Effects of retarders on the fluidity of pastes containing β-naphthalenesulfonic acid-based superplasticiser

The workability of a superplasticised cement paste can be improved by a higher fluidity and a lower flow loss. The effects of two retarders, sodium gluconate (SG) and citric acid (CA), on the workability of pastes containing 0·7 wt% β-naphthalenesulfonic acid-based superplasticiser (BNS) were determined. At the same BNS dosage, improved workability only occurred with the addition of 0·03–0·09 wt% SG, and not with 0·12–0·15 wt% SG or with 0·03–0·15 wt% CA. The reasons for this phenomenon were investigated by way of the adsorbed amount of BNS on C₃S, C₂S, C₄AF-gypsum, and C₃A-gypsum surfaces at different SG or CA dosages using ultraviolet spectrophotometry. SG and CA were found to reduce the adsorbed amounts of BNS on C₃S, C₂S, and C₄AF-gypsum surfaces at 5 and 60 min because of the competitive adsorption between the retarders and BNS. CA also had a more remarkable reducing effect than SG. On the other hand, the adsorbed amount of BNS on C₃A-gypsum was enhanced because the retarders accelerated the formation rate of ettringite. The adsorbed BNS amount was also higher in the presence of CA than that of SG, indicating that CA enhances the consumption of BNS in the structure of ettringite. Overall, 0·03–0·09 wt% SG distributes BNS better on different minerals and improves paste workability.