Foundry or green sand, a metal casting industry by-product, possesses significant opportunity as a sustainable alternative material with untapped potential. While traditionally viewed as a waste, this resource possesses valuable properties that can be leveraged across various civil engineering sectors, contributing to a more circular and environmentally conscious economy. The present study investigates the feasibility of substituting Portland cement with magnesium oxide (MgO) as the binding ingredient with foundry sand. To understand the suitability of this foundry sand-MgO mixture for various quick-strength geotechnical applications, the mechanical property of the mix is assessed by varying the MgO content from 3 to 15% and curing at ambient temperature for 24 hours. The water content required for the hydration of the mixture ranges from one to three times the OMC of the foundry sand. The strength of the sample increased as the amount of MgO in the mixture increased; however, beyond a threshold of MgO content, the strength was reduced. The samples’ strength decreased as the water-to-binder ratio increased, suggesting that lower water content and a higher reactive MgO percentage contributed to the strength. The X-ray diffraction and scanning electron microscope studies revealed the formation of cementitious compounds like magnesium silicate hydrate (M-S-H gel) and brucite (Mg(OH)2). This research focuses on developing a low-carbon method of utilizing waste foundry sand for a more sustainable infrastructure.