Cementitious Properties Of Steel Slag And Its Role In The Hydration And Hardening Process Of Complex Binder

Steel slag is a by-product from the conversion of iron to steel, the amount of which emitted takes about fifteen percent of the steel production. At the moment, part of steel slag is being used as aggregates for concrete, asphaltic paving, and road bases. But most steel slag has not been efficiently used yet. Using steel slag as a supplementary cementitious material during the manufacture of concrete is an effective and a high efficient way for its utilization. Basic oxygen furnace steel slag is the most common steel slag in China. The aim of present research work is to investigate the cementitious properties of basic oxygen furnace steel slag and its role in the hydration and hardening process of cement-steel slag complex binder. The main content and results are listed as follows:(1) The hydration process of steel slag is very similar with that of cement, but its hydration rate is much low. Raising the fineness of particles, alkaline condition and elevated curing temperature can all promote the hydration of steel slag at early age, of which elevated curing temperature is the most effective to excite the activity of steel slag.(2) C₃S, C₂S and RO phase are the main minerals of steel slag. The steel slag can be divided into two parts: cementitious phases (C₃S, C₂S, C₁₂A₇ and Ca₂Al₂Si₃O₁₂) and inert phases (RO phase, C₂F and Fe₃O₄). The proportion of RO phase in large particles (> 60μm) of steel slag is higher than that in small particles (< 6μm). The proportion of cementitious phases in small particles is higher than that in large particles. Ca(OH)₂, C-S-H gel, unhydrated C₂S and C₃S, RO phase, C₂F, and Fe₃O₄ are the main hydration products of steel slag. Though the hydration of the cementitious phase of steel slag is beneficial for the improvement of pore structure of hardened paste, its inert phase can not act as microaggregates to densify the hardened paste. Thus, steel slag does not have the effect of improving the pore structure at later ages as fly ash.(3) The alkalinity of steel slag determines the type of major minerals but does not determine the content of minerals. The difference of activity among different types of high-alkalinity steel slag can not be further distinguished by alkalinity. The hydration activity index (HAI= (SiO₂+ Al₂O₃)/ (FeO+Fe₂O₃+MgO+MnO)) is defined to evaluate the hydration activity of steel slag, which is verified to be effective by the results of examples. The higher the hydration activity index, the higher the hydration activity of steel slag.(4) The dormant period of binder is prolonged and its hydration rate at early age is reduced by replacing cement by steel slag. The hydration rate of complex binder is higher than that of cement after 7 days, which is attributed to the hydration of steel slag at later age, as well as the mutual promotion of hydration between cement and steel slag. The promoting factor is defined to evaluate the increased degree of the hydration of cement by the promotion of steel slag. It can be concluded from the promoting factor that the promoting effect becomes more obvious with the adding amount of steel slag and the hydration age.