Urban heat islands are developed areas that are warmer (2-10° F) than the surrounding undeveloped areas. Urban heat islands are the result of many causes, one of which is dark exposed surfaces, which absorb sunlight and retain heat. Concrete, for instance, is considered a superior surface, from an urban heat island perspective, than asphalt, because of its higher reflectivity. Negative effects of urban heat islands, especially in warmer climates, are increased air conditioning and refrigeration loads, increased mortality and illness due to elevated temperature, greater smog formation, and higher ozone concentration .
Slag cement is lighter in color than other common cementitious materials. It brightens exposed concrete surfaces, increasing reflectivity and reducing urban heat island potential. One industry study (Holcim) measured reflectivity differences in slag and non-slag concrete in equivalent mixtures. It showed that when comparing concrete mixtures with 35 and 70 percent slag cement to a non-slag mixture, the 35 and 70 percent mixtures increased concrete reflectivity by an average of 14 and 20 percent, respectively, over the wavelengths measured.
An EPA study (Levinson and Akbari, 2001) estimated that grey portland cement concrete albedo (a measure of reflectivity) was 0.41 to 0.52, depending on the mix. Mixtures made with white portland cement (a special cement formulated for white architectural exposed concrete) provided albedos of between 0.69 to 0.77. Slag cement concrete, although not included in the study, would fall somewhere between these values (as indicated in the aforementioned industry study), since it is whiter than portland cement, but not as bright as white cement.
Holcim (U.S.) Inc., Unpublished test data, developed by Concrete Technology Laboratories (Skokie, IL) on solar reflectance of concrete mixtures containing slag cement, 2000.
Levinson, R., and Akbari, H., “Effects of Composition and Exposure on the Solar Reflectance of Portland Cement Concrete,” Lawrence Berkeley National Laboratory Report LBNL-48334, 2001, Berkeley, CA.