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Recycling steel slag as a cement additive Murphy, Jeffrey N.


In Canada, approximately 1 million tonnes of steel slag are produced annually, but, at the present time, there is no economic outlet for the large scale recycling of this by-product. The overall objective of this work was to determine whether steel slag might be processed into a sufficiently cementitious material to allow it to be recycled as an additive to ordinary Portland cement clinker. Blending steel slag with with clinker in a modest ratio of 1:10, it would be possible to effectively recycle all of this steelmaking by-product. Although steel slag has a composition which is similar to that of Portland cement, (consisting mainly of lime, silica, and iron oxide), there are important compositional, mineralogical, and process related differences. Steel slag has limited cementitious properties due to both a lack of tricalcium silicate and the presence of wustite solid solutions as a predominant mineral phase. Even though wustite (FeO) possesses no inherent cementitious properties, it has been shown that hematite (Fe2C<3) will form hydraulic minerals when cooled from a slag melt. Thus, the valence state of the iron oxide in slag and cement systems will directly influence the cementitious properties of the material. This work explores the effects of Fe oxidation state, overall composition, and cooling rate on the mineral structure, crystalline formation, and glass forming ability of several synthetic slags and one commercial BOF slag. XRD, SEM, and EDX analyses were performed on both slow cooled and quenched slags, which had been oxidized. These analyses, of both synthetic and commercial slag, suggest that the trivalent iron (Fe2C«3) promoted both cementitious mineral formation in the slow cooled slags and glass formation in the quenched slags. The rate of cooling directly influenced the crystalline formation and mineral structure in the solidified product. The slow cooled slags tended to form highly crystalline polycomponent mineral systems, whereas the rapidly cooled granulated slags formed a predominantly amorphous material containing the primary Portland cement minerals.This granulated slag product was blended with Portland cement in varying ratios and hydrated to form cylindrical compression specimens. The preliminary compression results indicate that, for a hydration period of 1 to 35 days, blends with a 10% slag addition had increased strength relative to that of Portland cement. Blends with 20% slag maintained the same strength as Portland cement, while those with a 45% slag addition exhibited a significant loss in strength, for the hydration period tested. Therefore, the optimization of certain slag processing conditions enhanced the cementitious nature of the material allowing it to be blended with Portland cement, (at additions of up to 20% by weight), without affecting the strength performance of the material.

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