Abstract
High early strength mortars or concretes can be produced with high alumina cement. However, at later ages, particularly at temperatures higher than 20°C, there is a strength loss caused by a transformation of hexagonal hydrates into the cubic form.
Silica fume is capable of hindering or blocking this transformation. However, a water reducing admixture is required to counterbalance the higher amount of mixing water caused by the very high specific surface area of silica fume.
Since traditional water reducers (based on sulphonated naphtalene or melamine polymers) are not effective with high alumina cement, two alternative candidates have been tested: sodium tripoliphosphate and carboxylic acrylic ester.
The sodium tripoliphosphate admixture is capable of reducing the mixing water; however, the simultaneous presence of silica fume and sodium tripoliphosphate causes a surprising and not yet fully-explained strength loss (only at 20°C with liquid water), although the transformation of hexagonal hydrates into the cubic form is truly hindered.
In the presence of the carboxylic acrylic ester admixture, the amount of mixing water is reduced without any side effect on the strength loss of the high alumina cement-silica fume system. Therefore, the combination of silica fume with carboxylic acrylic ester appears to be a successful solution to produce mortars or concrete mixes based on high alumina cement content without any strength loss at later ages.
Résumé
Des mortiers et des bétons développant des résistances mécaniques élevées à court terme peuvent être produits avec les ciments alumineux. Toutefois, au cours du temps, en particulier pour des températures supérieures à 20°C, une diminution de la résistance mécanique survient suite à la transformation des hydrates hexagonaux en structure cubique.
La fumée de silice est capable de ralentir ou de bloquer cette transformation. Toutefois, il faut ajouter un réducteur d'eau afin d'équilibrer la plus haute teneur en eau de gâchage requise par la surface spécifique importante de la fumée de silice. Puisque les réducteurs d'eau traditionnels (basés sur le naphtalène sulphonate ou sur la mélamine) n'ont pas d'action sur les ciments alumineux, deux produits alternatifs ont été testés: le tripolyphosphate de sodium et l'ester carboxylique acrylique.
Le tripolyphosphate de sodium est capable de réduire l'eau de gâchage, mais la présence simultanée de fumée de silice et de tripolyphosphate de sodium conduit à une surprenante et non encore expliquée diminution de la résistance (seulement à 20°C avec l'eau liquide), bien que la transformation des hydrates hexagonaux en hydrate cubique soit vraiment limitée.
En présence d'ester carboxylique acrylique, la quantité d'eau de gâchage est réduite sans avoir de diminution de la résistance mécanique du système ciment alumineux-fumée de silice. C'est pourquoi la combinaison fumée de silice-ester carboxylique acrylique semble être une bonne solution pour produire des mortiers ou des bétons ne subissant pas de perte de résistance à long terme.
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Editorial note Prof, Saveria Monosi and Prof. Marío Collepardi are working at the Department of Materials and Earth Science, University of Ancona, Italy, which is a RILEM Associate Member. Prof. Collepardi is a member of the Board of Advisors for Materials and Structures.
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Monosi, S., Troli, R., Coppola, L. et al. Water reducers for the high alumina cement-silica fume system. Mat. Struct. 29, 639–644 (1996). https://doi.org/10.1007/BF02485972
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DOI: https://doi.org/10.1007/BF02485972