Green recycling of used motor oil and steel slag aggregate in concrete

Steel slags and used motor oil (UMO) are hazardous materials that are produced in high amounts and released into the environment by human activity. They adversely affect all types of life especially if they contaminate soil or ground when dumped. This research introduces a green clean safe sustainable efficient cheap method for recycling these materials. In the first step in this research, hazardous UMO is recycled as a concrete admixture. Previous research on UMO showed that the use of oil would result in higher air content in concrete, which in turn would result in a decrease in concrete strength. The research eliminates this problem by ensuring correct mixing procedures which will get rid of the increased air content. Hence, concrete strength can be maintained in the structure. The research suggests two simple economic methods to solve the problem. The first is to use an alternative mixing method of concrete to use UMO as a plasticizer without affecting the strength. Enhancement of the strength of concrete was attained by optimizing mixing time. The second method is to recycle steel slag aggregate (SSA) in concrete as coarse aggregate by replacing a certain proportion of natural coarse aggregate. The green use of SSA in concrete was beneficial in improving the properties of concrete containing UMO and was a sustainable solution for reducing the destruction of the environment caused by the depletion of natural resources when natural coarse aggregate is used. The change in mixing method and the use of SSA allowed the recycling of higher amounts of UMO without adversely affecting the fresh, hardened, or transitional plastic properties. Three states of concrete were studied in the research: fresh, plastic transitional, and hardened states. The study showed that the slump test alone is not sufficient to describe the workability of concrete containing UMO. In the transitional plastic state, setting time, bleeding, and finishability were studied. Results show that UMO and SSA can safely be used in concrete without adversely affecting its properties. The initial and final setting times increased by an average of 11 and 22 min respectively. Finishability tests show that the surface smoothness was excellent for UMO mixes containing 1.5% for SSA concretes and 1% for SSA-free mixes. The use of the modified mixing method reduced air content by about 3%. The use of SSA and the modified mixing improved the strength reduction of UMO concretes allowing dosages of about 0.8% to be safely used without losing compressive strength and 1.25% without losing tensile strength. No shrinkage problems are observed in all mixes. Lastly, a special study showed that the combined use will also reduce the overall material direct costs of about 5$ per cubic meter of concrete. Recycling UMO by this method would eliminate the CO₂ emissions produced by the firms that recycle UMO.