Material, Energy and Greenhouse Gas Savings

When slag cement reduces the amount of portland cement in concrete, three principal environmental benefits result:

• Avoided use of virgin raw materials
• Avoided energy
• Avoided greenhouse gas   

Quantifying these environmental impacts is important when comparing alternatives and when setting policy priorities.  A “life cycle inventory” (LCI) is the quantification, for a product or process, of all material and energy inputs, and all emissions outputs, including inputs/outputs of constituent materials, transportation and manufacturing operations.   An LCI analysis is systematic and documented, so that LCI results can be independently audited.  ISO Standards 14090 and 14091 provide guidelines for performing an LCI.

The SCA (Construction Technology Laboratories, 2003 (2 reports), and 2005) and the Portland Cement Association (Nisbet, Marceau and VanGeem, 2002) have developed LCI values for the constituent materials and processes involved in manufacturing various types of concrete.   Some of the results of these studies are shown in Table 1, which compares the inputs and outputs for manufacturing a ton of portland cement and slag cement.  The table shows that for each ton of portland cement replaced by slag cement, 100 percent of the virgin material, 86 percent of the energy and 98 percent of the carbon dioxide emissions can be saved.

Table 1: Life Cycle Inventory Results

for Portland and Slag Cements

Using the LCI values from Table 1, along with additional calculations for aggregates, transportation and plant operations, the LCI values of slag and non-slag concrete can be calculated and compared.   Table 5 quantifies the benefits of using concrete mixtures with slag cement in lieu of mixtures without slag cement.    These mixtures include: 5,000 psi ready mixed concrete, concrete block, 7,500 psi precast concrete, concrete pavement (based on a standard Maryland DOT mix), and mass concrete.

The LCI results show that, for these example mixtures, slag cement can save between 4 and 15 percent of the virgin materials used, 20 to 42 percent of the energy expended, and 29 to 59 percent of the carbon dioxide released in the production of concrete and its constituent materials.    This analysis is conservative, since it does not consider the benefits achievable when a mixture is optimized, and total cementitious material per cubic yard of concrete is reduced.

The figures shown in Tables 1 and 2 provide quantitative evidence that policies encouraging or requiring the use of slag cement in concrete, to the maximum extent possible within the construct of good engineering practice, can provide significant benefits in energy, emissions and virgin material reduction.

Table 2:  Life Cycle Inventory Savings for Concrete with Slag Cement

Compared with No/Low Slag Cement Concrete