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

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.

Impact of Annealing on the Early Hydration of Tricalcium Silicate

Amélie Bazzoni, Marco Cantoni, Karen Scrivener

It was recently proposed that the induction period observed during the hydration of tricalcium silicate could be explained by the build-up of ions in solution. Due to the importance of defects in this mechanism, this work describes the effect of different annealing effects on the defect structure and hydration behavior of C3S. The impact of annealing on the crystal structure was checked by X-ray diffraction and the defect structure studied by transmission electron microscopy. The hydration kinetics were followed by isothermal calorimetry of pastes. Scanning electron microscopy was used to look at the microstructure formation. It was observed that grinding created a highly deformed layer on the surface of the grains, which disappeared after annealing. The defect structure was closely related to the length of the induction period observed in pastes by calorimetry. There was no observable effect on the morphology of C-S-H during hydration, but the number of calcium hydroxide nuclei was less in pastes from annealed material.

Wiley-Blackwell2014

Microstructural development of early age hydration shells around cement grains

Emmanuel Gallucci, Prakash Chandra Mathur, Karen Scrivener

An important microstructural aspect of the early hydration of Portland cement (K) is the formation of a shell of hydration products around cement grains. There is, at present, limited information on the mechanism of formation of the shell and of the chemistry of the phases that constitute the shells. Through the use of STEM imaging of early age hydrated cement pastes as early as 2 h, the present work shows that the shells correspond to the first C-S-H type product formed which has a distinct morphology compared to C-S-H formed later when the main reaction occurs (nucleation and growth stage at setting time). The shells form only around the silicate part of the grain and are not empty but filled with a fragile fibrous C-S-H which appears to have a lower (packing) density than the rest of the hydration products. The cement grains underneath the shells are seen to react unevenly and the hydration seems to follow a reaction front, leaving striations up to 1 mu m deep on the grains. Over the long term, the original fragile product seems to densify and gives rise to the usual inner C-S-H. High resolution EDS chemical analysis and mappings were used to get insight into the chemistry associated with the formation of these early age products. The C/S ratio of all C-S-H (inner and outer shell) is the same (within the limits of the analysis accuracy) and evolves insignificantly over the first 24 h of hydration. High concentrations of sulfate are associated with the C-S-H formed during the early development of the microstructure, but these decrease later, the sulfate being mainly incorporated into ettringite. (C) 2009 Elsevier Ltd. All rights reserved.

2010

Fe-containing Hydrates and their Fate during Cement Hydration

Belay Zeleke Dilnesa

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.

EPFL2012

Fe-containing Hydrates and their Fate during Cement Hydration

Belay Zeleke Dilnesa

EPFL2012