Investigation Of Pozzolanic Binders Using Sulpho-Calcic Fly Ashes
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INVESTIGATION OF POZZOLANIC BINDERS USING SULPHO-CALCIC FLY ASHES J. PERA, A. SADR MOMTAZI and J. DEJEAN Laboratoire des Materiaux Mineraux, Bit. 307, INSA de Lyon, 20 Albert Einstein, 69621 Vileurbanne Cedex, France Received 21 November, 1988; refereed ABSTRACT Because of their high S03 content (-11%), sulfo-calcic fly ashes cannot be used as a cement admixture without pretreatment. The fly ashes contain mainly quicklime (CaO), anhydrite (CaSO 4) and larnite (13C 2 S). 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. INTRODUCTION To the present time, the use of sulfo-calcic fly ashes (SO3 content >10%) has been limited to road applications. The ettringite precipitation which occurs during the fly ash hydration prevents these ashes from being used in the concrete industry [1]. In this application, cracking quickly occurs, leading to the hydrated material splitting. On the other hand, in road applications, the porosity is high enough to enable a free development of ettringite crystals, without damage to the material. The present work aims to define the treatment conditions of the fly ash in order to use them as a Portland cement admixture or as a pozzolanic binder component. In these binders, fly ash can be associated with slag, siliceous ashes or calcined clays. RAW MATERIALS INVESTIGATION The chemical composition of the various raw materials is given in Table I. Figure 1 shows the XRD patterns of the sulfo-calcic fly ashes. Sulfo-calcic fly ash have been found to contain mainly quicklime (CaO), anhydrite (CaSO4) and di-calcium silicate (larnite: WC2S) associated with a vitreous phase. Calcined clay materials - i.e. metakaolin or calcined laterites - present a crystallographically disorganized phase after thermal treatment at moderate temperatures (700-800*C). This disorganization leads to an interesting reactivity with calcium hydroxide [2-61.
Mat. Res. Soc. Symp. Proc.
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