Stubble burnt ash (SBA), a silica-rich byproduct derived from agricultural residue, was evaluated as a supplementary cementitious material in MgO-based ternary binder system. SBA was produced through controlled burning at 500 ℃, 600 ℃, and 700 ℃ for 1 h, with reactivity found to be highly dependent on calcination temperature. X-ray diffraction (XRD) analysis confirmed that SBA calcined at 600 ℃ possessed the highest amorphous silica content, while increased crystallinity was observed at 700 ℃. A ternary mortar mix consisting of 50% ordinary Portland cement (OPC), 30% reactive MgO, and 20% SBA was prepared, incorporating a hydration agent to enhance MgO hydration. Mortar cubes (50 × 50 × 50 mm) were cast and tested for compressive strength and water absorption. The mix containing SBA calcined at 600 ℃ exhibited a compressive strength of 44.32 MPa at 28 days and water absorption of 0.86%. The mortar with SBA calcined at 700 ℃ showed reduced strength (30.43 MPa) and higher water absorption (1.85%) due to the presence of unreactive crystalline silica. M-S-H is the primary phase responsible for compressive strength, formed through the hydration of MgO and silica derived from SBA reported via XRD. These results demonstrate that SBA derived from stubble and calcined at 600 ℃ is more effective in enhancing the performance of the MgO-based binder system compared to SBA calcined at 500 ℃ and 700 ℃.