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Investigation Of Pozzolanic Binders Using Sulpho-Calcic Fly Ashes

Published online by Cambridge University Press:  21 February 2011

J. Pera ,
A. Sadr Momtazi  and
J. Dejean
J. Pera
Affiliation:
Laboratoire des Matériaux Minéraux, Bât. 307, INSA de Lyon, 20 Albert Einstein, 69621 Vileurbanne Cedex, France
A. Sadr Momtazi
Affiliation:
Laboratoire des Matériaux Minéraux, Bât. 307, INSA de Lyon, 20 Albert Einstein, 69621 Vileurbanne Cedex, France
J. Dejean
Affiliation:
Laboratoire des Matériaux Minéraux, Bât. 307, INSA de Lyon, 20 Albert Einstein, 69621 Vileurbanne Cedex, France
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Abstract

Because of their high SO3 content (∼11%), sulfo-calcic fly ashes cannot be used as a cement admixture without pretreatment. The fly ashes contain mainly quicklime (CaO), anhydrite (CaSO4) and larnite (BC2S). At ambient temperatures and at normal consistency, their hydration leads to the formation of ettringite, slaked lime and gypsum. As a result, cracking and splitting of the hydrated product quickly occurs due to the crystallization of the ettringite.

The ability of these fly ashes to produce Portlandite, Ca(OH)2, led the authors to investigate pozzolanic binders by the addition of artificial pozzolans — siliceous fly ashes, slag and clay minerals — which had been calcined at moderate temperatures.

The sulfo-calcic fly ashes are ground, then slaked either in ambient or hydrothermal conditions (2 or 6 hours at 130°C). In the case of slaking under ambient conditions, ettringite is quickly produced with a definitive crystallographical structure. As a result, no volume change is observed after re-hydration of the material. By comparison, autoclave slaking does not lead to this phenomenon, and only slaked lime is produced.

The rate of hydration is studied through the development of compressive strength and by analyses of the products by differential thermal analysis (DTA) and X-ray diffraction (XRD) at various hydration times. The best results are obtained using calcined laterite or blast furnace slag.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 136: Symposium O – Fly Ash and Coal Conversion By-Products V , 1988 , 169
Copyright
Copyright © Materials Research Society 1989

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References

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