Abstract
In this research, the plasticising mechanism of sodium gluconate (SG) retarder in cement paste containing a superplasticiser has been investigated in order to clarify the performance of the admixture combination in concrete. From adsorption and zeta-potential measurements, it is found that a lower viscosity occurs owing to (a) the electrostatic repulsion and (b) a lower particle volume fraction. Moreover, the gluconate anion is expected to enhance the formation of a homogenous polymer network on the cement grain surface, which explains the viscosity stabilisation time. In situ investigations of hydration using environmental scanning electron microscopy showed that SG induced modifications of the ettringite crystal morphology by reducing the amount and size of the ettringite needles. From pore solution analysis and energy-dispersive X-ray spectra, a mineral similar to syngenite is detected. These morphological changes lower the shear resistance and the paste viscosity. A thorough understanding of the plasticising mechanism will lead eventually to a more predictable concrete flow behaviour, which is particularly useful for concrete practitioners facing concerns related to combined use of admixtures.
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