Ettringite Precipitation Research Articles

Coupled chemo-transport-mechanical modelling and numerical simulation of external sulfate attack in mortar

We develop and apply in this study a chemo-transport-mechanical model for simulating the external sulfate attacks in Portland (CEM I) cement pastes and mortars. Basically, this degradation consists in the simultaneous decalcification of the hydrated phases resulting from leaching processes, and the migration of sulfate ions within the material and its subsequent interactions with these phases. The sulfate uptake leads generally to ettringite precipitation mainly from monosulfate, which in turn may produce intense macroscopic expansions and cracking. In our approach, crystallization pressures arising from the restrained growth of monosulfate crystals due to the confinement of the surrounding C–S–H matrix are assumed to initiate the observed macroscopic expansions. A macroscopic strain tensor evaluated from the volume fraction of supplementary precipitated ettringite is further introduced in the mechanical behavior law for explicitly reproducing the macroscopic expansions. Analytical homogenization schemes are applied to estimate both mechanical and diffusive properties from the local volume fraction of solid phases. The numerical platform Alliances is then used for solving both reactive transport and mechanical coupled problems, and is applied to the simulation of laboratory tests consisting in prismatic mortar specimens immersed in solutions containing sodium sulfate and subjected to free expansions. Comparison of the numerical results with experimental ones in terms of phase assemblage profiles, evolutions of mass changes and expansions shows a correct agreement. Finally, the extension of the model towards cases of restrained displacement conditions is discussed and some modifications regarding the kinetics of ettringite precipitation are proposed for such situations.

Reactive transport modeling of concrete-clay interaction during 15 years at the Tournemire Underground Rock Laboratory

Hydrated cement and concrete are major components of the engineered barrier system in proposed underground repositories for radioactive waste. Concrete was in contact with a clay-rich rock during 15 years in a borehole at the Tournemire Underground Rock Laboratory in France. Overcoring of the borehole and mineralogical analyses have shown a reduction of porosity at the interface due to the precipitation of ettringite, C-S-H/C-A-S-H and calcium carbonate, together with dissolution of portlandite in the cement. In the framework of the GTS-LCS project (JAEA, Japan; NAGRA, Switzerland; NDA, UK; POSIVA, Finland; SKB, Sweden), new reactive transport modeling (solute diffusion + mineral reaction) has been performed, including a sensitivity analysis with respect to several compositional and kinetic parameters. Results using the CrunchFlow code show sealing of porosity at the rock side of the interface (mm scale) due to the precipitation of C-A-S-H (calcium aluminum silicate hydrate), calcite and ettringite, together with some clay dissolution. The location of sealing is influenced by cation exchange. Inclusion of cation exchange results in sealing at the rock side of the interface. Without cation exchange, sealing is at the concrete side of the interface. Calculated alteration profiles along cm-scale fractures show increased alteration distances. Sealing of porosity is at the concrete side of the interface, due to the smaller effect of cation exchange.

Jet loop reactor application for mine water treatment using fly ash, lime and aluminium hydroxide

Recent studies have shown that a combination of coal fly ash (FA) and Al(OH)3 can be used to treat neutral mine drainage (NMD) and reduce sulphate concentration to within South African drinking water quality levels, Class II (400–600 mg/L). The shortcomings of this method were the large amounts of FA required to raise the pH to greater than 11 (3:1 liquid-to-solid ratio) so that Al(OH)3 can be added to facilitate removal of sulphate ions through ettringite precipitation. This requires large silos to store FA, making up-scaling of this treatment technology using normal mixing methods to be unrealistic. In the current study, a jet loop reactor was used to reduce the amount of FA needed to increase the pH to greater than 11. The pH was raised to greater than 11 by mixing 0.25 % of lime (w/v ratio) and 13 kg of coal FA with 80 L of NMD in a jet loop reactor. After the pH of the mixture was above 11, amorphous Al(OH)3 (83.2 g) was added to the mixture. This resulted in the sulphate concentration decreasing to less than 500 mg/L. Bench-scale studies using 0.25 % (w/v) of lime and 6:1 coal mine water to FA ratio could not reduce the sulphate concentration to below 500 mg/L. Therefore, the impingement and cavitation mixing techniques that happen in a jet loop reactor played an important role in enhancing sulphate removal.

Hydration of a silica fume blended low-alkali shotcrete cement

Ettringite and C–S–H are the main hydrates formed during the hydration of the low-alkali cement “ESDRED” consisting of 60% CEM I, 40% microsilica and 4.8% set accelerator. Small quantities of portlandite and hemicarbonate present as intermediate phases destabilise within a few weeks. The use of a set accelerator leads to massive ettringite precipitation, a moderate decalcification of C–S–H and reduction of pH due to presence of dissolved formate.The slow reaction of the silica fume during hydration decalcifies the C–S–H and decreases the alkali concentration to 30mM and the pH value of the pore solution to 11.5 after 1year and longer. The further reaction of the silica fume is expected to be slow and to result in a decrease of pH to 11. Further, the destabilisation of ettringite to thaumasite is expected. The long-term stability of C–S–H and the pH of approximately 11 make ESDRED a good candidate for usage in contact with the clay-based barriers of a repository for radioactive waste.

The Tournemire industrial analogue: reactive-transport modelling of a cement–clay interface

AbstractIn the post-closure period of a geological disposal facility for radioactive waste, leaching of cement components is likely to give rise to an alkaline plume which will be in chemical disequilibrium with the host rock (which is clay in some concepts) and other engineered barrier system materials used in the facility, such as bentonite. An industrial analogue for cement-clay interaction can be found at Tournemire, southern France, where boreholes filled with concrete and cement remained in contact with the natural mudstone for 15–20 years. The boreholes have been overcored, extracted and mineralogical characterization has been performed. In this study, a reactive-transport model of the Tournemire system has been set up using the general-purpose modelling tool QPAC. Previous modelling work has been built upon by using the most up-to-date data and modelling techniques, and by adding both ion exchange and surface complexation processes in the mudstone. The main features observed at Tournemire were replicated by the model, including porosity variations and precipitation of carbonates, K-feldspar, ettringite and calcite. It was found that ion exchange needed to be included in order for C-S-H minerals to precipitate in the mudstone, providing a better match with the mineralogical characterization. The additional inclusion of surface complexation, however, led to limited calcite growth at the concrete-mudstone interface unlike samples taken from the Tournemire site that have a visible line of crusty carbonates along the interface.

Reactive transport modeling of the clogging process at Maqarin natural analogue site

The Maqarin site in Jordan has been investigated for three decades as a natural analogue for the long term changes of materials in contact with hyper-alkaline solutions. Similar processes are expected in radioactive waste disposal sites, where cement based materials are in contact with natural rocks or other e.g. clay based materials. In this context, a numerical reactive transport model was used to study local geochemical alterations and induced porosity changes for the Maqarin marl rock in contact with the hyper-alkaline solution. The geochemical setup for the rock mineralogy and the pore water was calibrated to match measurements from the Maqarin site. The setup includes several clay and zeolite minerals, considers cation exchange processes, and a state-of-the-art model for cement phases. Similar to earlier calculations by Steefel and Lichtner (1998) who used a much simpler geochemical model, the pore clogging occurred after several hundred years at a distance of 5–10mm from the contact to the hyper-alkaline solution. In our calculations, this was caused by a massive precipitation of ettringite and C–S–H minerals. We performed a sensitivity study by varying the intrinsic diffusion coefficient, the Archie’s law exponential factor, and the mineral surface area available for dissolution and precipitation. We found that the dissolution of clay minerals controls the availability of Al, which is needed for ettringite and C–S–H phase precipitation. Thus, the amount and kinetically controlled dissolution of clay minerals controls the spatial and temporal evolution of porosity changes. The simulations reveal that neither cation exchange processes nor the formation of zeolite minerals strongly influence the geochemical evolution of the system.

The early hydration of Ordinary Portland Cement (OPC): An approach comparing measured heat flow with calculated heat flow from QXRD

Heat flow was calculated from XRD data and compared with measured heat flow from calorimetric experiments. It was shown that the heat released during the hydration of a commercial Ordinary Portland Cement can be assigned mainly to three mechanisms, the silicate reaction (sum of dissolution of alite and precipitation of C-S-H-phase and portlandite), the dissolution of C 3A, and the precipitation of ettringite. The contributions made by anhydrite dissolution and gypsum dissolution to the heat released during hydration turned out to be quite small. It is possible to explain, on the basis of the data produced, the origin of the heat flow curve of the cement used.

Coupled Thermochemical Effects on the Strength Development of Slag-Paste Backfill Materials

In this paper, the coupled effects of sulfate (chemical factor) and curing temperature (thermal factor) on the strength development of cemented paste backfill (slag-CPB) that contains ground granulated blast furnace slag (slag) as a mineral admixture is studied. Almost 200 slag-CPB samples, cured at temperatures of 2, 20, 35, and 50°C at 28, 90, and 150 days with sulfate concentrations of zero ppm, 5,000 ppm, 15,000 ppm, and 25,000 ppm are tested for uniaxial compressive strength (UCS). The results show that the effect of sulfate on the strength of slag-CPB is significantly dependent on the curing temperature, initial sulfate content, and curing time. The coupled effects of sulfate and temperature can lead to an increase or decrease of the slag-CPB strength and significantly influence the type and amount of minerals formed within the cemented matrix of slag-CPBs. The strength-increasing factors are (1) refinement of the pore structure of slag-CPBs due to the precipitation of ettringite within the empty po...

Excessively Wet Subgrade Improvement with 100% Industry By-Products for Heavy Traffic Pavement – II. Mechanical Behavior and Durability of Stabilized Soils

The workability of excessively wet subgrade can be effectively improved by using a “green materials” based entirely on industry byproduct of combination of fly ash (FA) with carbide lime (CL) and flue gas desulfurization gypsum (FGDG). This paper presents the results of a further systematic investigation on the mechanical behavior and durability of FA/CL/FGDG treated silt from Dezhou in Shandong province. FA, CL and FGDG were added at 3 levels in percentages of 8%-24%, 2%-6% and 1%-3% by dry weight of the soils, respectively. The unconfined compressive strength as well as splitting tensile strength (STS), volumetric expansion (VE), California bearing ratio (CBR), dry-wet cycles test and freeze-thaw cycles test were performed. The best strength of 0.802MPa was achieved for 24%FA/6%CL/1%FGDG treated soil, while the 16%FA/6%CL/1%FGDG treated soil also produce good strengths of 0.742MPa, after 7 days curing. STS keeps increasing with curing period in the first 8 weeks throughout the investigation. According to the VE test, a mellowing period may help prevent swelling from ettringite precipitation by letting these minerals form and hydrate before compaction. The FA/CL/FGDG treated soil has a better durability with strength loss ratio less than 15% after 10 wet-dry cycles, and the maximum weight loss around 25% after 10 freeze–thaw cycles. CBR decreases with decrease of compaction degree, but the values are still around 70 even in a poor compaction degree of 93%, meeting the general requirements of subgrade. So, if the silt subgrade is in excessively wet state and could not be compacted to the required density, the modified subgrade would still serve as a strong platform and could provide necessary support for pavement.

Physico-chemical investigation of clayey/cement-based materials interaction in the context of geological waste disposal: Experimental approach and results

Within the concepts under study for the geological disposal of intermediate-level long-lived waste, cement-based materials are considered as candidate materials. The clayey surrounding rock and the cement-based material being considered differ greatly in their porewater composition. Experiments are conducted on the diffusion of solutes constituting those porewaters in a confined clay/cement composite system using cells. The test temperature was set at 25 °C and 2, 6 and 12 months. Results supply new information: carbonation is low and not clog the interface. Such absence of carbonation allows for the diffusion of aqueous species and, thus, for the degradation of the cement paste and the illitisation of illite/smectite interstratifications. The cement material is subjected to a decalcification: portlandite dissolution and a CaO/SiO 2 reduction in the calcium silicate hydrate. The sulphate in diffusion induces non-destructive ettringite precipitation in the largest pores. After 12 months, about 800 µm of cement material is concerned by decalcification.

Immobilization and Long-term Evolution of Selenate in Portland Cement

AbstractThe long-term selenate uptake capacity of leached cement was studied by means of a replenishment batch experiment with cement pore water (CPW) doped with selenate. The corresponding blank experiment (without Se) was also done. The systems were studied for 31 cycles (18-days each cycle) to understand the long-term selenate immobilization in leached cement. Results showed that the retention capacity of leached cement exponentially decreases with cycle evolution. Precipitation of ettringite, identified by SEM and XRD, occurred along the experiments. The characterization of the cement solid phases indicated that selenate was only retained in the precipitated ettringite.Experimental data have been successfully modeled by assuming that selenate incorporates into the precipitating ettringite. Precipitation of ettringite is controlled by the kinetic dissolution of the initially present monocarboaluminate.

Effects of seawater on AAR expansion of concrete

Recently, AAR was identified in submerged piles of some bridges in tidal waters. Microstructural examination detected chloroaluminate salts in some cracks. To clarify whether seawater had influenced the deterioration an experimental program was planned to examine the effects of sodium chloride on AAR under various curing conditions. Concrete prisms containing either of highly-reactive, slowly-reactive or nonreactive aggregate, and either low or high alkali contents, were stored in saltwater (representing seawater) or at 100% RH, at temperatures of 38, 60 and 80 °C, for expansion measurement over 600 days, after which the temperature for those stored in saltwater was lowered to 23 °C, to check its effect on further expansion, which could be attributed to precipitation of ettringite and/or Ca-chloroaluminate. The results indicate that the type of aggregate and concrete alkali content had the greatest effect on AAR expansion. Exposure to saltwater did not have any significant effect on the AAR expansion.

Early C 3A hydration in the presence of different kinds of calcium sulfate

Hydration reactions of C 3A with various amounts of calcium sulfate hemihydrate, gypsum or a mixture of the two, were investigated by isothermal microcalorimetry, and a monitoring of the ionic concentrations of diluted suspensions. This study shows that sulfate type used modifies the early C 3A–CaSO 4 hydration products and the rate of this hydration. The fast initial AFm formation observed before ettringite precipitation in the C 3A–gypsum system is avoided as soon as hemihydrate is present in the suspension. This was attributed to higher super saturation degrees and then higher nucleation frequency with regard to the ettringite obtained in the presence of hemihydrate. Moreover, replacement of gypsum by hemihydrate also leads to an increase of the ettringite formation rate during at least the five first hours under experimental conditions.

Simplified coupled chemo‐mechanical modeling of cement pastes behavior subjected to combined leaching and external sulfate attack

AbstractIn the context of radioactive waste storage, underground concrete structures may be subjected to external sulfate attack due to penetration of sulfate combined with leaching. This paper proposes a coupled chemo‐mechanical modeling relying on a simplified chemical description of the solid phase composing the cement paste to address this issue. The sulfate and calcium species diffusing through the pore solution are supposed to completely control the chemical system, in particular the precipitation of gypsum and ettringite, which may lead to significant macroscopic expansions and cracking. The crystallization pressure generated by the growing crystals is introduced in the modeling through a poroelasticity analogy. Based on a representation of the cement paste of the matrix‐inclusion type, the application of the Mori–Tanaka (MT) scheme provides estimates of the mechanical and diffusive properties and of the interaction coefficient as a function of the degradation state. The obtained coupled system is applied to two‐dimensional numerical simulations of laboratory tests; the confrontation between experimental and computational results shows a good agreement in terms of both hydrated product evolutions as a function of time and macroscopic cracking appearance. Copyright © 2008 John Wiley & Sons, Ltd.

Cryo‐FIB‐nanotomography for quantitative analysis of particle structures in cement suspensions

Cryo-FIB-nanotomography is a novel high-resolution 3D-microscopy technique, which opens new possibilities for the quantitative microstructural analysis of complex suspensions. In this paper, we describe the microstructural changes associated with dissolution and precipitation processes occurring in a fresh cement paste, which has high alumina and sulphate contents. During the first 6 min, precipitation of ettringite leads to a general decrease of the particle size distribution. In the unhydrated cement paste almost no particles smaller than 500 nm are present, whereas after 6 min this size class already represents 9 vol%. The precipitation of ettringite also leads to a significant increase of the particle number density from 0.294*10(9)/mm(3) at t(0min) to 20.55*10(9)/mm(3) at t(6min). Correspondingly the surface area increases from 0.75 m(2)/g at t(0min) to 2.13 m(2)/g at t(6min). The small ettringite particles tend to form agglomerates, which strongly influence the rheological properties. The particular strength of cryo-FIB-nt is the potential to quantify particle structures in suspension and thereby also to describe higher-order topological features such as the particle-particle interfaces, which is important for the study of agglomeration processes.

Effects of sucrose and sorbitol on cement-based stabilization/solidification of toxic metal waste

The effects of sucrose or sorbitol addition on the hydration, unconfined compressive strength and leachability of Portland cement pastes containing 1% Pb and 1% Zn were studied as a function of time. Whereas Pb and Zn were found to shorten the time to achieve maximum hydration of Portland cement, the combination of these metals with 0.15 wt% sucrose or 0.40 wt% sorbitol retarded the setting of cement by at least 7 and 28 days, respectively, without affecting the strength at 56 days. The leachability of Pb and Zn evaluated by the TCLP 1311 protocol at 56 and 71 days was slightly reduced or unchanged by the addition of sucrose or sorbitol. SEM-EDS and XRD analyses revealed that ettringite precipitation was favored whereas the formation of CSH gel, which accounts for most of the strength of hydrated cement, was delayed in cement pastes containing both metals and sucrose or sorbitol. These results indicate that controlled additions of sucrose or sorbitol can add flexibility to the handling of cement-treated metal waste, particularly when it needs to be transported by truck or pipeline between the treatment plant and the disposal site, without affecting its long-term performance.

Treatment of mine tailings using ALFA® technology

To evaluate the success of the ALFA technology to stabilise and solidify the acid-producing tailings by means of ettringite formation, a tailings sample from a mine in the province of Quebec, Canada, was subjected to treatment and subsequent evaluation. The tailings sample was initially subjected to physical and chemical analysis and the optimised condition for the formation and precipitation of ettringite within the sample was established via X-ray diffraction analysis. To establish whether the pozzolanic reactions has occurred in the treated samples and monitor the growth of ettringite in the specimens under investigation, samples were cured for 3, 28, 90, 360, and 660 days and were analysed for their mineralogical constituents. The treated samples with the aluminium, lime and fly ash (ALFA) technology were subjected to different leaching tests. The leachants were simulated groundwater with a pH of 5·5; acid rain with a pH of 4·5, which contained sulphuric acid; a sulphuric acid solution with a pH of 2·5 (Modified TCLP); and acetic acid solution with pH of 2·88. In addition, treated samples were further evaluated for their stability via durability tests (strength, hydraulic conductivity, and cyclic freeze-thaw). Pour évaluer le succès de la technologie ALFA (Aluminium, Lime and Fly ash ou aluminium, chaux et cendres volantes) pour stabiliser et solidifier des stériles produisant de l'acide au moyen de formation d'ettringite, des échantillons de résidus miniers issus de la province de Québec, au Canada, ont été traités puisé tudiés. Des essais chimiques et physiques ont tout d'abord été réalisés sur les échantillons. Une analyse par diffraction de rayons X a permis d'établir les conditions optimales de formation et de précipitation d'ettringite dans le spécimen. Pour déterminer si les réactions pozzoloniques se sont produites dans les échantillons traités et contrôler la formation d'ettringite dans les spécimens àl'étude, les échantillons ont été mûris pendant 3, 28, 90, 360 et 660 jours avant que leurs constituants minéralogiques ne soit analysés. Différents tests de lixiviation TLCP (« toxicity characteristic leaching procedure ») ont été réalisés sur les échantillons traités avec la technologie ALFA. Les lixiviats considérés ont été les suivants: eau souterraine simulée de pH 5,5, pluie acide de pH 4,5 contenant de l'acide sulfurique, solution d'acide sulfurique de pH 2,5 (TLCP modifié) et solution d'acide acétique (TLCP) de pH 2,88. On a également évalué la stabilité des échantillons traités au moyen d'essais de durabilité (résistance, conductivité hydraulique, cycle de gel-dégel).

External sulphate attack by ground water

ABSTRACT Materials based on Portland cement react chemically with solutions containing sulphates, even at very low concentrations. The originality of the work described in this paper is to have characterised the kinetics of evolution of mineralogy and solution-material chemical exchanges, for a concentration 10.10−3 mol/L of sulphate, representative of underground conditions, which is a deep geological site made up of argillite. Under these chemical conditions which are maintained as constant as possible at the laboratory scale, the main experimental fact established is the precipitation of gypsum in the damaged zone for two cements with very different C3A contents, despite the low sulphate concentration of the aggressive solution. The gypsum forms immediately behind the dissolution front of portlandite, which itself spreads towards the core of the material at a rate of 0.15 mm.d−0.5. The amplitude of the precipitation of secondary ettringite is consistent with the C3A content of the cement. However, such...

Cementation of ILW ion exchange resins: Impact of sulfate ions released by radiolysis on hydrated matrix

Some of the ion exchange resins used during treatment of spent nuclear fuels are intermediate level radioactive wastes which may be damaged by radiolysis process, releasing sulfate ions directly into the cement-based encapsulating material. This work consists in an experimental study of the resulting sulfate attack on the properties of the hydrated matrix: dimensional stability, mineralogy and microstructure of the samples, as well as variations in the chemical composition of the curing solution, were studied during six months. Three sites of delayed ettringite formation were detected: into the cement matrix near the surface exposed to solution, localized in the interfacial transition zone between cement matrix and resins, or progressively replacing the portlandite that initially fulfilled the cracks of anionic resins. During the experiment period, the ettringite precipitation and the expansion detected were moderate, and did not lead to cracking. The material involved was considered as having a good resistance to sulfate attack.

Behavior of B, Cr, Se, As, Pb, Cd, and Mo present in waste leachates generated from combustion residues during the formation of ettringite

The behavior of B, Cr, Se, As, Pb, Cd, and Mo in the leachates generated from two combustion residues, coal-fired power plant fly ash and municipal solid waste incineration ash, during precipitation of ettringite is presented. Experiments also were performed using modeled waste leachates as well as controlled solutions containing all the investigated elements. Moreover, to determine the possible effect of pH, lime treatment was conducted using the waste and modeled leachates. Results indicated the removal of B, Se, and Cr from the leachates because of incorporation of their oxyanions in the ettringite structure. The removal of B could further be explained by considering the structure of ettringite and monosulfoaluminate. The removal of Pb also occurred, probably because of lime-induced precipitation of Pb(OH)2. Similarly, the removal of As was observed as a result of lime-induced precipitation of Ca3(AsO4)2 and incorporation into the ettringite structure. On the other hand, the precipitation of ettringite did not reduce the concentrations of Mo and Cd because of the complex nature of these elements in the leachates. The results also suggest that the presence of high amounts of constituents such as Cl-, an alkali metal in the solution, also affected ettringite precipitation behavior. Moreover, elemental speciations as well as the presence of other constituents in the solution affected the incorporation by ettringite.