Effects of alkali metal hydroxides on alkali-free accelerators

Abstract

Alkali-free accelerators are relatively new admixtures which allow concrete to be projected onto the rock wall during tunnel excavation. This sprayed concrete layer acts as a consolidating shield avoiding the convergence (the tendency of the section to squeeze) or dangerous structural collapses. Their efficiency could be related to: cement class and its chemical composition; environmental conditions (humidity; temperature); concrete mix design. A loss of accelerator efficiency is extremely dangerous for workers' safety as sudden collapses of the sprayed material can occur. Only a few studies have been published on the effects of these factors on the accelerating properties of alkali-free accelerators. In the present study, the influence of alkali metals (sodium and potassium) on cement pastes to which different alkali-free accelerators had been added was studied. It was discovered that the higher the alkali content the worse was the accelerator performance. In order to elucidate this phenomenon an analytical investigation was carried out. This included morphological (environmental scanning electron microscopy – field emission gun), chemical (induced coupled plasma – atomic emission spectroscopy), crystal-chemical (X-ray powder diffraction) and physical–chemical (hydration temperature profile; thermogravimetry/differential scanning calorimetry) evaluations. The results revealed significant morphological and crystal chemical differences among the investigated samples.