Effect of competitive adsorption between sodium gluconate and polycarboxylate superplasticizer on rheology of cement paste

- Effect of SG on dispersion of PCE is decided by the added dosage of SG.
- Combination between PCE and SG via Ca2+ has been confirmed to increase the dispersion.
- Competitive adsorption between PCE and SG would reduce the dispersion of PCE.

Sodium gluconate (SG) is usually employed as a supplementary material in polycarboxylate superplasticizer (PCE) system to enhance the dispersion and dispersion retention. In real concrete, this enhancement cannot always be achieved as expected, and sometimes, even a negative effect can be found. This uncertainty is mainly because SG would perturb the dispersion of PCE, while the mechanism behind has not been completely understood. The objective of this study is to investigate the interaction between PCE and SG. The rheology was studied to assess the dispersion ability, and adsorption behavior was characterized with total organic carbon analyzer. The combination between PCE and SG was analyzed with conductivity, X-ray photoelectron spectrometer and dynamic light scattering. The dispersion model was then proposed to explain the main mechanism behind. The results show that two positive effects of SG on dispersion of PCE-SG system can be found: one is the formation of water film caused by adsorption of SG, and the other is more interesting, namely the combination between SG and PCE via the bridging Ca2+ to enhance the steric hindrance. The negative effect of SG can also be found, mainly because of the hindrance to adsorption of PCE resulting from competitive adsorption. Accordingly, the dispersion of PCE-SG depends on the predominant effect: when the dosage of SG less than 0.10%, the contribution to dispersion predominates; while the competitive adsorption effect predominates when the dosage more than 0.10%, with a decline in dispersion ability. Such results would provide guidance on the design of high efficient dispersant system in real engineering practice.