Pandemic-related supply chain problems have hit everyone, and a lot of industries continue to suffer long wait times and low inventory. This is just as true for paving as it is everywhere else. Traditional cement is in short supply. Fortunately, Portland-limestone cement, or PLC, is poised to pick up the slack.
Portland-limestone cement is sometimes called type 1L cement. The U.S. standard for traditional Portland cement allows for 5% of its clinker to be made up of limestone. PLC, on the other hand, can contain between 5-15% limestone.
PLC was first developed in the mid-'60s in Europe and was first used in paving applications in the U.S. in 2007. Despite the difference in composition, PLC performs very similarly to Portland cement.
Though Portland cement and PLC behave similarly, the increased percentage of limestone does give PLC some unique benefits and drawbacks:
Even before supply chain issues caused problems sourcing traditional cement, it was coming under fire for contributing to carbon emissions. Making cement means firing clay and other materials at high temperatures, which takes a lot of energy. The main energy sink in cement production is clinker. By using less clinker and making up the difference with limestone, you can create a product that requires less energy to produce.
As a result, PLC is about 10% less carbon-intensive to make than Portland cement.
This is a pretty controversial statement, but the strength of PLC may occasionally suffer. Overall, there's a lot of pretty robust research that suggests that this isn't the case -- PLC generally performs just as well as conventional cement. An analysis of the data in this research has led critics to say that a loss of strength is possible.
While this may be the case, it's important to note that a lot of critics are concerned about liability. Many of the pros of PLC don't benefit manufacturers and pavers directly, and they don't want to be left dealing with the aftermath if a project fails. Therefore, it's best to view this criticism as an abundance of caution. PLC has been used successfully for a long time outside of the U.S.
The physical and chemical properties of the limestone powder enhance the hydration efficiency of PLC. Since limestone is a soft mineral, it grinds very fine. This means that you get better particle packing and paste density with PLC over traditional Portland cement.
The tiny limestone particles also fit neatly in the intermediate spaces between the larger clinker pieces. There, they provide nucleation sites for calcium silicate hydrate crystals.
Chemically, limestone leaches calcium pretty readily. This means that it releases calcium compounds that are then taken up by aluminates in the cement, producing tough carboaluminate hydrate crystals.
Anyone who works with cement knows that hydration causes an exothermic reaction -- that is, it begins generating heat. Portland-limestone cement is more strongly exothermic than conventional cement. As it forms bonds and hardens, it gives off more heat. Depending on the application, this heat may need to be managed.
The increased concentration of limestone in PLC doesn't preclude the use of recovered resources like fly ash and slag cement. This is important because fly ash is a waste product of the coal industry. Using it instead of natural pozzolans reduces the depletion of these resources, and cuts down on the most energy-intensive aspects of cement production.
Piggybacking on the concern about the loss of strength relative to conventional cement, critics of PLC say that, if you can increase the limestone composition of cement without a loss of strength, you should be able to use cement that meets ASTM C 150 specifications, reduce cement content and rely on sand or fines produced by grinding during mixing to achieve equivalent concrete strengths. By reducing the about of cement used, you'd achieve the same environmental benefits as PLC.
On the other hand, if PLC is less strong than conventional cement, then you'd have to use more of it and negate the environmental benefits.
Despite the concerns raised by skeptics, PLC isn't a new idea. It's been used in Europe and Canada for decades and is gaining wider acceptance within the U.S. Some states' Departments of Transportation began using it in the 2000s, and its use has only increased since.
The paving industry has suffered from both environmental and supply chain concerns. In this case, we may not need an innovative solution -- the answer might lie in something as simple as Portland-limestone cement. While its critics raise valid points, PLC has already been used for over half a century, and expanding its use may help us achieve carbon neutrality without a net loss in durability.
POSTED: July 18, 2022
TAGS: Cement Paving