Globally, waste management and reduced carbon emission concepts have drawn wide attention, while the circular economy can synergistically coexist to benefit from each other. These concepts align with using waste materials like foundry sand and low-carbon binders like reactive magnesia mixed to serve the civil engineering industry in low-load-bearing applications. Foundry sand has been combined with various proportions of MgO salt (3–15%) with varying water content (WC) in multiples of OMC (OMC, 2*OMC, and 3*OMC) of the FS. Furthermore, curing conditions were maintained at 60 and 105 °C for 24 h before testing the cooled samples on day one after casting. The effects of binder content in conjunction with WC and curing temperature (CT) were systematically assessed for short-term strength gain through mechanical and microstructural analysis of the diverse mixes. The increasing MgO and WC added during the mixing process influenced the compressive strength and had varying impacts due to changes in CT. The peak compressive strength in the CTs meets the requisite standards for civil engineering applications such as soil subgrade improvement, indicating potential usefulness in medium load-bearing geotechnical contexts. The microstructural characterization via XRD and FESEM provides insights into the mechanical performance of the mixes.