Influence of magnesium oxide expander on mechanical and shrinkage properties of UHPC incorporating recycled sand

The incorporation of MgO expansive agents and internal curing mechanisms has individually exhibited positive effects on mitigating shrinkage in Ultra-high performance concrete (UHPC), yet studies on their synergistic performance remain scarce. This paper explores the utilization of recycled sand as a substitute for quartz sand in UHPC production, incorporating various reactive (Type R and Type S) MgO expansive agents at concentrations of 4%, 6%, and 8% (by mass) into the recycled sand UHPC. The primary objective is to investigate the impacts of these MgO agents on the fluidity, mechanical properties, shrinkage behavior, chloride ion permeability, and the macroscopic and microscopic characteristics of the recycled sand UHPC. Utilizing Scanning Electron Microscopy (SEM), Thermogravimetric Analysis (TG), and Mercury Intrusion Porosimetry (MIP), we analyze the phase transformations, microscopic morphology, and pore structures of the recycled sand UHPC with different MgO expansive agents. Furthermore, we delve into the underlying mechanisms governing the effects of these MgO agents on the performance of recycled sand UHPC. The results reveal that both Type R and Type S MgO expansive agents reduce the fluidity, compressive strength, and chloride ion permeability of recycled sand UHPC, while significantly mitigating autogenous and drying shrinkages. Notably, Type R MgO demonstrates superior performance, reducing autogenous shrinkage by 26.4% after 7 days and drying shrinkage by 76.9% after 150 days. Microstructural analyses indicate that the highly reactive Type R MgO hydrates to form a greater amount of Mg(OH) ₂ crystals, which contributes to a decrease in porosity, particularly in pores smaller than 50nm, thereby enhancing the compactness of the UHPC matrix. Based on a comprehensive evaluation of workability, mechanical properties, and shrinkage behavior, the incorporation of Type R MgO expansive agent at an optimal dosage of 6% is recommended for recycled UHPC.