Influence of retarders on hydration and microstructure development of cement containing high-volume limestone powder

Abstract

Limestone powder is widely used in concrete for efficient reduction in hydration heat in the place lacking common supplementary cementitious materials. The sodium gluconate and citric acid have been commonly used as retarders to extend the setting time of concrete during summer construction. To study the influence of retarders on hydration and microstructure development of cement containing high-volume limestone powder, multi-technique methods are employed. Isothermal calorimetry, thermogravimetric analysis, X-ray diffraction, mercury intrusion porosimetry and energy-dispersive X-ray analyses in the scanning electron microscopy are used to investigate the hydration process, the content of hydration products, the species of the hydration products, the pore structure of cement pastes and the composition of C–S–H. The results show that the retarders have a better inhibition effect on the hydration of cement containing high-volume powder limestone than on that of cement without limestone powder. At a given dosage, the citric acid has a stronger interaction with the cement containing 30% limestone powder than the sodium gluconate. For the cement containing a high volume of limestone powder, the presence of retarders induces a higher porosity, a larger critical radius and a lower average Ca/Si ratio of C–S–H at 28 curing days. The porosity and the volume fraction pore size smaller than 50 nm have a linear relationship with the hydration degree and the gel/space ratio, respectively. However, the critical radius of cement pastes has an exponential relationship with the hydration degree and gel/space ratio, respectively.