Mitigation on the autogenous shrinkage of ultra-high performance concrete via using MgO expansive agent

Due to the use of high binder content and low water-to-binder ratio, ultra-high performance concrete (UHPC) demonstrates relatively high autogenous shrinkage or even cracking at early age. This paper aims to evaluate the efficiency of various MgO expansive agents (MEA) with different activity on the early age shrinkage deformation of UHPC. The effects of three types of MEA on the fluidity, hydration characteristics, mechanical properties, and microstructure of UHPC were investigated. The results indicated that the fluidity, compressive strength and expansion of UHPC were affected by the activity and dosage of MEA. The higher activity of MEA led to a fasted hydration and expansion speed without significant sacrifice for strengths. The UHPC incorporated MEA with activity value of 75 s (represented as MEA-R) had smaller autogenous shrinkage and higher compressive strength than that of with reactivity value of 130 s (MEA-M) and 220 s (MEA-S). The crystal growth pressure of brucite forced the pastes to expand and increased the porosity of the UHPC matrix, thus prominently mitigating the early age autogenous shrinkage and slightly reducing the strengths. This study helps to provide an effective approach for mitigation the autogenous shrinkage of UHPC.