The studies of the effect of sulfates added as chromium(VI) reducers in portland cement

Abstract

The calorimetric measurements were applied in testing the effect of some sulfates, used as Cr(VI) reducers in cement, as setting and hardening modifiers. The iron(II) sulfate is most commonly added as Cr reducer to cement on grinding. This was taken as a reference in the studies of the other potential chromium reducers, such as tin(II) and manganese(II) sulfates on cement hydration. The high percentage of admixtures was reduced steadily from very high overdosage—to find the possible effect of non-homogeneity resulted from the hygroscopic character of compounds used and to detect the possible products which can be formed—to relatively small quantity, as used in practice. The progress of cement hydration was investigated by calorimetry and chemical shrinkage measurements. The rheological properties of cement paste admixtured with iron, tin, and manganese sulfates were investigated, as well as the phase composition of hydrated pastes was studies by XRD. The compressive strength of the small paste cylinders was measured. Finally, the hydrated samples were subjected to the SEM observations. The tin sulfate showed the strongest retarding action as it was proved by calorimetry and chemical shrinkage data, as well as by strength and rheological measurements; however, at small quantities, this compound has a positive impact on setting and hardening. The detrimental effect of overdosed Mn and Fe sulfates due first of all to the formation of higher amount of ettringite at very early age was found. This can be proved additionally by the change of rheological parameters—higher yield stress and viscosity.