Preparation and accelerating mechanism of aluminum sulfate-based alkali-free accelerating additive for sprayed concrete

The aluminum sulfate-based alkali-free liquid accelerating additive (AFa) was prepared by dissolving aluminum sulfate in mother liquor (ML). The setting time and strength of different cements with AFa were fully investigated. Moreover, the early hydration behavior and morphology of the hydration products of the accelerated cement paste were explored. Furthermore, the effect of the AFa on the early hydration of the C(3)A and C3S was also studied, and the hydration model was established. The mechanisms of hydration and the microstructure development were investigated by Isothermal calorimetry, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The results showed that the addition of the AFa shortened the setting time of the cement paste and improved the early and late strengths of cement mortar. When the AFa was added to the cement paste, the Al3+ converted to [Al (OH)(4)](-) and reacted with gypsum to form the AFt and AFm crystals, whereas the CaF2 and MgF2 crystal nucleus promoted the crystallization and growth of Ca(OH)(2).Through the experimental results, the effect of the AFa on the early hydration behavior of the cement paste was explained. In addition, the early hydration process of the C(3)A and C3S mixed with the AFa was identified. (C) 2019 Elsevier Ltd. All rights reserved.