For the past decade, the concrete industry has been getting bad publicity over the carbon footprint of cement, and it’s true that cement manufacturing does have an impact on the environment. So if cement manufacturers come up with a way to improve concrete sustainability with no negative impacts on constructability, performance, or durability, why would we resist using it?
While the construction industry if oftentimes cautious with new materials, the evidence shows that there is no reason now not to adopt portland-limestone cements (PLC) with open arms.
Portland-limestone cement is approved by the majority of DOT agencies in the United States and is approved for use by the Alabama Department of Transportation beginning December 1, 2019. It is a slightly modified version of portland cement that improves both the environmental footprint and potentially the basic performance of concrete. It is now described in ASTM and AASHTO specifications and is used just like traditional portland cement in mix designs. It can be made at any portland cement manufacturing plant. While ordinary portland cement (OPC) may contain up to 5% limestone, PLC contains between 5% and 15% limestone.
To make this product, a metered proportion of crushed, dried limestone is fed to the finish-grinding mill, along with clinker and gypsum. In almost all cases, the limestone used in PLC is the same limestone used as a raw material for cement manufacturing at that plant. The limestone is more easily ground than the clinker, which is harder, and becomes concentrated in the finest particles.
A key component for the development of U.S. specification requirements was the interest in providing options to continue to improve the sustainability of cement and concrete construction. Typically, PLC has about 10% lower CO2 footprint compared to portland cement, due to the replacement of clinker by about 12% fine limestone, which is not pyro-processed. This directly helps lower the initial sustainability impact of concrete roads, bridges and other structures.
How does adding 10% to 15% of limestone work? Three primary reasons are particle packing, improved hydration, and some slight chemical reaction.
Since limestone is a softer material than clinker, it is easier to grind. Finer limestone particles can fill in gaps between larger clinker grains in the cement and can reduce the porosity of the paste. In addition, the higher surface area of fine limestone particles provides surfaces for hydrating cement phases to form and grow, and they develop away from the reactive grains, potentially allowing more complete reactions of the clinker. A small amount of limestone does chemically react (it is essentially inert), but the solids that form can reduce the porosity and add to strength development. All of these changes are small but positive.
Effect on concrete properties
Fresh concrete properties using portland limestone cements should be the same as OPC. End users report equal performance of concrete with PLC in terms of workability, placing, and finishing. Even though PLC is finer than Type I or II OPC, all of the extra fineness is due to fine limestone particles, which are relatively inert in fresh concrete.
PLC performance is essentially equivalent to that of non-limestone cements from the same plants in terms of shrinkage, permeability, freeze-thaw resistance, and resistance to salt scaling, with even slight enhancements in some cases.
Improving the sustainability of concrete
PLC substitution for OPC is the most significant improvement to concrete sustainability with current technology. When OPCs with up to 5% limestone are replaced with PLCs containing 10% to 15% limestone, the resulting impact per million tons of cement produced equates to 443,000 to 664,000 million BTU less clinkering energy used and 189,000 to 283,000 tons reduction of CO2 emissions.
PLC has the potential to significantly improve concrete sustainability with performance equal to or better than OPC and may be used seamlessly as a direct substitution for OPC in mix designs. Although concrete’s durability makes it an inherently sustainable construction material choice, use of PLC is an option to make concrete even more environmentally friendly.