Effect of cement composition on the reactivity of alkali-free accelerating admixtures for shotcrete

The evolution of mechanical strength in cement matrices is strongly dependent on cement properties and on the admixtures used. Limestone additions and accelerators, used in shotcrete, alter the hydration mechanisms of the cementitious material, influencing its strength development. Hence, it is extremely important to evaluate the cement-accelerator compatibility and reactivity, in order to obtain proper mix designs for determined applications. The objective of this study is to evaluate the reactivity between alkali-free accelerating admixtures and cements with different compositions. The main focus of the research was the analysis of the effect of the limestone filler present in the type II cement on the kinetics of hydration of the accelerated systems. A type-I and a type II cements and two alkali-free accelerators were studied. Isothermal calorimetry, X-ray diffraction and setting time tests were performed on cement pastes produced with accelerating admixture contents of 5.0% and 7.0% by cement weight. Results showed that the type II cement presents higher reactivity with the accelerators used, due to the calcium carbonate addition. This leads to the formation of a higher quantity of ettringite, shortening setting times. The conclusions from this study provide valuable evidence towards explaining variations in the behavior of the accelerated systems due to the characteristics of the cement used