The effect of nanosilica on early hydration of Portland cement

Concrete can be applied to a surface in for example tunnel construction and rock support by spraying it on the surface. When using sprayed concrete, a fast setting of the cement is desirable to reduce the amount of rebound and achieve a good contact with the sprayed surface. Therefore the effect of an alkali-free accelerator that is commonly used in sprayed concrete, in combination with nanosilica on the hydration of Portland cement during the first two hours was investigated. This was done with Vicat measurements, a method to measure the setting of cement, and diffuse reflectance Fourier transform infrared spectroscopy (DR-FTIR). The cement paste contains water which has a very strong absorbance in the infrared spectra. This made it hard to see anything in the DR-FTIR spectra. Therefore a method to dry the samples had to be developed. It was concluded that freezing the samples with liquid nitrogen and then freeze dry them made it possible to achieve good enough spectra to follow the early hydration process. Nanosilica had almost no accelerating effect on the cement used in this work during early hydration if it was the only additive. This is due to the formation of an ettringite layer on the cement grains that prevents the interaction between nanosilica and cement. However, the accelerator was found to greatly decrease the setting time of the cement by removing the ettringite layer. When this accelerator was combined with nanosilica an even greater decrease of the setting time was achieved. The fast setting is not due to an undesired flash set of the aluminate phase in the cement, but an increased formation of C-S-H gel