High Alumina Cement-Silica Fume Mixtures in the Presence of Superplasticizers

In the presence of silica fume (SF), the hydration of high alumina cement (HAC) produces hexagonal hydrates (CaH10 and C2AH8) which do not convert into the cubic hydrate (C3AH6) and therefore the related strength loss does not occur: this is due to the formation of C2ASH8 which blocks the conversion process. However, due to the presence of SF, the required mixing water significantly increases. Therefore, an effective water reducing admixture is needed to compensate for the presence of SF and to allow the mixture to attain the same strength level of pure HAC at early and later ages. Two alternative admixtures - sodium tripolyphosphate (TPP) and/or acrylic polymer (AP)< instead of other traditional superplasticizers - were studied as water reducers for the blended HAC-SF binder. Both were very effective in reducing the amount of mixing water. However, in the presence of TPP there was a quick fluidity loss after about 45 min. This was a sort of flash set of the binder followed by a sudden heat development. Due to the restrained thermal expansion of the hotter nucleus with respect to the colder surface areas, microcracks formed. Upon contact with liquid water microcracks changed to macrocracks. This severe distress caused a strength failure of the specimens. This change was produced by the pressure exerted by crystal growth of hexagonal hydrates in the water-filled microcracks. By reducing the fluidity loss - for instance by using AP instead of TPP - the heat development became more gradual and therefore microcracks as well as macrocracks disappeared. Consequently any strength loss was removed in the blended HAC-SF binder.