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Effects of superplasticising admixtures used with blended cements: part two

World Cement,

Cements with fly ash

Readily soluble fly ash constituents, e.g. alkali and sulfate ions, counteracted the previously mentioned dilution effect. Two siliceous flyashes, one with more readily soluble sulfates (V11) and one with less (V21) were used to produce cements with 55 mass % fly ash (CEM IV/B (V)). In combination with both cements the duration of plastification of the PCE 11 was tested. On account of the virtually unchanged concentration of sulfate ions in the pore solution of the cement with V11 (Z55V11) compared to the Portland cement CEM I and the more or less identical zeta potential, the effect of PCE 11 used along with this cement was similar to that with Portland cement. Initially, PCE 11 remained for the most part not adsorbed in the pore solution. Sorption took place over the test duration and the cement paste was plasticised. Used along with the cement with V21, the lower sulfate ion concentration and the less negative zeta potential led to greater sorption of PCE 11 and to an intensified stiffening of the cement paste. Compared to the readily soluble constituents, the content of residual coke of fly ash (V11: 0.9 %, V21: 3.3 %) did not significantly influence the dispersing effect of the PCE.

Cements with calcined clay

The clays were calcined at the particular temperature necessary to meet the requirement of EN 197-1 of at least 25 mass % reactive SiO2. The calcined clays exhibited hardly any readily soluble constituents. Due to the flaky platelet particle structure, their BET specific surface was larger compared to e.g. fly ash. An increasing proportion of calcined clay in the cement resulted in an increased superplasticiser demand. Not decomposed, residual clay minerals in calcined clay further increased the cement’s PCE demand. Cements with properly calcined clay (low BET specific surface) could be plasticised with almost comparable or even lower PCE dosages. This emphasises the importance of adjusting the calcining conditions to the particular clay to reduce the water requirement and superplasticiser demand of the corresponding cement and thus enhance the workability of concrete.

This is part 2 of a 2-part article that was originally published in Newsletter 3/2015 of the European Cement Research Academy and is reproduced by kind permission of ECRA.

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