Mechanical and shrinkage behavior of magnesium oxide based concrete with recycled aggregates

ABSTRACT This study investigates the combined effect of reactive Magnesium Oxide (MgO) and Recycled Aggregates (RA) on the shrinkage and mechanical behavior of High-Performance Concrete (HPC), with a focus on sustainable and eco-concrete development. Unlike previous studies that typically examined MgO or RA in isolation, this work uniquely explores their interaction across varying RA levels (0%, 25%, 100%) and two curing periods (7 days and 6 months). Ten concrete mixes were prepared, with 10% Ordinary Portland Cement (OPC) replaced by MgO in half of them. The findings reveal that MgO reduced autogenous shrinkage by up to 93% due to Mg(OH)2 formation, while overall shrinkage was lowered by 22–39% depending on RA content. However, MgO also increased drying shrinkage due to greater water demand, especially at higher RA levels. A key novelty is the identification of threshold RA contents—35% at early age and 42% at maturity—beyond which the shrinkage contribution from RA offset MgO’s benefits. Mechanically, MgO caused a modest 5–8% reduction in strength properties. However, at 100% RA, the combination with MgO led to up to 21% strength loss, attributed to poor aggregate–matrix interaction. This study provides new insights into the synergistic and competing effects of MgO and RA in HPC, helping define optimal material combinations for shrinkage control without severe compromise in strength. The results contribute valuable data for sustainable concrete design involving both recycled materials and alternative cementitious components.