Abstract: Due to the different hydration mechanism ofcement mixed with alkaline and alkali-free accelerators, there are obvious differences in application performance. In this paper, the application performance and early hydration process of two accelerators were comprehensively analyzed through the test of macroscopic properties such as setting time and compressive strength of mortar, and the microscopic methods such as hydration heat analysis, XRD quantitative analysis, thermogravimetric analysis and scanning electron microscope observation. The results show that the [Al(OH)₄]^- accelerates the consumption rate of gypsum in cement and a large amount of ettringite (AFt) are generated in the initial hydration stage after the alkaline accolerator is added to the cement. At the same time, the alkaline accolerator also promotes the hydration of C₃S minerals, shortens the setting time of cement paste and improves the early compressive strength of mortar. However, the accelerated consumption of gypsum also makes the hydration products such as monosulfur-type hydrated calcium sulfoaluminate (AFm) and hydrated calcium aluminate (C-A-H) form in advance, which negatively affects the long-term compressive strength development of cement-based materials. While the alkali-free accelerator is added to the cement, the [Al(OH)₄]^- and SO^2-₄ form a large amount of AFt in the liquid phase, which promotes the hydration of C₃A and C₃S minerals, and affects the crystallization and precipitation of Ca(OH)₂(CH). It is worth noting that SO^2-₄ from the alkali-free accolerator not only promotes the process of C₃A to generate AFt, but also delays the consumption of gypsum in cement and the formation of hydration products such as AFm and C-A-H. Therefore, the 28 d compressive strength is not negatively affected in addition to the obvious improvement of the early compressive strength.

Key words: accelerator, quick setting mechanism, early compressive strength, ettringite, gypsum, hydration heat