ABSTRACT
Alkali-free accelerators are a new and growing technology for the safe application of shotcrete in underground support applications. Alkali-free accelerators provide many benefits over conventional alkalibased accelerators, such as improved setting and strength development characteristics with safe operating conditions. However, the chemistry of alkali-free accelerators, the mechanism of acceleration, and interactions with different cement and admixture chemistries is not well understood. Therefore, considerable time and effort is frequently spent to qualify the accelerator for a specific mix design through live spray trials, which can be a costly and difficult task in an underground mining environment. This paper addresses these issues by presenting a new calorimetric technique for characterization of the performance of shotcrete accelerators with different cement and admixture chemistries in terms of the heat of hydration profile. The key aspects of the accelerator and cement chemistry are discussed along with the interactions generated with different cement and admixture types. Some attempts are also given to explain the effect seen in terms of the impact on cement hydration. Results from this laboratory technique are shown to correlate well with strength development curves taken from field spray trials using a corresponding shotcrete mix design. Guidelines for developing an optimized mix design with an enhanced acceleration profile are briefly reviewed.
