Water reducers for the high alumina cement-silica fume system

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 naphtalenc 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 2O0C 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 cementsilica 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.