Humidity Sensitivity of Hydration of Expansive Agent and Its Expansive Efficiency in Ultra-High Performance Concrete
Yujiang Wang, Yujiang Wang, Qian Tian, Hua Li, Yang Wang, Yang Wang
Abstract
Ultra-high performance concrete (UHPC) has a potential cracking risk due to its large autogenous shrinkage. The use of an expansive agent is an effective approach to reduce autogenous shrinkage of UHPC. However, different kinds of expansive agents show different expansive efficiency in UHPC. To study the cause for the difference in expansive efficiency, this study selected three expansive agents, namely highly reactive MgO-based, medium reactive MgO-based, and CaO-based expansive agents, and carried out the following experiments: autogenous shrinkage, hydration heat, hydration process of expansive agent under different relative humidity (RH), and micrographs. The results showed that the CaO-based expansive agent has high hydration activity at RH of more than 44.0%, while the hydration activity of two kinds of MgO-based expansive agents, especially a medium reactive MgO-based expansive agent, decreases significantly when RH drops below 80%. Meanwhile, the CaO-based expansive agent had higher expansive efficiency in UHPC than the MgO-based expansive agent. This study suggested that the CaO-based expansive agent is more suitable for compensating autogenous shrinkage of UHPC due to its low humidity sensitivity compared to the MgO-based expansive agent.