The air-cooled blast furnace slag (ACBFS), the by-product of the pig iron making process, is often used as coarse aggregate in portland cement concrete (PCC) pavements, especially in the areas located in the vicinity of the iron mills. The utilization of this by-product as an aggregate in concrete offers environmental and economic benefits in the form of elimination of waste, decrease in the disposal costs, and reduction in need for mining of the natural materials. However, concerns exist with relation of the influence of these aggregates on the long-term durability of pavement concretes, especially at locations exposed to freezing and thawing environment.
The objective of this research was to evaluate the influence of using the ACBFS aggregate (slag aggregate) as a replacement for natural aggregates on the properties of pavement concrete designed to meet the standard specifications of the Indiana Department of Transportation. A total of eight different concrete mixtures, four containing air-cooled blast furnace slag (ACBFS) as coarse aggregate and additional four containing natural dolomite, were used in this study. These mixtures included plain concrete mixture (100% of Type I portland cement), two mixtures with binary binder systems (one containing 20 wt.% of Class C fly ash + 80 wt.% of Type I portland cement and the other containing 25 wt.% slag cement + 75 wt.% of Type I portland cement) and one mixture with a ternary binder system (23% slag cement + 17% of Class C fly ash + 60% of Type I portland cement). Specimens produced from each of these mixtures were ponded with each of the three different types of chloride based deicers (CaCl2, MgCl2 and NaCl) while being subjected to either freezing-thawing (FT) cycles or wetting-drying (WD) cycles.
The mechanical and durability properties of these concretes were assessed by conducting series of tests prior and after the exposure to FT and WD cycles. In addition, changes in the overall physical appearance of the test specimens were also documented to aid in the evaluation of the effects of given exposure conditions on the deterioration process of concretes. In the case of a plain concrete, the analysis of the data collected during the study (i.e. the observed loss of strength, reduction in the dynamic modulus of elasticity, and physical changes in the appearance of the specimens) indicated that the calcium chloride (CaCl2) deicer caused the most severe distress, followed by the magnesium chloride (MgCl2). Specimens exposed to sodium chloride (NaCl) experienced the least damage and performed comparably to those exposed to distilled water (DST). The scanning electron microscopy studies of the microstructure revealed the presence of deposits of Friedel’s salt and zones of chloride rich C-S-H in all concretes used in this project, irrespective of the type of deicer the specimen was exposed to. The deposits of brucite (Mg(OH)2) and of magnesium-silicate hydrate (M-S-H) were found in specimens exposed to MgCl2 while calcium oxychloride was detected in concretes exposed to CaCl2 under FT conditions.
The use of either Class C fly ash, slag cement or the combination of both as part of the binder was found to greatly increase the resistance of concrete to damaging effects of deicers.