Abstract
Alkali-free, aluminium-based accelerators are often used for shotcrete linings in tunnels and mines, where external sulfate attack can occur. Therefore, it is essential to know, if the influence of the accelerators on the cement hydrate assemblage negatively impacts sulfate resistance. This study focuses on the effect of aluminium-based accelerators on cement hydration and the consequences for external sulfate attack. Aluminum sulfate-based accelerators cause rapid setting due to the very early formation of ettringite and accelerate alite hydration. At late ages, significant more AFm phases are formed compared to the reference without accelerator, which during storage in sodium sulfate solution react to ettringite. A significantly higher volume increase due to the formation of additional ettringite during sulfate exposure was calculated by thermodynamic modelling for the accelerated paste compared to the reference. Alkaline accelerators based on sodium aluminate form mainly amorphous calcium (sulfo-) aluminate hydrates at very early age, while ettringite seems to be destabilized. The ettringite quantities formed during storage in sodium sulfate solution are significantly higher than for the reference and the paste with the alkali-free accelerator. This finding compares well to experiments on concrete specimens, where in the mixture with the alkaline accelerator deleterious expansion was observed.
Highlights
Shotcrete or sprayed concrete is frequently used in tunneling and mining for the stabilization of tunnel walls and their protection against the penetration by ground water [1,2,3]
The paste with the alkali-free accelerator shows a much higher initial peak compared to the plain paste, which can be attributed mainly to formation of ettringite and other calcium aluminate hydrates from the constituents of the accelerator and the available calcium ions provided by the free lime and the calcium sulfates originating from the cement [11,12,14]
The influence of shotcrete accelerators on the hydration kinetics and phase assemblage of cement pastes and their interaction with 5% sodium sulfate solution was investigated by isothermal calorimetry, X-ray diffraction analyses (XRD), thermogravimetry (TGA) and thermodynamic modelling
Summary
Shotcrete or sprayed concrete is frequently used in tunneling and mining for the stabilization of tunnel walls and their protection against the penetration by ground water [1,2,3]. Alkali-free accelerators formulated mainly based on mixtures of aluminum sulfate and aluminum hydroxide [4,5,8,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28], which contain other ingredients such as formates or amines, are applied at high dosages in the order of approximately 5–10 mass-% of the cement. Alkaline accelerators often show a decrease of long-term strength, and the high alkali contents may cause health issues, and alkali-free accelerators are increasingly used [29]. The sulfate resistance of shotcrete is an issue. In-depth knowledge on the influence of these accelerators on cement hydration, the formed hydrate assemblage and sulfate resistance is needed
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