Utilization of the black tea powder as multifunctional admixture for the hemihydrate gypsum

Abstract

The purpose of this work was to investigate the feasibility of using black tea powder produced through green technology (plant-source material and biotechnology) as multifunctional admixture for the hemihydrate gypsum. The hemihydrate gypsums were mixed with black tea, and the setting times, densities, hydration characteristics and strengths of different plasters were discussed, and the mechanisms responsible for their performance variations were explored by isothermal calorimeter, X-ray powder diffractometry and scanning electron microscopy. Besides, the economic and environmental compatibilities were also considered. The results indicate that black tea lowers the water requirement and improves the normal consistency, and then the high density is acquired. It prolongs the setting times. Moreover, this prolongation is more remarkable as more black tea is introduced. The gypsum strength is related to the content of black tea. Moderate black tea has the positive role in the improvement of strength, but the excessive black tea poses a negative effect. Furthermore, black tea alters the gypsum hydration process. It decreases the hydration rate and delays the hydration time of gypsum. Although more black tea seriously delays the gypsum hydration, all hemihydrate gypsum thoroughly converts into dihydrate gypsum after 28d. SEM results show that black tea changes the gypsum crystal morphology. All gypsum crystals are long and needlelike in the hardened pastes with less than 1% black tea. However, these crystals become shorter and larger as more black tea is mixed. Various crystal morphologies mainly contribute to the strength alteration of gypsum with black tea. These results show that black tea can serve as an alternative to the traditional functional admixture and then acquire a green and sustainable technology for the preparation of hemihydrate gypsum functional admixture, but also expand the commercial utilization of black tea in the building materials industries and foster the development of building industry towards green-oriented and sustainability.