Barbara Lothenbach

Solid Solutions between CrO4- and SO4-Ettringite Ca6(Al(OH)6)2[(CrO4)x(SO4)1-x]3*26 H2O

Chromate is a toxic contaminant of potential concern, as it is quite soluble in the alkaline pH range and could be released to the environment. In cementitous systems, CrO4(2−) is thought to be incorporated as a solid solution with SO4(2−) in ettringite. The formation of a solid solution (SS) could lower the soluble CrO4(2−) concentrations. Ettringite containing SO4(2−) or CrO4(2−) and mixtures thereof have been synthesized. The resulting solids and their solubility after an equilibration time of 3 months have been characterized. For CrO4-ettringite at 25 °C, a solubility product log K(S0) of −40.2 ± 0.4 was calculated: log K(CrO4−ettringite) = 6log{Ca2+} + 2log{Al(OH)4(−)} + 3log{CrO4(2−)} + 4log{OH−} + 26log{H2O}. X-ray diffraction and the analysis of the solution indicated the formation of a regular solid solution between SO4- and CrO4-ettringite with a miscibility gap between 0.4 ≤ XCrO4 ≤ 0.6. The miscibility gap of the SO4- and CrO4-ettringite solid solution could be reproduced with a dimensionless Guggenheim fitting parameter (a0) of 2.03. The presence of a solid solution between SO4- and CrO4-ettringite results in a stabilization of the solids compared to the pure ettringites and thus in an increased uptake of CrO4(2−) in cementitious systems.

Uptake of oxo-anions by cements through solid-solution formation: experimental evidence and modelling

Summary Uptake experiments were carried out with selenate and chromate on fresh and leached Portland and high-alumina cements; and in addition with selenate and selenite on synthetic ettringite. In all experiments with cements, exceptionally high uptake could be observed under conditions where significant amounts of secondary ettringite were formed. Experimental data obtained for pure ettringite corroborated the important role of this mineral phase. However, uptake kinetics show opposite trends in these two systems, which can be viewed as end-members of the same process: Where a fast precipitation of secondary ettringite occurred, initial uptake was high, with Kd values in the range of ≍1-5 m3/kg for Se(VI), but decreased with time. Uptake by pre-formed (primary) ettringite initially gave lower Kd values (≍0.1 m3/kg), which increased with time. After prolonged equilibration times, the two systems started to approach each other. Concurrent measurements of sulphate concentrations allowed to extract a mean partition coefficient for the solid-solution formation of selenite and selenate with ettringite. Based on this simple solid-solution model, a pragmatic quantitative relation was developed that permits to estimate Kd for Se(VI) on whole cement as a function of the concentrations of sulphate in the cement and in the solution. A test against experimental data shows reasonably good agreement between measurements and calculations. This relation can also be directly applied to estimate Kd values of chromate (and perrhenate) by different cements, indicating the same uptake mechanism. The approach may be less well suited for Se(IV), whose uptake on cement appears to be related mainly to minerals other than ettringite.

Quantification of fly ash in hydrated, blended Portland cement pastes by backscattered electron imaging

An automated image analysis procedure for the segmentation of anhydrous fly ash from backscattered electron images of hydrated, fly ash blended Portland cement paste is presented. A total of six hundred backscattered electron images per sample are acquired at a magnification of 2000. Characteristic features of fly ash particles concerning grey level, shape and texture were used to segment anhydrous fly ash by a combination of grey level filtering, grey level segmentation and morphological filtering techniques. The thresholds for the grey level segmentation are determined for each sample by semiautomatic histogram analysis of the full image stack of each sample. The analysis of the presented dataset reveals a standard deviation of the reaction degree of fly ash of up to 4.3%. The results agree with a selective dissolution method to quantify the reaction degree of fly ash showing the potential of the presented image analysis procedure.

Influence of slag composition on the hydration of alkali-activated slags

The hydration of different blast-furnace slags with sodium metasilicate or NaOH as activator was investigated. Sodium metasilicate shows poor strength after 1 day, but high strength after 7 days and beyond, whereas NaOH activation leads to high strength after 1 day , but only moderate strength development at later ages. At the same hydration degree, sodium metasilicate activation gives a much higher compressive strength than NaOH activation. This is due to the formation of a dense hydration product in the NaOH-activated system at the early age, which leads to a more porous microstructure than in the case of sodium metasilicate. A higher MgO content in the slags is beneficial with respect to faster hydration kinetics and a more rapid strength development. Increasing Al2O3 contents lead to slightly slower hydration kinetics and slightly lower compressive strengths. The best correlation between compressive strength and slag composition was found using the extended basicity (CaO + MgO)/SiO2 index.

Sensitivity analysis of radionuclide migration in compacted bentonite: a mechanistic model approach.

Mechanistic model calculations for the migration of Cs, Ra, Am and Pb in compacted bentonite have been carried out to evaluate sensitivities with respect to different parameter variations. A surface chemical speciation/electric double layer model is used to calculate: (i) porewater composition and radionuclide speciation in solution and at the bentonite surface, yielding the distribution of mobile and sorbed species and (ii) interaction of diffusing species with negatively charged pore walls to obtain diffusion parameters. The basic scenario considers the interaction of compacted bentonite with a fresh-type groundwater; variations include the presence of bentonite impurities and saline groundwater. It is shown that these scenarios result in significant variations of porewater composition that affect migration via three mechanisms that can partly compensate each other: (1) effects on sorption through radionuclide complexation in solution, and competition of major cations for surface sites; (2) changes in radionuclide solution speciation leading to different diffusing species under different conditions; (3) effects on diffusion through changes in the electric double layer properties of the clay pores as a function of ionic strength.

Thermodynamic data for the solubility of tin(IV) in aqueous cementitious environments

The solubility of Sn(IV) in cementitious systems is directly related to the presence of dissolved Ca. In the presence of typical Ca concentrations, solid Ca[Sn(OH)6](s) is precipitated, resulting in dissolved tin(IV) concentrations that are about four to six orders of magnitude lower than observed in alkaline, Ca-free systems. From different experiments carried out in initially oversaturated and undersaturated solutions, solubility products have been evaluated. A constant of 109.85 at I=0 has been derived for the formation of fresh Ca[Sn(OH)6](precip) according to the reaction Ca2++{Sn(OH)62- ↔ Ca[Sn(OH)6](s). A constant of 1010.93 has been derived in undersaturation experiments for the equilibrium with crystalline Ca[Sn(OH)6](cr).

Crystal Chemistry of Iron Containing Cementitious AFm Layered Hydrates

The crystal structure of the three main Fe-containing AFm phases (Al2O3-Fe2O3-mono: family of lamellar calcium alumina-ferrite hydrates) encountered in cement hydration process are characterized and compared with their Al-analogs. This includes AFm phases containing sulfate (which is present in Portland cement to regulate the hydration kinetic), carbonate (which is present in Portland cements, or originates from atmospheric carbon oxide) and chloride (either from the water used or from the environment). The results show that Fe-AFm and Al-AFm compounds are not (or rarely) isostructural. Iron in AFm phases does not simply substitute aluminium. Fe-carbonate has a rhombohedral symmetry whereas Al-carbonate has a triclinic symmetry, with carbonate anions located in different crystallographic sites in both compounds. Fe-Friedel’s salt corresponds to a 3R polytype whereas Al-Friedel’s salt corresponds to a 6R polytype. Both compounds have a temperature dependent transition with two different HT- and two different LT-polymorphs descriptions (HT: high-temperature, LT: low-temperature). Only Fesulfate and Al-sulfate are isostructural. Despite this isostructural feature, only limited solid solutions have been observed between both sulfate end-members. In a general way, this system (when considering sulfate, carbonate and chloride with aluminium and iron) leads to extremely complicated subsystems with limited solid solutions. The crystallographic studies and comparisons developed here have been fully completed by thermodynamic characterisations in order to make possible thermodynamic modelling of the hydrates assemblage during the hydration process and the aging of Portland concrete.

Mechanisms and Modelling of Waste/Cement Interactions – Survey of Topics Presented at the Meiringen Workshop

Cementitious matrices are being used worldwide as a containment medium for radioactive and non-radioactive waste in order to retard the mobility of contaminants. The present thrust of research is to further the understanding of contaminant binding in the cementitious matrix in order to predict the long-term behaviour and the potential impact of the waste on the environment. The workshop “Mechanisms and Modelling of Waste/Cement Interactions”, held in Meiringen, Switzerland, between May 8 and 12, 2005, focused on the chemical understanding and thermodynamic modelling of the processes responsible for the retention of radioactive and non-radioactive species in cementitious systems. The objectives of the workshop were to bring together scientists from different disciplines, i.e. cement chemistry, radioactive and non-radioactive hazardous waste disposal, to stimulate discussions on current developments and to identify future needs in this field of research. The topics treated in the workshop were chosen to maximize the benefit to the different fields of research. Cement chemists reported on developments in the understanding of cement mineralogy and thermodynamic modelling of cement systems. The hazardous and radioactive waste management communities presented their ideas on the mechanisms of contaminant binding to cement minerals as well as field, laboratory and modelling results from practical applications. In this paper important areas of research on waste/cement interactions presented in the workshop will be outlined and briefly discussed. The following overview reflects a subjective perception of the workshop and does not lay claim to deal comprehensively with all the papers that were presented in the workshop.

Durability of Portland Cement Blends Including Calcined Clay and Limestone: Interactions with Sulfate, Chloride and Carbonate Ions

The durability has been investigated for mortars made from a pure Portland cement (CEM I) and five Portland cement – SCM blends, using a cement replacement level of 35 wt% and the following SCM’s: (i) pure limestone, (ii) pure metakaolin, (iii) metakaolin and limestone (3:1 w/w), (iv) metakaolin and silica fume, and (v) metakaolin, silica fume and limestone. The blends with metakaolin and silica fume employ a fixed ratio for these components which mimics the alumina-silicate composition of a 2:1 clay (i.e., montmorillonite). All mortars were demoulded after hydration for one day and cured saturated in water at 20 °C for 90 days prior to exposure. Expansions induced by sulfate attack, chloride profiles, and carbonation depths were measured to investigate the durability performances of the mortars. Porosity and pore connectivity were analysed before exposure by mercury intrusion porosimetry. The results show that mortars incorporating metakaolin, independent of additional silica fume or limestone, all exhibit very high resistance towards sulfate attack and chloride ingress, but are vulnerable to carbonation. The binary Portland cement – limestone blend is most susceptible to all types of studied chemical attacks, as expected. The pure Portland cement exhibits poor resistance to sulfate attack and chloride ingress, but high resistance to carbonation. The observed performances for the different blends can be explained based on their microstructure and phase assemblages. For example, the presence of metakaolin increases the chloride-ion binding capacity and enhances chloride resistance by the low pore connectivity present in the hydrated blends with metakaolin.

An internally consistent thermodynamic dataset for aqueous species in the system Ca-Mg-Na-K-Al-Si-O-H-C-Cl to 800 °C and 5 kbar

This study presents an internally consistent thermodynamic dataset for aqueous species in the system Ca-Mg-Na-K-Al-Si-O-H-C-Cl, obtained by adding species of calcium, magnesium and carbon to the core system Na-K-Al-Si-O-H-Cl (Miron and others, 2016). Critically evaluated experimental data on mineral solubility (Ca and Mg hydroxides, Ca and Mg silicates, anorthite, Ca and Mg carbonates) in water and aqueous electrolyte solutions over wide ranges in temperature and pressure were added to the database of experimental data. The complete experimental dataset was then used to simultaneously refine the standard state Gibbs energies of all aqueous ions and complexes in the framework of the revised Helgeson-Kirkham-Flowers (HKF) equation of state. The thermodynamic properties of the solubility-controlling minerals were accepted from the internally consistent dataset of Holland and Powell (1998; updated Thermocalc dataset ds55). The association equilibria of important hydroxide, chloride, carbonate and silicate complexes were critically reviewed, and their standard state properties and HKF parameters were independently derived from conductance, potentiometric and, in a few cases, solubility measurements. In a global optimization of standard Gibbs energies of aqueous species, performed with the GEMSFITS code (Miron and others, 2015), the association equilibria for aqueous complexes were always maintained. The new thermodynamic dataset reproduces all available fluid-mineral phase equilibria and mineral solubility data in the system Ca-Mg-Na-K-Al-Si-O-H-C-Cl with good accuracy over wide ranges in temperature (25–800 °C), pressure (1 bar – 5 kbar) and composition (salt concentrations up to 5 molal). This makes it possible to perform geochemical and reactive transport modeling of processes in natural and engineered georeservoirs over wide ranges of conditions with an unprecedented level of accuracy and reliability and to address processes of fluid flow and fluid-rock interaction in the Earths crust from a new perspective. Using the same strategy as applied in the present study, the internally consistent thermodynamic dataset can be further extended with additional major and trace elements, and the data refinement process can be repeated when new experimental data or next-generation equation of state or activity models for aqueous solutions become available.