Portland Limestone Cement
Portland Limestone Cement (PLC) is being discussed as a substitute to replace regular Portland cement in Louisiana. The transition will require cooperation of the cement industry, contractors, ready mix producers and government and industry specifiers. Louisiana currently allows a PLC referred to as Type 1L on their Approved Materials List (AML).
Ton for ton, PLC mixtures performance is very much like ordinary Portland cement (OPC). Tests show that strength, density, durability and resilience are comparable. What is different is the 10 percent reduction in carbon emissions. The use of SCMs like fly ash or slag reduces a project’s carbon footprint even further.
Calcination accounts for approximately 60 percent of all carbon emissions in cement production. It generates more than 525 kg of CO2 per ton of clinker. Therefore, using less clinker is key to more sustainable concrete. Type IL cements do just that. Portland cement is about 91 percent clinker, while PLC is just 83 percent clinker. Limestone content is up to five percent in ordinary Portland cement (OPC), but as high as 15 percent in PLC.
PLC blends appear in residential, commercial and government projects. Specifiers list PLC concrete for bridges, pavements and a wide variety of structures. Schools, hotels, parking garages are some examples. In 2007, Colorado became the first state to allow the use of PLC concrete for paving. By 2018, there were more than 900 lane miles of PLC paving in Colorado, Utah and Oklahoma combined. Now, thirty state transportation departments permit the use of these blended limestone mixes.
European producers embraced Portland-limestone cement (PLC) decades ago. In the mid-1960s, German producers experimented with PLC for certain specialty applications. They experimented with mixtures including up to 20 percent limestone. By 1990, blended limestone cements were widely used in Germany. In 1992, the United Kingdom followed suit. It allowed up to 20 percent limestone in PLC formulations.
In 2012, ASTM C595/AASHTO M 240 allowed for Type IL cements with 5-15 percent limestone.
How is it possible to add this much limestone to a cement blend without compromising performance? Both physical properties and chemical reactions enhance performance. Limestone grinds finer than the clinker it replaces. This promotes denser particle packing and more even particle distribution, increasing nucleation.
The use of finely milled limestone particles introduces another small, yet positive advantage. It provides more surfaces on which cement hydrates may form. This frees up the clinker grains for extra hydration.
In the United States, PLC production is increasing. From 2012 to 2016, producers delivered 2 million tons of PLC. More specifically, Type IL cement production reached 348,000 metric tons in 2013. In 2016, plants produced 890,000 metric tons, an increase of 155 percent in the span of four years.