By Claude Goguen, P.E., LEED AP
The term “green cement” may sound like an oxymoron to some, but it’s not. Many companies are hard at work reducing the environmental footprint of cement. From startups to the world’s largest cement manufacturers, it appears everyone is on the bandwagon.
Concrete is the second most used material in the world, trailing only water. According to the U.S. Geological Survey, domestic production of Portland cement increased to more than 80 million tons in 2014 and 4.5 billion tons worldwide. Most of that production is in China, which consumed more cement in the last four years than the United States has in 100 years.
The cement industry accounts for more than 5% of global carbon dioxide emissions. The majority of these emissions are due to the decarbonation of limestone and the energy required to heat materials in a rotating kiln to temperatures exceeding 2,600 degrees Fahrenheit. According to the U.S. Department of Energy, domestic cement production accounts for 2.4% of energy consumption, which is lower than iron and steel mills at 11% and paper mills at 15%.
The U.S. cement industry has been focusing on continuing to manufacture a superior product while improving energy efficiency and minimizing emissions. Portland Cement Association members are committed to reducing CO2 emissions 10% by 2020 from a 1990 baseline. The association is working to improve energy efficiency by 20% by the same deadline. Many companies have their own goals that exceed these benchmarks. Large companies have already reported they have achieved more than 20% reduction in CO2 emissions. Manufacturers are also progressing toward these goals by employing innovative practices. Most involve improving kiln energy efficiency, using alternative fuels rather than fossil fuels and producing less-carbon-intensive cements. Some are even experimenting with capturing waste heat to generate electricity.
While producers are continuing to work on reducing environmental impacts, research is taking place to find other ways to reduce emissions. Researchers at the Massachusetts Institute of Technology Concrete Sustainability Hub have conducted a five-year study examining the molecular properties of Portland cement. The study, titled “Combinatorial Molecular Optimization of Cement Hydrates,” was released in 2014. It focuses on enhancing the molecular properties of calcium silicate hydrate in order to develop a concrete with greater stiffness and strength. This could yield a more durable concrete that would reduce greenhouse gas emissions by as much as 60%. The study states that this could be achieved by reducing the ratio of calcium to silica, thus lowering the energy required to produce cement and reducing the amount of CO2 released from limestone during the manufacturing process.
Portland cement alternatives
There are many products on the market that are alternatives to OPC which are manufactured to enhance performance but reduce impact on the environment. Lower-carbon cements are an example. This technology involves reducing the amount of clinker used to produce ordinary Portland cement. Since clinker is made with limestone and produced in kilns, less clinker means less emissions and less energy. Byproducts from other industries, including slag and fly ash, are used as substitutes for clinker.
Limestone calcined clay cement is also a new product that claims to reduce CO2 emissions by 30% versus OPC. This technology, also referred to as LC3, was developed by researchers in Switzerland. Other lower-carbon cements are being developed using magnesium silicates. The resulting material can be heated at lower temperatures than limestone.
Portland limestone cement has been used in Europe for more than 25 years, often as an alternative to OPC in applications that do not require sulfate resistance (even though some tests have shown improved sulfate resistance for cements containing limestone). Limestone is a low-cost, readily available material that is easier to grind than clinker and leads to improved particle packing and hydration. It improves workability and reduces “bleeding” in PLC compared to OPC, but its main attributes are reduced cost and reduced CO2 emissions. PLC containing up to 20% limestone, manufactured in accordance with current standards, can reduce energy requirements and carbon emissions by as much as 10% compared to other Portland cements.
To use another oxymoron, the topic of green cement is not really old news. Companies have been working on environmentally friendly cements for decades. New regulations are an obvious driver for research, but the cement industry has been proactive in recognizing its contribution to pollutant emissions and working to limit those emissions.
A trusted material
NPCA members are dedicated to providing sustainable construction products. Advances in cement technology are helping them deliver a material that has been trusted for thousands of years, and will likely continue to be the material of choice for thousands more.
For more information on this or any other precast concrete sustainability related topic, please visit the sustainability page on the NPCA website, or contact Claude Goguen, director of sustainability and technical education at [email protected].
Claude Goguen, P.E., LEED AP, is NPCA’s director of sustainability and technical education.