Mode of action of alkali-free sprayed shotcrete accelerators

摘要

Accelerating admixtures for shotcreting in general commercial use today are mixtures of sodium carbonate and sodium aluminate. While they produce acceptable early strengths relative to plain concrete, they typically reduce the ultimate strength by more than half, requiring the use of thicker layers than would be necessary in the absence of the accelerator to achieve the same structural strength. Additionally, these admixtures are highly caustic and great care must be taken in handling them. The new generation of accelerators that are free of alkali metal ions and chlorides contain at least one water-soluble sulfate and further constituents. The sulfate is mostly aluminium sulfate. The sulfate ions react with calcium hydroxide and with the surface of the tricalcium aluminate particles, producing a compound called ettringite. The mode of action of aluminium sulfate on the hydration of different cements from the time of mixing up to 7 days was investigated using X-ray diffraction.