CHARACTERIZATION OF BASE AND SUBBASE IRON AND STEEL SLAG AGGREGATES CAUSING DEPOSITION OF CALCAREOUS TUFA IN DRAINS

Tufaceouslike materials are observed clogging pavement drains along highways in northeastern Ohio. These materials have also been found in catch basins and spillways. Previous studies suggest that the original free lime (CaO) in slags used as subbase materials is responsible for the deposition of the tufa. To characterize these slags, X-ray diffraction, energy dispersive X-ray, surface area measurements, CaO solubility in anhydrous ethylene glycol ("sugar test"), and other physicochemical tests were conducted on air-cooled blast furnace (ACBF), open-hearth (OH), and basic oxygen furnace (BOF) slags. ACBF iron slag is composed of crystalline akermanite (Ca2MgSi2O7). Results of the sugar test indicate that this slag does not contain any residual or easily available free lime. The ACBF sample exhibited the lowest surface area and therefore is the least reactive with CO2-charged waters. The OH and BOF slags, however, exhibit the presence of CaO or its weathering equivalents such as Ca(OH)2 and CaCO3. All these steel slags exhibit a high tufa-generating potential. Analysis of the physicochemical tests of these steel slags indicates that they are composed mainly of Beta-dicalcium silicate (larnite), calcium ferrites, wustite (FeO), free lime (CaO), periclase (MgO), portlandite [Ca(OH)2], and calcite (CaCO3). These slags also exhibit high surface areas and are more reactive with CO2-charged waters. Most highway departments require that steel slags be aged or cured for at least 6 months before they are used. Sugar tests conducted on samples obtained from different horizontal depths (up to 10 ft) in a stockpile showed that the CaO (free lime) content increases with increasing depth into the stockpile. Evidently some of the free lime is encapsulated by insoluble silicates or is in occluded pores and has not come in contact with CO2-charged porewater. Therefore, aging of slags by exposure to weathering does not necessarily decrease the free lime content enough to prevent the formation of tufa. It is concluded that the presence of CaO, MgO, Ca(OH)2, and CaCO3 in slags is critical in determining their tufa potential for use as base and subbase aggregates.