Avoided "Waste" (Disposal, Energy, and Emissions)
There are no comprehensive industry statistics on slag produced versus slag utilized in the U.S. However, the U.S. Geological Service (USGS) estimates that in 2005, between 9 and 11 million metric tons of blast furnace slag (in all forms) was produced (USGS 2004 Iron and Steel Slag Minerals Yearbook). The amount of slag utilized, and not imported, is approximately 10.8 million metric tons (8.1 million metric tons of slag aggregate per USGS, plus 2.7 million tons of granulated slag, Table 2). This equates to either a small amount of slag disposed (0.2 million metric tons) or a decrease of existing slag stockpiles (1.8 million metric tons). In either case, the statistics attest to successful programs and promotions throughout the years to utilize most or all blast furnace slag.
Avoiding disposal for nearly all blast furnace slag produced is, to a great extent, accomplished through the current levels of beneficial use in the U.S. It is important to continue, and even strengthen, existing policies and programs to ensure continued success for all forms of blast furnace slags.
However, looking at avoided material landfill disposal is only one aspect of “waste.” Another aspect is the waste that occurs when slag is not granulated, that of wasted energy (stored as chemical potential energy in granulated slag). Chemical potential energy is the energy stored in the bonds of atoms and molecules. There are many forms of chemical potential energy, including oil and natural gas. Portland cement stores chemical potential energy by converting earthen materials in a kiln to a form (clinker) that stores the energy until it is released in the chemical hydration process to produce concrete. So too, slag granules store some of the energy expended in the blast furnace as chemical potential, released during hydration. If slag is not granulated, that chemical potential energy is lost forever, essentially emitted as waste heat into the environment, and formation of non-reactive crystalline compounds in slag aggregate.
The corollary to energy wasted due to non-granulation is the “waste” of avoided greenhouse gas. If slag is not granulated, it cannot become slag cement, and it cannot reduce the use of portland cement in concrete and associated avoided greenhouse gas production.
In this era where energy and greenhouse gas production are critical environmental, national security, human health, and public policy issues, the concept of “disposal” and “waste” must be expanded beyond merely landfill disposal. It must also encompass situations where energy is “disposed” and greenhouse gas avoidance is “wasted.” Granulation of blast furnace slag in the U.S. is certainly one example where increased demand and beneficial utilization of slag cement could spur higher rates of granulation, ultimately avoiding the disposal of energy and avoided greenhouse gas production.