Early hydration of ordinary Portland cement with an alkaline shotcrete accelerator

The early hydration (< 24 h) of ordinary Portland cement (OPC) with an added alkaline shotcrete accelerator was investigated by means of differential calorimetric analysis, an environmental scanning electron microscope, inductively coupled plasma-optical emission spectroscopy and X-ray diffraction analysis. Compared with normal cement paste, the presence of the accelerator first changes the hydration conditions, such as the ion concentrations and the pH value of the liquid phase of the paste directly after mixing. It influences the reactions of cement clinkers and the development of the hydrate phases. A foil-like hydration product composed of calcium aluminate hydrate (CAH) phases, long fibre-like ettringite crystals and tabular monosulphates was detected in the microstructure of the paste with the accelerator. The additional Al(OH)₄⁻ from the accelerator takes part in the hydration reactions to form CAH phases. The reactions of the accelerator are dependent on the concentration of Ca²⁺ ions in the liquid phase of the paste in the early course of hydration.