Optimizing the Use of a Constrained Resource to Minimize Regional Greenhouse Gas Emissions: The Case Study of Slag in Ontario’s Concrete

Green policies currently incentivize
concrete producers to replace
portland cement with industrial byproducts to reduce their greenhouse
gas (GHG) emissions. However, policies are based on attributional
life cycle assessments (LCAs) that do not account for market constraints
and consider byproducts either available burden-free to the user (cutoff
approach) or partially responsible for the emissions generated in
the upstream processes (allocation). The goal of this study was to
investigate whether these approaches (and incentives) could lead to
a mismanagement of byproducts and to suboptimal solutions in terms
of regional GHG emissions. The use of ground granulated blast-furnace
slag (GGBS) in Ontario was studied, and an optimization model to find
the least GHG-intense way of using GGBS was developed. Results showed
that producers should replace 30 to 40% of portland cement in high-strength
concrete to minimize the regional GHG emissions associated with concrete.
However, traditional LCA approaches do not suggest this solution and
are estimated to lead to up to a 10% increase in concrete GHG emissions
in Ontario. The substitution method, which assigns emissions or credits
to byproducts based on emissions associated with the products they
may displace, can yield decisions consistent with the regional emission
optimization model. A revision of current policies is recommended
to include market constraints.