Industrial activities generate significant amount of hazardous waste from refinery, chemical manufacturing, cosmetic, drug and pharmaceutical industry. Petroleum sludge is immobilized using ordinary Portland cement to stabilize and solidify the contaminant in the sludge as an alternative treatment for final disposal of the waste. The waste has significant amount of organic material which normally interfere with the cement hydration process. A way to improve is by addition of cement replacement material. Rice husk ash (RHA) was added at 5, 10 and 15 cement replacement. The 5 RHA exhibited the best performance with regards to unconfined compressive of 24.9 N/mm2. The strength is better than the sludge cement of 19.2 N/mm2. Porosity was found to increase with increasing RHA content. The surface morphology of solidified cement with voids was found to be in the range of 10 to 15 μm for 15 RHA. RHA addition promotes carbonation. Its high silica content reacts with CO 2 to form calcite and silica gel. © 2010 IEEE.