Influence of alkali-free and alkaline shotcrete accelerators within cement systems:: Hydration, microstructure, and strength development

The strength development of mixtures accelerated with. alkali-free and alkaline admixtures is clarified. The alkali-free admixtures do not interfere with the calcium silicatehydrate gel development. A good dissolution of the admixture-for instance, the calciumsulfoaluminate-rich one-causes the ettringites to be homogeneously distributed within the cementitious mass, thus acting as a crack arrester and enhancing the strength. On the other hand, an inhomogeneous distribution of the admixture results in the formation of high voluminous ettringite enrichments that weaken the microstructure and result in the strength decrease of the alkali-free accelerated mixtures. The addition of the alkaline accelerator delays the hydration of the calciumsilicate components to calciumsilicatehydrate gel and its densifying process. Consequently, a general high porosity persists up to 6 months, resulting in the lowest strength among the accelerated mixtures.