MgO is added during warm lime softening (WLS) to remove silica from steam-assisted gravity drainage produced water. Although it is a general practice in the oil sand industry to avoid MgO slaking to ensure maximum silica removal, there has been no study on how slaking of MgO affects silica removal. This work aims to understand the effect of MgO slaking on silica removal and silica removal mechanism by MgO. Silica removal by slaked and nonslaked MgO was studied at different pH values (8.0-11.3) with different dosages (100-1000 ppm) and contact time (15-120 min) at WLS operating temperatures (65-85 degrees C). Silica removal takes place through two possible competing mechanisms: adsorption on the formed Mg(OH)(2) or precipitation by forming magnesium silicate precipitates. Slaked MgO achieves lower silica removal percentage than nonslaked MgO at SAILS conditions because MgO slaking makes silica adsorption on the formed Mg(OH)(2) more predominant.