Abstract
A microstructural study of mortars prepared with a low-alkali, low-C3A cement and a Class F fly ash, both of Swedish origin, was carried out using the scanning electron microscopy-energy-dispersive X-ray analytical technique. Supplementary phase analyses were made by X-ray diffraction and thermogravimetry-differential thermal analysis. Normally, CH crystals in the transition zone grow with their c axis parallel (or the (0 0 1) cleavage plane perpendicular) to the aggregate surface. The encapsulation of the fly ash particles by the growing CH reduces the amount of orientated CH at the aggregate-paste interface. The growth mechanism of these crystals is discussed. The reduction of CH, most significant after 28 days of hydration, is mainly due to the reaction of CH with the fly ash glass phase. Initially, the replacement of cement by fly ash weakens the paste-aggregate interfacial zone due to reduction of contact points, and increases the local water-to-cement ratio. This, however, improves significantly when the fly ash has reacted. In order to enhance the reaction of fly ash, extra gypsum was added. The results show that gypsum can accelerate the fly ash reaction, but the products formed, and the beneficial effects of gypsum, are mainly determined by the total amount of gypsum in the paste.
Resume
Une étude microstructurale de mortiers préparés à partir de ciment à faible teneur en alcali et en C3A ainsi que de cendres volantes de classe F (tous deux d’origine suédoise) a été menée en se servant du microscope électronique à balayage et de la technique d’analyse de rayons X à dispersion d’énergie. Les phases supplémentaires étaient étudiées par thermogravimétrie et analyse thermique différentielle.
Normalement, les critaux de CH croissent dans la zone de transition en sorte que leur axe est parallèle à la surface du granulat (ou le plan de clivage (0 0 1) perpendiculaire à la surface). En croissant le CH recouvre les cendres volantes, ce qui réduit la quantité de CH orienté à l’interface pâte-granulat. La diminution de CH, très importante après 28 jours, semble dépendre surtout de la réaction entre les cristaux de CH et la phase vitreuse des cendres volantes.
Au début de l’hydratation, l’addition de cendres volantes affaiblit la zone d’interface entre la pâte et le granulat en réduisant les points de contact. Le rapport eau-ciment local est aussi augmenté. Une fois que la réaction des cendres volantes commence, la situation s’améliore de façon significative. Afin d’augmenter la réactivité des cendres volantes, on a ajouté du gypse. Les résultats démontrent que, bien que le gypse puisse accélérer la réaction des cendres volantes, la formation de produits de réaction, ainsi que les avantages, dépendent de la quantité totale de gypse dans la pâte.
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Xu, A., Sarkar, S.L. & Nilsson, L.O. Effect of fly ash on the microstructure of cement mortar. Materials and Structures 26, 414–424 (1993). https://doi.org/10.1007/BF02472942
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DOI: https://doi.org/10.1007/BF02472942