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The mineral carbon sequestration capacity of basic oxygen furnace (BOF) slag offers great potential to absorb carbon dioxide (CO₂) from landfill emissions. The BOF slag is highly alkaline and rich in calcium (Ca) containing minerals that can react with the CO₂to form stable carbonates. This property of BOF slag makes it appealing for use in CO₂sequestration from landfill gas. In a previous study, CO₂and CH₄removal from the landfill gas was investigated by performing batch and column experiments with BOF slag under different moisture and synthetic landfill gas exposure conditions. The study showed two stage CO₂removal mechanism: (1) initial rapid CO₂removal, which was attributed to the carbonation of free lime (CaO) and portlandite [(Ca(OH)₂)], and (2) long-term relatively slower CO₂removal, which was attributed to be the gradual leaching of Ca²⁺from minerals (calcium-silicates) present in the BOF slag. Realising that the particle size could be an important factor affecting total CO₂sequestration capacity, this study investigates the effect of gradation on the CO₂sequestration capacity of the BOF slag under simulated landfill gas conditions. Batch and column experiments were performed with BOF slag using three gradations: (1) coarse (D₅₀ = 3.05 mm), (2) original (D₅₀ = 0.47 mm), and (3) fine (D₅₀ = 0.094 mm). The respective CO₂sequestration potentials attained were 255 mg g⁻¹, 155 mg g⁻¹, and 66 mg g⁻¹. The highest CO₂sequestration capacity of fine BOF slag was attributed to the availability of calcium containing minerals on the slag particle surface owing to the highest surface area and shortest leaching path for the Ca²⁺from the inner core of the slag particles.