Fe-containing Hydrates and their Fate during Cement Hydration

Belay Zeleke Dilnesa
EPFL, 2012
Thèse EPFL

Résumé

Thermodynamic modeling is a versatile tool for predicting the chemical composition cement during the hydration of cement. The quality of the thermodynamic modeling depends directly on the quality and completeness of thermodynamic database used. One of the main limitations of modeling the hydration of cement is the lack of thermodynamic data for Fe containing hydrates. In addition, the formation of solid solutions between Fe- and Al-containing hydrates could stabilize mixed solids. However, it is unclear to what extent such solid solution formation occurs. Also experimentally it is very difficult to identify Fe-containing hydrates in hydrating cements by standard analytical techniques as the signals from Fe-containing phases significantly overlap with those from the corresponding Al-containing phases. Thus, in this study, potential Fe-containing hydrates like Fe-hemicarbonate (Fe-Hc), Fe-monocarbonate (Fe-Mc), Fe-monosulphate (Fe-Ms), Fe-Friedel's salt (Fe-Fr), Fe-strätlingite (Fe-St), Fe-katoite (C3FH6 ) and Fe-siliceous hydrogarnet (Fe-Si-Hg) were synthesised at 20, 50 and 80 °C. The solid phases were characterized by X-ray powder diffraction (XRD), Thermogravimetric analysis (TGA), scanning electron microscopy (SEM), vibrational spectroscopy (Raman and Infrared spectroscopy) and Extended X-ray absorption fine structure spectroscopy (EXAFS). The compositions of the liquid phases were analyzed using inductively-coupled plasma optical emission spectrometry and mass spectrometry (ICP-OES and MS). At ambient temperature Fe-Mc, Fe-Ms, Fe-Fr and Fe-Si-Hg were stable, while Fe-Hc, Fe-katoite and Fe-St were metastable. Fe-Mc, Fe-Ms, Fe-Fr and Fe-Si-Hg were stable also at 50 °C, but the Fe-AFm phases were unstable at 80 °C while Fe-Si-Hg were stable up to above 100 °C. The measured composition of the liquid phase was used to calculate the solubility products at 20 and 50 °C and to derive the data for standard conditions (25 °C, 1 atm). The solubility products of Fe-Fr was similar to the solubility product of Al-Fr, while the solubility products of Fe-Mc and Fe-Ms were about 3 log unit lower than that of Al-Mc and Al-Ms indicating that in Fe-Friedel's salt is probably not stable in cements. The very low solubility product of Fe-Si-Hg (5 to 7 log units lower than that of Al-Si-Hg) implies that Fe-Si-Hg could be a stable phase in hydrated cements. Also the mixed Al- and Fe-containing hydrates were synthesized to study the extent of solid solution formation. Both XRD and thermodynamic modelling of the liquid compositions indicated that Al- and Fe-monosulphate and Al- and Fe-Friedel's formed solid solutions with a miscibility gap, while Al- and Fe- monocarbonate existed as two separate hydrates due to their different crystal structure (Al-Mc: monoclinic, Fe-Mc: rhombohedral). The formation of solid solution between Al and Fe-siliceous hydrogarnet seemed probable. To understand to what extent the findings from the synthesised hydrates were relevant for real cements, the speciation of iron was determined in hydrating cement using EXAFS spectroscopy. Identification of Fe-containing hydrates and quantification of their contributions was achieved by combining principal component analysis with iterative target tests, and linear combination. The results show that several Fe species already contributed to the overall Fe K-edge spectra of cement pastes during the first day of hydration. While ferrite was the dominant Fe-containing phase in the unhydrated cement, Fe-hydroxide was detected shortly after starting the hydration process. With time the formation of stable Al/Fe-siliceous hydrogarnet was observed, while the amounts of Fe-hydroxide and ferrite clinker slowly decreased. The latter finding agrees with results from thermodynamic modeling of the hydration process, which predicts formation of stable Al/Fe-siliceous hydrogarnet in cement system. The determination of the solubility products of these hydrates will help to extend the thermodynamic data base of cement minerals and establish whether and to which extent Fe-containing hydrates are stable in fresh and in leached cementitious systems. The results from this study on the Fe speciation in cementitious systems are important for a better understanding of cement-water interactions with a view to the durability of cementitious materials.

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https://infoscience.epfl.ch/record/170276?ln=fr

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Belay Zeleke Dilnesa
EPFL, 2012
Thèse EPFL

Résumé

Official source

https://infoscience.epfl.ch/record/170276?ln=fr

À propos de ce résultat

Concepts associés (12)

Concepts associés

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Publications associées (17)

Publications associées

Chargement

Impact of water activity on the mineralogy of hydrated cement

Luis Guillermo Baquerizo Ibarra

Cement hydrates and their chemically bound water content are sensitive to changes in relative humidity (RH) and temperature. This may cause specific solid volume changes affecting dimensional properties of hydrated cement paste such as shrinkage, swelling and expansion, and therefore impact the performance and the transport properties of cementitious materials. The present thesis studies the impact of drying conditions on the structure and thermodynamic properties of crystalline cement hydrates in different hydration states (i.e. varying molar water contents). The cement hydrates studied include the most important AFm and AFt phases present in different types of cements. A novel multi method approach, including XRD, TGA, DSC, sorption balance measurements, sorption calorimetry and the hydrate pair ¿ humidity buffer method, was used to derive physico-chemical boundary conditions and the thermodynamic properties of the studied hydrated phases. The stability and hydration states of AFm phases depend on the anion content and the exposure conditions. Some phases such as monosulfoaluminate and hydroxy-AFm are very sensitive to different relative humidities. On the other hand monocarboaluminate and strätlingite present very good volume stability with varying RH. Ettringite shows a strong hysteresis during desorption/adsorption, and the thermodynamic properties associated with its decomposition and reformation were determined. The experimental results were included into a thermodynamic model capable of predicting the response of cementitious systems considering the ¿true¿ crystal water content on the final phase assemblage in two different scenarios: i) during hydration, even at conditions where the amount of water added was insufficient for complete hydration, and ii) during drying and re-wetting processes in already hydrated systems. Our work hence opens the possibility to model the response of various cement materials exposed to different climatic conditions and to engineer cementitious systems with respect to minimizing volume changes in the course of drying, which may positively impact properties like drying shrinkage and total porosity.

EPFL2015

Chargement

Effect of mixing on the early hydration of alite and OPC systems

Emmanuel Gallucci, Patrick Juilland, Aditya Kumar, Karen Scrivener

The kinetics of hydration of cementitious materials is sensitive to the mixing procedure. High shear mixing conditions lead to an increase in the kinetics of hydration at early age compared to low shearing conditions such as hand mixing. In this study the effect of mixing speed and procedure was studied on alite and Portland cement in the presence or not of aggregates. The kinetics of hydration was monitored using isothermal calorimetry at 20 degrees C. The early reactivity was enhanced both with an increase in the speed of mixing and the shearing conditions. The principal features are a shortening of the induction period; a higher rate of hydrate precipitation during the acceleration period as well as an increase in the height of the main heat evolution peak. Analysis of the results in terms of dissolution theory, coupled with quantitative simulation with the plc modelling platform indicate different effects of mixing prior to and after the end of the induction period. Before the end of the induction period mixing has an impact on the rate of dissolution in the fast dissolution regime and high undersaturation, which appears to be (at least partially) controlled by the rate of transport of ions away from the alite surface. After the end of the induction period the main effect of mixing appears to be the production of more C-S-H nuclei, due to the possible detachment of the primary C-S-H (metastable) by mechanical action. This higher nucleation density leads to a denser microstructure for systems mixed at high intensities. (C) 2012 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2012

Chargement

Calcium silicate hydrate (C-S-H) is the main hydration product in Portland and blended cements, and greatly affects durability and mechanical properties of the hydrated cement. In the presence of Al-rich supplementary cementitious materials (SCMs), C-(A-)S-H can contain more Al than C-S-H in plain Portland cements. The use of SCMs in cementitious system not only lowers CO2 emission, but may also enhance the mechanical properties in the long term, its durability and can help to suppress alkali silica reaction. However, important questions regarding the effect of Al, alkali and equilibration time on C-A-S-H structure and solubility are still not fully understood. This thesis focuses on the interplay between aluminium, alkali hydroxides in solution, the structure of C-A-S-H and aluminium, alkali uptake by C-A-S-H and kinetics of C-A-S-H formation. For aluminium free C-S-H, KOH and NaOH have a similar effect, they increase pH values and silicon concentrations, and decrease calcium concentrations. At higher alkali hydroxide concentrations, more portlandite precipitates, while amorphous silica dissolves. This increases the Ca/SiC-S-H at low Ca/Sitarget but lowers the maximum Ca/SiC-S-H from 1.5 to 1.2 in 1 M KOH/NaOH. The amount of alkalis bound in C-S-H increases with increasing alkali hydroxide concentrations and is higher at low Ca/SiC-S-H. KOH/NaOH lead to a structural rearrangement in C-S-H, increasing the interlayer distance, number of layers stacked in c direction and shortening the silica chains. Comparison with the independently developed CASH+ thermodynamic model showed a good agreement between the observed and modelled changes, including the shortening of the MCL.For C-A-S-H with Ca/Si=1, the presence of Al led to higher amounts of secondary phases, larger interlayer distance, longer silicate chain, higher dissolved Al and Si, and lower Ca concentrations. The amount of secondary phases is reduced at higher pH; strätlingite and Al(OH)3 dominate at lower pH, and katoite at higher pH values. High pH is also associated with shortened silicate chain length, increased Al and Si concentrations, maximum Al/SiC-S-H and lower Ca concentrations. The Al uptake in C-A-S-H increased with aluminum concentration in solution, indicating the predominance of a common Al sorption mechanism independent of dissolved Al concentration or pH values. XANES data showed the presence of both Al(IV) and Al(VI) with different chemical environments in C-S-H.The effect of equilibration time on C-A-S-H was investigated from 6 hours to 3 years. Rapid changes within the first day were observed in both the solid phase and the aqueous phase. Portlandite was formed in all samples with different Ca/Si and its amount decreased rapidly within the first day and remained stable after more than 7 days. Gismondine-P1 precipitated between 1 and 3 years and destabilized the C-A-S-H phases at target Ca/Si 0.6. In the solid phase, the Al migration from secondary phases to C-A-S-H and an increasing fraction of Al(V) and Al(VI) in C-S-H were observed with longer equilibration time. The Ca/Si ratio of C-S-H played an important role on the extent of changes in aqueous concentration: an increasing of Al concentrations with time was observed in C-A-S-H with Ca/Sitarget=0.6, while a decrease was observed for C-A-S-H with Ca/Sitarget=1.0. The uptake of Al and Na in C-A-S-H was observed to decrease after 4 days and 1 day separately, which indicates a rearrangement of C-A-S-H with time.

EPFL2022

Chargement

Impact of water activity on the mineralogy of hydrated cement

Luis Guillermo Baquerizo Ibarra

EPFL2015

EPFL2022