Coupled Effects of Time and High Temperature on Rheological Properties of Cement Pastes Incorporating Various Superplasticizers

The time-dependent rheological behavior at high temperature of cement paste incorporating various chemical admixtures is critical for hot-weather concreting applications. This paper investigates the combined effects of temperature and time on the performance of polycarboxylate-, melamine sulfonate- and naphthalene sulfonate-based high-range water-reducing admixtures used to enhance the flow of cement paste. Rheological parameters, including yield stress, plastic viscosity, and thixotropy of cement pastes with a water/binder ratio of 0.38, were measured as a function of the superplasticizer dosage and temperature $(22–45°\mathrm{C})$ from $20\text{to}110\min$ from the time of mixing with $30\min$ between successive measurements. The rheological tests were conducted using an advanced shear-stress/shear-strain controlled rheometer. This paper aims at developing a better understanding of the coupled time and temperature effects on the performance of various superplasticizers, and formulating more realistic recommendations for using such admixtures in hot weather. Current technical information on chemical admixtures used in concrete has traditionally been developed in countries with mild climates and is often unreliable when transposed to extreme weather conditions. Thus, the findings of this study should have significant implications in practice. Results indicate that polycarboxylate-based superplasticizers should be used at a dosage close to the saturation in order to ensure adequate rheological behavior at high temperature and prolonged mixing time, while melamine sulfonate- and naphthalene sulfonate-based superplasticizers should be used at dosages beyond the saturation level.