Effects of calcium-carbonate-based aggregates on alkali-free accelerators

Alkali-free flash setting accelerators are widely used in the area of underground construction as they allow a very rapid hardening of cementitious systems. These admixtures are particularly used for wet-mix shotcrete, which involves pneumatically pumping a previously prepared concrete to a nozzle, where liquid accelerator is added. Compressed air is used to ‘shoot' the mixture onto the receiving surface. The resulting concrete layer acts as a consolidating shield that helps to avoid the tendency of the excavated section to squeeze (convergence) or collapse. The accelerator's efficiency is related to many parameters such as cement class, environmental conditions (humidity, temperature), concrete mix design and aggregate type. A limited number of papers have been published on the effects of these factors on the accelerating properties of alkali-free accelerators. In this paper, the influence of the aggregates chemical nature is studied. Setting time and mechanical strength development of mortar samples containing calcareous or siliceous sands, with the addition of an alkali-free accelerator, are analysed. It is discovered that calcium-carbonate-based aggregates strongly improve accelerator performances. In order to better understand this phenomenon, the influence of calcium carbonate on accelerated hydrating cement mortar is investigated through thermo-chemical hydration profiles, thermogravimetry (TG), environmental scanning electron microscopy – field emission gun (ESEM-FEG), induced coupled plasma – atomic emission spectroscopy (ICP-AES) and X-ray powder diffraction (XRPD). From the results, a significant increase of ettringite is evident in the accelerated mortars containing calcite.