faible soit 1,68 est dénommé epsomite par les minéralog

Deterioration of mortar bars immersed in magnesium containing sulfate solutions

Wolfgang Kunther, Barbara Lothenbach, Karen Scrivener

Mortars prepared with a CEM I and a CEM III/B binder were investigated in different magnesium sulfate solutions. The main deterioration mechanism for the CEM I was expansion, while surface erosion was dominant for CEM III/B. The presence of sodium, potassium and calcium in a magnesium sulfate solution led to less expansion and less surface deterioration for both, CEM I and CEM III/B, than which was observed in solutions containing only sodium or magnesium sulfate. The presence of a mixture of different cations seems to lower both the surface deterioration and the expansion and might explain why sulfate attack damages are not as frequent in the field as in laboratory tests. Sulfate binding before cracking/expansion is similar in the presence of all different solutions investigated, indicating that the speed of sulfate ingress and the amount of bound sulphate depends during the first months mainly on the binder.

Springer Verlag2013

Resistance of cementitious materials to sulfate attack: quantifying performance with a reliable unidirectional approach

Qiao Wang

Depending on the environmental conditions, concrete materials can come into contact with sulfate ions which are widely present in rivers, underground water, sewers, seawater and soil. Sulfates can react with the cement paste in concrete and cause damage which impacts the service life of concrete structures. Cement chemistry, transport of sulfate ions, and physical properties of the host material all interact in the complex process of sulfate degradation. Given different conditions of concrete exposure, sulfate attack is commonly divided into "chemical" and "physical" sulfate attack. However, the processes are not fully understood and there is still a need for in-depth investigations of the whole deterioration process. The current standard testing method is adapted for Portland cement systems affected by the "chemical sulfate attack" in submerged conditions. However, only expansion is monitored, which is not suitable for characterizing the increasingly common usage of supplementary cementitious materials in the cement. Moreover, several key factors (e.g., RH & temperature) are usually overlooked so lab tests do not well represent the field conditions. Particularly, the understanding of the damaging mechanism of "physical sulfate attack" is very limited even if different testing approaches have been established. The division into "chemical" and "physical" attack is in itself misleading. In this context, this research project was launched to establish a unidirectional penetration approach for the in-depth comprehensive investigation of joint sulfate attack mechanisms. First, the feasibility of this newly established approach was studied for Portland cement mortars and pastes. A high concentration of sulfate solution and a high water-to-cement ratio was chosen to accelerate the degradation and to obtain results in a reasonable period. Results showed that this new concept of combining chemical and physical sulfate attack mechanisms into a single experiment was viable. Moreover, results showed that using cement paste samples can accelerate the degradation, compared to mortar samples. The sulfate degradation process was more rapid with the capillary rise and water evaporation. A comparison of the macroscopic manifestation of damage (visual appearance and expansion) with the underlying microscopic changes was made considering different key parameters, i.e., water-to-cement ratio, exposure solution concentration, cement type, temperature and humidity conditions (constant or cyclic). Regarding the different cement types, Ordinary Portland cement pastes and sulfate-resisting cement pastes showed higher expansion and cracking, whereas, spalling damage was more pronounced in blended-cement pastes/mortars (slag Portland cement and limestone calcined clay cement). Damage from the chemical attack was more extensive and rapid when high concentrations and cyclic exposure conditions were applied. However, lower water-to-cement ratios leading to lower porosity and/or higher mechanical strength could usually reduce the extent of the damage. Thermodynamic modelling was also conducted with GEMS to better explain the pore solution environment and the link between supersaturation and expansion. The interacting factors of porosity and strength were decoupled. The effect of the curing temperature on the expansion mechanism was also investigated in sulfate-resisting cement mortars under submerged sulfate exposure.

EPFL2022

Influence of pH on dehalorespiring consortia & first steps in the development of a methodology for acid neutralization in enhanced in situ bioremediation

Nicole Munz

Enhanced in situ bioremediation of chlorinated solvents is, especially in source zone remediation, often limited through the accompanied high acidification of groundwater. This acid built-up originating from the dechlorination itself and fermentation products, such as organic acids, presents a draw-back in the efficiency of dehalorespiring bacteria. It is therefore indispensable to develop and implement appropriate buffering strategies to ensure a constant transformation of toxic chloroethenes to the final non-toxic product ethene. In this study pH tolerance of different dehalorespiring bacteria was tested. The limiting acidic pH varies significantly among the consortia. The consortium SL23b4 (PCE to ethene) grows at all the examined pH (pH 5 to pH 7.5) with a pH optimum between 6 and 7 (depending on the dechlorination step), whereas Maroc 6b (PCE to ethene) was only found to grow at incubation pH of 7, and very slowly at pH 6.5. The consortium 2SO (cis-DCE to ethene) seems to be inhibited at acidic pH (no growth at pH 5 and 5.5) having a pH optimum above neutral pH. In most cases incomplete dechlorination was observed in cultures growing at acidic pH or below the pH optima, accumulating toxic intermediate dechlorination products as a consequence. Additionally, high rates of dechlorination were always accompanied by pH drops up to 1.2 pH units, giving evidence of high acid production. pH had also influence on the community composition. In the consortium SL23b4 a shift from a Dehalococcoides- (pH 6-7.5) to a Sulfurospirillum spp.- (pH5 -5.5) dominated community was observed. Consortium Maroc 6b showed significantly different community compositions due to a pH change of only 0.5 pH units. Dehalococcoides spp. and Dehalobacter spp. were dominating at pH 7 and pH 6.5, respectively. Consortium 2SO is dominated by Dehalococcoides spp. at all pH, although at pH 7 a much higher diversity seems to prevail, with the threefold number of OTUs. Tests on the tolerance of the dehalorespiring consortia for elevated PCE concentration showed similar patterns for the different dechlorination steps, as for pH cultures, and confirmed a generally higher sensitivity towards PCE and pH changes for the last steps of dechlorination. Conclusively, Dehalococcoides spp., the only known genera able to perform complete dechlorination, seem to be highly sensitive to pH and PCE changes. Further, abiotic experiments on silicate mineral dissolution, using forsteritic olivine, were performed to simulate the buffer capacity of the minerals for the acid production during microbial reductive dechlorination. In these experiments, the buffer capacity was insufficient to counterbalance the constant acid titration rate, resulting in a final pH between 3 and 4 (initial pH of 7). The formation of precipitates, such as iron oxide, and the re-precipitation of magnesium were observed in some cases, showing a decrease in magnesium concentrations.

2010