Investigation on expansion effect of the expansive agents in ultra-high performance concrete

In this study, the expansion mechanisms of calcium-sulfoaluminates -CaO based expansive agent (CSA-CaO EA) on the volume stability of ultra-high performance concrete (UHPC) were systematically studied. The test results show that the incorporation of CSA-CaO EA is beneficial to reduce the autogenous shrinkage of UHPC effectively due to the formation of additional Ca(OH)2 and ettringite. However, the reduction of early autogenous shrinkage is limited and the cracking risk is still very high when the addition of CSA-CaO EA reaches 15% of binders. According to the results, the factors hindering the expansion effect of CSA-CaO EA include the low w/b ratio, compact microstructure, out-sync of time “window” of expansion and shrinkage and the potential exchange of water between solids. Strategies and experimental validations on improving the shrinkage compensation are discussed. It indicates that the expansive UHPC can be prepared by using an CaO based EA with high reactivity and low water consumption via controlling the time “window” of expansion.