Gypsum | EPFL Graph Search

Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, drywall and blackboard or sidewalk chalk. Alabaster, a fine-grained white or lightly tinted variety of gypsum, has been used for sculpture by many cultures including Ancient Egypt, Mesopotamia, Ancient Rome, the Byzantine Empire, and the Nottingham alabasters of Medieval England. Gypsum also crystallizes as translucent crystals of selenite. It forms as an evaporite mineral and as a hydration product of anhydrite. The Mohs scale of mineral hardness defines gypsum as hardness value 2 based on scratch hardness comparison. Etymology and history The word gypsum is derived from the Greek word γύψος (), "plaster". Because the quarries of the Montmartre district of Paris have long furnished burnt gypsum (calcined gypsum) used for various purposes, this dehydrated g

Typology of recent groundwaters from different aquifer environments based on geogenic tracer elements

A key problem in environmental science and in engineering geology is the often incomplete understanding of the origin of dissolved components in groundwater. The dissolved contents of trace elements in groundwater are of special importance for groundwater quality control. The AQUITYP project aims to establish a detailed typology of recent groundwaters based on their geogenic trace element compositions, and to derive a so-called "geo-reference" for groundwaters from five principal aquifer lithologies in the Alpine belt. This geo-reference provides a database for investigations related to groundwater contamination, groundwater resources management and engineering geology. Groundwaters from crystalline, carbonate, and evaporite rocks, as well as molasse and flysch sediments in Switzerland and in neighbouring countries were sampled and documented by previous researchers (Dubois, 1993; Dematteis, 1995; Mandia, 1993; Hesske, 1995; Basabe, 1993). Based on a statistical analysis of the data and examination of the relationship between aquifer lithology and chemical groundwater composition these researchers identified a number of characteristic tracer elements (geogenic tracers). The present study includes (1) a synthesis of the hydrogeology and the hydrochemistry of recent groundwaters in these five aquifer types based on groundwater data acquired within the AQUITYP project, and (2) a hydro-geochemical investigation of the origin and chemical behaviour of the geogenic tracer chromium, based on a comprehensive field and laboratory study. To enable a comparison of the chemical groundwater data gathered and analysed over a time span of 20 years, a rigorous quality control of the entire database was made. An assessment was made of the sampling techniques, the sample treatment and the analytical methods applied in the AQUITYP project since 1981. Different tests were carried out to evaluate the quality and comparability of the data, including geochemical model calculations, comparison of different analytical techniques, and tests to estimate the influence of the filtration procedure. In order to make this large number of quality controlled data accessible also for future investigations, a groundwater data storage system was developed (AQUITYP-DataBase). 1.) Typology of recent groundwaters In the synthesis of groundwater hydrochemistry, the emphasis was laid on the hydro-geochemical evolution leading to the characteristic groundwater composition in each of the five aquifer types. Chemical characteristics and differences between the groundwaters from the different aquifer types were identified and natural concentration ranges for each aquifer type derived. The proposed geogenic tracers were evaluated and the potential sources of these tracers identified. The dominant processes leading to the typical mineralisation of recent groundwaters were investigated using geochemical modelling strategies. Finally, the concentrations of chemical elements in the groundwaters from the different aquifer types were compared to the Swiss drinking water standards in order to assess the quality of the investigated groundwaters. It has been found that each rock type contributes in a characteristic way to the major and trace element composition of the corresponding groundwater: The groundwaters derived from the crystalline Mont-Blanc and Aiguilles-Rouges massifs are characterised by a low total mineralisation (TDS 22 to 158 mg/L) dominated by Ca2+, Na+, Mg2+, alkalinity, SO42-, and F- (Ca-Na-HCO3,-SO4, waters). Elevated amounts of Mo, U, W, and As occur. These groundwaters derive their mineralisation mainly from the interaction with hydrothermal minerals present along fractures. Fractures act as major groundwater flow paths. Minerals relevant for groundwater mineralisation include carbonates (Ca2+, Mg2+, HCO3-), clay minerals (Ca-Na ion exchange), fluorite (F-, Ca2+), Fe-, As-, and Mo-sulphides (SO42-, As, Mo), and U- and W-minerals (U, W). In these crystalline groundwaters the natural concentrations of F- (23% of the investigated springs) and As (7%) exceed the Swiss limits for drinking water. In addition, the WHO guideline values for U are exceeded in 65% of the cases and for Mo in 15% of the cases. The carbonate karst groundwaters obtain their low to intermediate mineralisation (TDS 161 to 547 mg/L) from the dissolution of calcite (Ca-HCO3, waters), as well as in certain regions dolomite (Ca-Mg-HCO3, waters) and gypsum (Ca-Mg-HCO3,-SO4, waters). Together with their very short residence times, the carbonate karst groundwaters generally contain very low trace element contents. Nevertheless, geogenic trace elements occur in specific regions in relation with fossil organic matter (I, V) and accessory minerals such as barite in deep sea limestones (Ba), evaporite minerals (gypsum, celestite: Sr, Li), clay- and Fe-minerals (V), and Mo-sulphides and U-minerals in dolomitic limestones (Mo, U). In 18% of the carbonate karst springs atmospheric derived Pb exceeds the Swiss drinking water quality target value. The groundwaters from Triassic evaporites in the Swiss Rhone basin are characterised by a high total mineralisation (TDS 760 to 2788 mg/L) expressed by elevated amounts of Ca2+, Mg2+, Sr2+, SO42-, and alkalinity (Ca-Mg-SO4-HCO3 waters). Elevated amounts of the trace elements Mn, Ni, Cu, Li, Rb, Y, and Cd occur. The hydrochemical evolution of these groundwaters is governed by incipient dedolomitisation, involving dissolution of gypsum, celestite and dolomite and simultaneous precipitation of calcite. The Na+ and K+ contents probably controlled by ion-exchange reactions on clay minerals. Characteristic trace elements originate mainly from the dissolution of dolomite (Mn, Ni) and small amounts of apatite (Y, Cd), and from the oxidation of sulphide minerals (Cu, Ni, Cd). The elevated concentrations of the highly soluble Li and Rb may be related to brine inclusions in evaporite minerals and eventually to clay minerals. These evaporite groundwaters contain SO42- concentrations exceeding the Swiss quality target for drinking water in all springs, and the concentrations of U and Ni exceed the WHO guideline values in respectively 58% and 2% of the cases. In addition, Mn, Cd, and As concentrations exceed the Swiss quality targets in respectively 11%, 10%, and 7% of the investigated springs. Recent groundwaters circulating in the porous and fissured molasse sandstones and conglomerates acquire their intermediate mineralisation (TDS 48 to 714 mg/L) primarily by dissolution of calcite and minor dolomite (Ca-Mg-HCO3 waters). The particular mineralogy of certain Molasse formations is reflected in specific trace element compositions of corresponding groundwaters: ophiolite detritus in OMM sandstones in western Switzerland (Cr), barite fracture mineralisations in subalpine and folded Molasse units (Ba), granitic detritus containing sulphides (Mo), U-minerals (U) and abundant mica (Li) in the "Glimmersand" (OSM, Ca-Mg-HCO3-SO4 waters), and evaporite minerals (Li, Sr), sulphides (Mo), and U-minerals (U) in the "Gypsum-bearing Molasse" (USM, highly mineralised Ca-Mg-SO4-HCO3 waters, TDS 984 to 1346 mg/L) . In these molasse groundwaters the Swiss quality target values for drinking water of Cr and Pb are exceeded in 36% and 6% of the springs, respectively. The U concentrations of 14% of the molasse groundwaters ("Gypsum-bearing Molasse" and "Glimmersand"), exceed the WHO guideline value. The groundwaters from the "Gypsum-bearing Molasse" display similar quality problems as the evaporite groundwaters. Groundwaters derived from the shallow flysch aquifers in the Niesen and Gurnigel nappes are poorly evolved Ca-(Mg)-HCO3 waters. Their low to intermediate total mineralisation (TDS 160 to 459 mg/L) is acquired primarily by dissolution of calcite and to a lesser degree dolomite. The low trace element content is dominated by Ba originating from barite fracture mineralisations. The poor chemical evolution of recent flysch groundwaters results from (1) their short residence time in the fractured flysch rocks, and (2) the absence of readily dissolving minerals except carbonates and barite. In 32% of the flysch springs Pb derived from atmospheric sources exceeds the Swiss quality target value. 2.) Case study on the chemical weathering of molasse sandstone: Sources and chemical behaviour of Cr For the characterisation of the potentially toxic tracer element Cr a comprehensive field study was carried out on a selected catchment (Lutry spring catchment near Lausanne) situated in a molasse sandstone (OMM, Burdigalian). The investigation of the processes controlling the dissolved Cr content in these groundwaters was based on the groundwater chemical data, as well as on mineralogical, geochemical, and hydrological data. Relic Cr-bearing spinel and pyroxene in the sandstone were identified to be the primary sources of Cr. An electron microscope study showed that in the Burdigalian sandstone and overlying soil these minerals are strongly weathered. The slow weathering of these minerals is the major Cr releasing process. Under the oxidising conditions reigning in the investigated groundwaters, Cr prevails in solution in its highly soluble and toxic hexavalent state (CrO42-). In this case, retention by secondary Cr-hydroxide phases does not occur, as can be shown by geochemical model calculations. Laboratory leaching experiments were carried out with Burdigalian molasse sandstone from the field site, in order to support the field study findings and to quantify the processes responsible for the Cr mineralisation observed in the Lutry groundwater. Two experiments with mountain-wet oxidised and reduced sandstone were carried out over a time span of 2 months each, to obtain information about the influence of the oxidation state of the substratum on the mobilisation of Cr. The experiments clearly showed that the Cr-releasing processes are fast enough to explain the Cr contents found in the groundwater, and that the release of Cr into the groundwater depends on the weathering state of the sandstone. In the oxidised, Fe-hydroxide-coated sandstone, Cr is faster released into solution than in the less altered reduced sandstone. This indicates that the Cr contained in the reduced sandstone is in a more stable state, i.e. mainly incorporated in detrital minerals, while in the oxidised sandstone, the Cr is partly in an unstable state, i.e. adsorbed on the surface of secondary Fe-hydroxides, from where it is more easily leached.

EPFL2001

Hydration of C3A with Calcium Sulfate Alone and in the Presence of Calcium Silicate

Alexandra Quennoz

Tricalcium aluminate (C3A) is one of the main constituents of Portland cement. Even though it represents less than 10% of the total composition, its strong reaction with water can lead to a rapid setting, called flash set. Gypsum is added to regulate this reaction and preserves the workability of the cement paste at early ages. The understanding of the C3A-gypsum reaction is therefore crucial for the comprehension of the early hydration of cement. The role of the amount of C3A and the sulfate balance of cement hydration are of major interest since two important routes for the development of new cementitious materials are the increasing rate of substitution materials and the increasing level of aluminate in clinker. This thesis aimed to investigate the C3A-gypsum reaction alone and in the presence of alite in order to provide basic knowledge on the C3A-gypsum reactions and study the interactions that occur between the cement phases when hydration occurs in alite-C3A-gypsum systems. Alite and C3A as well as clinkers of controlled composition were synthesized. Model systems composed of C3A with different gypsum additions and alite-C3A-gypsum systems were studied in terms of hydration kinetics, phase assemblage and microstructural development. This work confirmed the findings of previous works on the mechanism that controls C3A-gypsum hydration when sulfate ions are present in solution and gave new results on the reaction when gypsum is depleted. It was shown that AFm phases do not crystallize only as platelets that fill the space between the C3A grains but also form an "inner" product within the original C3A grain boundaries and that hydrogarnet (for which the presence depends on the gypsum addition) crystallizes as a rim around C3A grains. Moreover the influence of the gypsum addition on the morphology of the AFm platelets and the role of their morphology on the hydration rate were highlighted. In the presence of alite the hydration kinetics of C3A-gypsum systems was subject to change due to the adsorption of sulfate ions on C-S-H and the reduction of the space available for the reaction. In addition, with the correlation of calorimetric, XRD and SEM analyses it was possible to observe a second formation of ettringite from C3A and sulfate ions released from C-S-H after the depletion of gypsum. Finally, the rate of alite hydration related to the growth of C-S-H was shown to be modified in the presence of C3A and gypsum.

EPFL2011

Hydration, microstructural development and dimensional stability of cementitious materials made of portland cement clinker and fluoroanhydrite

Carolina Prieto Gomez

The production of Portland cement accounts for 5 to 8% of the global anthropogenic CO2 emissions. In the past years, the cement industry has attempted to reduce this contribution by the use of mineral additions on the cement or in the concrete mix. The typical additions in cement are materials with pozzolanic properties such as fly ash, silica fume, etc. Other attempts have been done using materials obtained as byproducts from other industrial processes. This is the case of the chemical gypsums i.e. calcium sulfates. Calcium sulfate can be obtained in the production of several acids such as boric (borogypsum), hydrofluoric (fluorgypsum or fluoroanhydrite), phosphoric (phosphogypsum), etc. or in desulfurization process in coal burning power plants. In this work, the reaction mechanisms of a binder made of 60% fluoroanhydrite and 40% Portland cement clinker have been studied. This binder is currently being produced in Mexico in the preparation of aerated concrete mixes intended to be used in low-cost housing. This work aims to gain insight on the mechanisms of reaction of the binder made of fluoroanhydrite- Portland cement clinker on the one hand and the dimensional stability aspects related with this binder and its application in the construction industry on the other. Model mixes prepared with different proportions of materials were also studied to understand the effect of the fluoroanhydrite on the properties of the binder. The hydration kinetics of the binder was studied by means of isothermal calorimetry and chemical shrinkage. The microstructural development was studied by a combination of techniques such as SEM, XRD, mercury intrusion and others. The dimensional stability of the binder was approached mainly by length change tests carried under different conditions. As this product is a calcium- sulfate based binder the results under high relative humidity conditions were highlighted. The reaction kinetics of the binder were found to be affected by the impurities present in the fluoroanhydrite; hydrofluoric acid traces particularly retard the hydration of both fluoroanhydrite and the clinker. Other aspects such as aging and the ethanolamine-based grinding agent used on the production of the binder were found responsible of the delayed reaction of the binder compared to Portland cement clinker. The hydration of the binder in water immersion is not very different from that observed under medium relative humidity; however, the dimensional stability of the binder is importantly affected by this parameter. The composition of the binder was found to play an important role.

EPFL2010