Effects of moisture on surface chemistry of steel slags and steel slag-asphalt paving mixes

Random cracking and premature failure have been observed in some steel slag-asphalt pavements in Ontario. The causes of the failure were investigated. Surface chemistry and morphology of steel slags and asphalt mixes and the effect of moisture on these characteristics were studied using scanning electron microscopy, scanning electron microscopy coupled with energy-dispersive x-ray analysis, and optical microscopy. X-ray diffraction and electron microprobe analysis were used for mineralogical studies. Experiments were also performed to determine moisture uptake and volume expansivity of steel slags and steel slag-asphalt mixes. Basic oxygen furnace slags and an electric arc furnace slag were used in this investigation, and asphalt mixes were prepared from some samples. The results indicate that failure in steel slag-asphalt mixes in moist environments is due to the formation of calcium-rich deposits, mainly calcium carbonate, along the surface and the interface. This affects interfacial bonding and can cause excessive expansion of the slag leading to cracking of the mixture. This reactivity was also observed in field core samples, even after several years of service. The mineralogical data showed that the distribution and microstructure of the two main phases (an iron oxide-rich mixed oxide phase and a calcium silicate phase in steel slags) were different among the slags examined. The calcium silicate phase appears to be one of the sources responsible for the reactivity of steel slags in moist environments.