Hydration processes of accelerated cementitious systems governing early strength development

Mixes used for sprayed concrete differ from those used for cast concrete in various aspects, the most important one being the presence of setting accelerators, which control the chemical reactions during the first hours of hydration. In particular, the use of alkali-free aluminium sulfate based accelerators promotes the almost instantaneous and massive formation of ettringite, which facilitates rapid setting
and contributes to the strength gain needed to ensure good adherence between the shotcrete and the substrate and to provide adequate mechanical support.
The use of supplementary cementitious materials and fillers in shotcrete has not been fully explored yet. However, the same environmental criteria as for conventional concrete should be considered and the development of low clinker formulas for shotcrete with a lower ecological footprint is of primary importance.
In this paper data on the hydration of sprayed concrete are presented. Characterization techniques such as compressive strength tests, isothermal calorimetry, shear modulus measurements, SEM and in-situ XRD allowed to follow phases’ formation and strength development during the first 24 hours
after spraying, both in laboratory and real scale tests. In particular, the acceleration of the calcium silicate hydration in the presence of fine limestone powder is discussed.