Test and simulation of cement hydration degree for shotcrete with alkaline and alkali-free accelerators

In this paper, adiabatic temperature rise test was used to evaluate the effect of the type and content of accelerator on cement hydration in shotcrete. Two kinds of accelerators, alkaline and alkali-free accelerators were used in the experiments. For each type of accelerator, two different concrete mixture were used, and the dosage of the accelerator was 3% and 7% respectively. The experimental and theoretical results show that the use of alkaline or alkali-free accelerator can significantly improve the hydration rate in the first and second stage for shotcrete, but the alkaline accelerator will reduce the final hydration degree obviously, however the effect of alkali-free accelerator is small or even slightly increase. With the increase of the content of accelerator, the final hydration degree of concrete decreases, whether for alkali-free or alkaline accelerators. The mechanism may present in two aspects. For alkali-free accelerator, the hydration products are mainly rod-shaped AFt which will make concrete lose fluidity but do not completely cover the surface of the cement particles, and the acid substances in alkali-free accelerator will accelerate the dissolution of C3S, thus promoting the hydration reaction. However, for the alkaline accelerator, the hydration products are mainly composed of C-A-H, AFm and a small amount of C–S–H gel which will wrap on the surface of cement particles, thus affect the further hydration of cement particles. The proposed cement hydration rate model can well predict the development process of the hydration degree for shotcrete, especially for the early age concrete.