Posted on March 17, 2022
All eyes are on greener cements that reduce emissions. Any gain ripples across a very large global concrete industry. Global cement production was 4.1 billion tons in 2020, up from 1.39 billion tons 25 years earlier. China accounts for 2.2 billion tons of the total. India’s annual production is 340 million tons. There are more than 2,000 active cement plants around the world.
According to the United States Geological Survey (USGS), this country’s annual production is about 90 million tons. There are approximately 100 active cement plants in the United States.
The production of Portland cement generates significant CO2 emissions. As one industry source notes, “The calcination process, combined with the gas generation to provide the high heat, are the sources of CO2 generation in the production of Portland cement clinker.” Compared to ordinary Portland cement (OPC), emissions related to Portland limestone cement (PLC) are about 10% less.
PLC reduces industry emissions without compromising performance. It has withstood the test of time. PLC was first used in Europe in the 1960s and Canada in the 1980s. In the United States, the first PLC paving project occurred in Colorado in 2007. Since then, real-world performance assessments of roads in Oklahoma, Colorado and Utah confirm its viability in the transportation sector.
Caltrans PLC approval
DOTs that authorize the use of PLC to help reduce the industry’s overall emissions. The California Department of Transportation is commonly referred to as Caltrans. The state agency is responsible for 50,000 miles of highways/freeways.
On January 25, 2022, Caltrans announced it would allow the use of PLC for construction and repair. Caltrans Director Toks Omishakin says, “Using low-carbon cement can cut Caltrans’ concrete-related carbon dioxide emissions annually by up to 10 percent. This is a big step in supporting California’s efforts to achieve carbon neutrality by 2045.”
In 2017, Caltrans consumed 325,000 tons of cement. Fully converting to PLC would reduce CO2 emissions by 28,000 tons per year.
Caltrans did its due diligence before it made the decision. The DOT funded research at Oregon State’s College of Engineering, for example. Researchers established that PLC met the performance standards of ordinary Portland cement (OPC). The findings came with a bonus — PLC not only meets those standards, it does so at a lower cost.
Ordinary Portland Cement: Details
Portland cement is a product of mining, crushing and then heating limestone to high temperatures. The high heat and rotation in a kiln combine to form a rock-like clinker.
Specific production steps include crushing, pre-heating, heating, cooling, milling and packaging. First, quarried limestone gets crushed and combined with clay and other components. The material is heated in the multi-stage pre-heater before it goes into the kiln. There, it is subjected to temperatures of 1250-1400° C. Carbon dioxide (CO2) breaks free, leaving calcium oxide (CaO). The formula for this chemical process is: CaCO3 - CO2 = CaO.
The calcium oxide (lime) combines with silica and alumina products to form clinker. Each nodule is up to one inch in diameter. The hot clinker is then cooled. Heat is often recycled to the pre-heater. Next, the clinker is milled together with gypsum to form a fine powder. The finished product is then bagged and shipped.
Portland Limestone Cement: Details
Cement plants produce PLC in accordance with ASTM C595, the “Standard Specification for Blended Hydraulic Cement.” Many DOTs have allowed blended cement in some form for decades.
In PLC, the extra limestone particles don’t just take up space otherwise occupied by cement particles. Rather, they become nucleation sites promoting hydration. Strength is comparable to standard Type I and Type II cement. The advantages max out at about 15% concentration. Above that level, nucleation gains are increasingly offset by the dilution of the cement. This compromises strength and reduces set times.
For PLC to make a worthwhile contribution to reduced emissions, it must perform even with the addition of cement replacements referred to as pozzolans. Otherwise, gains would be offset by the reduced use of those emissions-reducing substances.
Pozzolans are nothing new. The ancient Greeks and Romans used volcanic ash and finely ground volcanic pumice as pozzolans. Today, ground granulated blast furnace slag (GGBFS) and fly ash are popular pozzolans. They deliver essential hydraulic cementitious properties while reducing cement content in mixes. Other examples of pozzolans are silica fume, silica-glass, burnt clay and burnt shale.
GGBFS is a byproduct of steel production. It must be periodically removed from the blast furnaces. Penn State notes it contains silicate and aluminosilicate of melted calcium. GGBFS enhances workability and finishability. It also delivers thermal stress reduction, higher compressive strength and higher flexural strength.
Fly ash is a byproduct of coal combustion in power plants. In the plant’s boiler, molten particles rich in silica, alumina, and calcium solidify into microscopic, glassy spheres. Fly ash increases concrete’s strength, durability and chemical resistance.
The PLC Transition: How Fast?
The pace of the transition to Portland limestone cement is accelerating. Various cement producers have announced plans to convert plants to 100% PLC. Lafarge Holcim, Argos USA, Titan America, GCC and Lehigh Hanson are some examples. PLC production is also increasing at Cemex plants in Florida and Alabama. Together, PLC-producing plants will account for more than 10% of total projected cement production in 2023. This percentage will undoubtedly increase as more plants announce their PLC plans in 2022.
By 2023 or 2024, OPC production may come to an end in certain markets, leaving PLC as the sole option.
The Pennsylvania Aggregates and Concrete Association (PACA) monitors developments across the global concrete industry. This site reviews innovations pertinent to our members and to the general public. Should you have any questions about your next concrete project, please contact us.