Effects of alkali metal hydroxides on alkali-free accelerators

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 analyti...