Fig. 1. Lost Creek Road in Morgan UT utilizes PLC cement, and is a major transportation route handling heavy truck traffic for a cement plant
Portland-Limestone Cement—A Choice Material for Sustainable Concrete
Paul D. Tennis, Portland Cement Association
Portland-limestone cement (PLC) is a relatively new technology for the US, with requirements defining Type IL adopted in 2012 in specifications ASTM C595 and AASHTO M 240 (in parallel thanks to efforts of a joint AASHTO-ASTM task group). However, experience with this type of cement has been developed over several decades in Europe and other countries around the world. A key driver for the development of US 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 pyroprocessed and is easier to grind. This directly helps lower the initial sustainability impact of concrete roads, bridges and other structures.
Why would adding 10% to 15% of limestone, an essentially inert material, improve strength and other properties? 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. This can reduce the volume of water required for workability and 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, possibly allowing more complete reactions of the clinker. A small amount of limestone does chemically react (it is essentially inert), but the solids that form also can reduce the porosity, improving strength and reducing permeability and shrinkage. All of these changes are small but positive.
Early- and later-age concrete strengths are similar when made with PLC or portland cement. For example, see Figure 2 with data for concretes made with a water:cement ratio of 0.40, for specimens with no supplementary cemeticious material SCM, or a water:cement ratio of 0.45 for specimens with 35% slag cement or 20% Class F fly ash. Note that the ability to use SCMs in concrete is not adversely impacted, which further improves concrete sustainability characteristics.
Fig. 2. ASTM C39/AASHTO T 22 compressive strengths of concretes at various ages made with PLC or portland cement, with or without SCMs (Thomas and Hooton, 2010).
Permeability, a durability indicator, is also comparable in concretes made with PLC and portland cement, with and without SCMs. Figure 3 provides ASTM C1202/AASHTO T277 data.
Fig. 3. ASTM C1202/AASHTO T 277 results for concretes made with PLC or portland cement, with and without SCMs (Thomas et al. 2010).
A common question for those new to PLC is how shrinkage is impacted. PLC is generally about 100 m2/kg (Blaine) finer that comparable portland cements. However, this does not increase shrinkage, in part because the finer ingredients in a PLC are relatively inert and help with particle packing. Data from Bucher et al. (2009) are provided in Figure 4. Total shrinkage is the autogenous shrinkage plus the free (drying) shrinkage. A concrete made with a PLC with 10% limestone lowered the shrinkage measurably.
Fig. 4. ASTM C157/AASHTO T 160 shrinkage of mortars made with cements with 0%, 5% or 10% limestone (Bucher et al. 2009).
Portland-limestone cement is a relatively new option for North American concrete, but has a long-history of use elsewhere in the world. Generally, a PLC will have a CO2 footprint about 10% less than a portland cement with comparable performance characteristics. 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.