Effect of replacement of silica fume with calcined clay on the hydration and microstructural development of eco-UHPFRC

Wei Huang, Karen Scrivener, Wei Sun
Elsevier Sci Ltd, 2017
Journal paper

In this paper, the replacement of high cost silica fume with calcined clay was investigated in UHPFRC mixes. The impact of two grades of calcined clay on the hydration and microstructural development of UHPFRC matrices was investigated, where 54% of the cement by volume has already been replaced by a limestone with similar PSD. It was seen that comparable mixes were obtained when silica fume was replaced with calcined clay by volume. The experimental results revealed a competition for water between the cement hydration and pozzolanic reaction of kaolinite to form more AFt and AFm phases, which affected the compressive strength development of the UHPFRC matrices. This competition is more pronounced in mixes where the kaolinite content increases in the calcined clay. (C) 2017 Elsevier Ltd. All rights reserved.

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Wei Huang, Karen Scrivener, Wei Sun
Elsevier Sci Ltd, 2017
Journal paper

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https://infoscience.epfl.ch/record/228657?ln=en

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Pozzolanic reactivity of low grade kaolinitic clays: Influence of calcination temperature and impact of calcination products on OPC hydration

Adrian Alujas Diaz, Rodrigo Fernandez Lopez, Karen Scrivener

The influence of thermal activation temperature in the pozzolanic reactivity of low grade kaolinitic clay and the impact of calcination products in OPC hydration are assessed in this paper. The raw material, with approximately 40% kaolinite and 40% of 2:1 clay minerals, was calcined to temperatures ranging between 500 and 1000 degrees C. The clay and its calcined products were characterized by XRD, XRF, FTIR, TGA-DTA, Al-27 NMR, PSD and BET. The pozzolanic reactivity of the calcined products and its impact on hydration of blended systems was assessed by compressive strength in mortars with 30% substitution of OPC and heat output from lime-pozzolan pastes. The influence of calcined clays on OPC hydration was characterized by XRD, Al-27 NMR and Si-29 NMR, and Isothermal Calorimetry. The increase of pozzolanic reactivity with the increase of activation temperature is limited not only by recrystallization above 900 degrees C, but also by the decrease of the specific surface that occurs between 800 and 900 degrees C. Highest pozzolanic reactivity is reached when calcined at 800 degrees C, a temperature representing the best compromise between structural disorder of the clay fraction and its specific surface. The impact of calcined clays on the hydration of blended systems is a combination of a filler effect at early ages and the pozzolanic reaction. Calcined clays favor the formation of AFm-type phases and C-A-S-H with longer chains, in which Al is substituting for Si. The experimental results indicate that clays with moderate contents of kaolinite constitute a potential source of high reactivity pozzolanic materials. (C) 2015 Elsevier B.V. All rights reserved.

Elsevier Science Bv2015

Pozzolanic activity of mechanochemically and thermally activated kaolins in cement

Karen Scrivener, Ruben Anton Snellings

This paper reports on the mechanochemical activation by intensive grinding as an alternative way to produce pozzolanically reactive metakaolin. The physicochemical properties, pozzolanic reactivity and impact on cement hydration of mechanochemically and thermally activated kaolinitic clays were compared. Mechanochemical activation of kaolin led to an amorphous hydrous material with increased specific surface area and high water content. Unlike in thermally activated kaolins, the Al coordination environment was partially retained during mechanochemical treatment. At low clinker replacement levels of 10%, the use of mechanochemically activated kaolins accelerated the hydration of the blended cement. The pozzolanic reaction reduced portlandite contents strongly and from early on. The combined effect of enhanced cement hydration and pozzolanic reaction at early ages was reflected in a higher early age strength development (up to 7 days). The differences between mechanochemically and thermally activated kaolins largely disappeared at later ages. (C) 2015 Elsevier Ltd. All rights reserved.

Pergamon-Elsevier Science Ltd2015

Influence of limestone on the hydration of Portland cements

Emmanuel Gallucci, Barbara Lothenbach, Karen Scrivener

The influence of the presence of limestone on the hydration of Portland cement was investigated. Blending of Portland cement with limestone was found to influence the hydrate assemblage of the hydrated cement. Thermodynamic calculations as well as experimental observations indicated that in the presence of limestone, monocarbonate instead of monosulfate was stable. Thermodynamic modelling showed that the stabilisation of monocarbonate in the presence of limestone indirectly stabilised ettringite leading to a corresponding increase of the total volume of the hydrate phase and a decrease of porosity. The measured difference in porosity between the "limestone-free" cement, which contained less than 0.3% CO2, and a cement containing 4% limestone, however, was much smaller than calculated. Coupling of thermodynamic modelling with a set of kinetic equations which described the dissolution of the clinker, predicted quantitatively the amount of hydrates. The quantities of ettringite, portlandite and amorphous phase as determined by TGA and XRD agreed well with the calculated amounts of these phases after different periods of time. The findings in this paper show that changes in the bulk composition of hydrating cements can be followed by coupled thermodynamic models. Comparison between experimental and modelled data helps to understand in more detail the dominating processes during cement hydration.

2008

Cement

A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand

Clay

Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, Al2Si2O5(OH)4). Clays develop plasticity when wet but can be harde

Experiment

An experiment is a procedure carried out to support or refute a hypothesis, or determine the efficacy or likelihood of something previously untried. Experiments provide insight into cause-and-effe