Mechanical properties, hydration behavior and shrinkage deformation of ultra-high performance concrete (UHPC) incorporating MgO expansive agent

Abstract

To reduce the extremely high autogenous shrinkage deformation of ultra-high performance concrete (UHPC) caused by low water-to-binder ratio and high binder materials content, this work aimed to designed and prepared low shrinkage UHPC containing MgO-based expansive agents (MEA). The effects of UHPC with different reactivity of MEA (R-MEA and S-MEA) were compared in terms of fluidity, mechanical properties, hydration characteristics, shrinkage deformation, pore structure and microstructure. The results indicated that the incorporated of MEA decreased the fluidity and mechanical properties of UHPC, while the autogenous shrinkage deformation was compensated significantly. When 6 % R-MEA was incorporated, the flow and compressive strength decreased by 16 % and 7.9 %, respectively, but the autogenous shrinkage at 7 days also decreased by 70.1 % at the same time. The same amount of S-MEA showed lower compensation for the autogenous shrinkage of UHPC than R-MEA, but with the same reduction in compressive strength loss. This was due to the highly reactive R-MEA showed higher hydration degree and larger swell increment, which caused more compensation for shrinkage in UHPC, while it had slight adverse effect on the strength development. It is worth noting that the compensation effect of autogenous shrinkage is very limited when the doping is higher than 6 % for either MEA, but the loss of mechanical properties is serious. In addition, the microstructure showed that the crystal growth pressure of brucite forced the pastes to expand and thus resulted in increase on the porosity for UHPC. This study provides a feasible and effective way for mitigating the autogenous shrinkage of UHPC.