When there are fundamental changes to our human technologies that can affect global warming on a large scale, we should move as quickly as possible in that direction. But the fiddling continues, while the proverbial Rome burns. An eye-opening example is the manufacture and use of Portland cement vs. magnesium-based cements. Almost every aspect of the built environment, from bridges to factories to tower blocks, and from roads to sea walls, could be turned into structures that soak up carbon dioxide, the main greenhouse gas behind global warming. All we need to do is change the way we make cement.
When the Intergovernmental Panel on Climate Change examined the greenhouse gas emissions from various sectors, cement manufacturing and use was by far the largest contributor of carbon dioxide in the industry sector. In the big scheme of things, cement manufacturing worldwide causes significant amounts of CO2. The figures and estimates range from 3% to 7% of the total greenhouse gas emissions worldwide. This figure rises above 10 percent in fast-developing countries such as China, which currently manufactures one in every three tons of cement made around the world. In the United States, there was an increase in greenhouse gas emissions from 1990 to 2000, and again from 2000 to the present. This was attributed mainly to the production of electricity and more fossil fuels burned in the transportation sector, but the other large cause was an increase in cement manufacturing.
Portland cement involves extensive use of limestone (CaCO3), which is over 90% of the raw material. The process of making Portland cement releases extraordinary amounts of CO2, first from the extensive use of fossil fuels to produce the very high temperatures in the cement kilns, up to 1300 to 1450 °C, needed to create the chemical reactions needed, and the release of CO2 from the chemical reduction of limestone, which releases large amounts of CO2 derived from the decarbonation of limestone in which calcium carbonate (CaCO3), reacts with silica-bearing minerals to form a mixture of calcium silicates and forms CO2 as a by-product. The emissions of CO2 continue throughout the life of Portland cement.
Every year, some 1.7 billion tons of Portland cement are now produced worldwide, a staggering quarter of a ton for every six persons on Earth. For every ton of Portland cement emerging from the kilns, roughly a ton of CO2 escapes into the atmosphere. Cement manufacture is responsible for around 3-7 percent of total man-made CO2 emissions worldwide, a figure that rises above 10 percent in fast-developing countries such as China, which currently manufactures one in every three tons of cement made around the world.
Magnesium-based cements are created using different raw materials and a different chemical formula than Portland cement. Magnesium-based cements replace the calcium carbonate in the kilns with magnesium carbonate, a rock that occurs widely on its own, as the mineral magnesite, or in mixtures with calcium carbonate, such as dolomite. This magnesium-based cement has a number of major environmental advantages. The kilns don't need to be run so hot, which means that emissions of CO2 from the energy used to fire kilns are roughly halved. (Magnesium carbonate converts readily to magnesium oxide at around 650 °C, not the 1450 °C needed to create Portland cement.) During setting and hardening, a process called carbonation reabsorbs most of the CO2 created during the kiln from the air as the magnesium-based cement hardens and strengthens. Magnesium-based cements continue to absorb CO2 throughout their lifetime along exposed surfaces.
Magnesium-based cements with phosphates (MPC or artificial stone made from an acid-base reaction of magnesia and phosphates) have many superior properties to Portland cement such as high early strength, excellent volume stability, better durability, good fire resistance, and easy manufacture process. MPC has been utilized for many years for rapid repair of concrete structure, such as highway, airport runway, and bridges, etc. due to its rapid setting and high early strength. MPC can incorporate with a lot of non-toxic industrial wastes and translate it into useful construction materials. For example, the addition fly ash in MPC can be over 40% by mass of MPC, about double that of Portland cement. Non-toxic materials such as recycled industrial waste and organic materials can be added to make the final products. The resulting product has very good durability, including chemical attack resistance, deicer scaling resistance, and permeation resistance. Due to the high alkali environment of Portland cement, it cannot be used with some components, like natural fibers, without the risk of degradation and loss of strength. The sugar in some natural fibers, such as sugarcane and corn stalks, can prohibit the setting of Portland cement, but the set of MPC is not influenced by sugar, lending itself to innovative, insulative block made using natural insulative materials.
The switch to magnesium–based cements obviously could have significant impacts. And if magnesium–based cements gained a foothold in our cities, they could immediately reduce the cement industry's contribution to global warming, reabsorbing much of what was emitted in their creation, with some estimates showing we could eliminate over a billion tons of CO2 each
I discovered that Bindan Corporation, in Oak Brook, Illinois, offers magnesium-based cements. And George Swanson of Swanson Associates, in Austin, Texas, is like a guru of this product and its uses. Interviewing George Swanson, I found that other countries in Europe, and now China, “get it” about the magnesium-based cements and are already moving in that direction with large-scale projects and innovative uses and products.
The main problem in the U.S., other than the “leaders” who just don’t “get it,” is that the building materials industry is intensely conservative. It prefers what it knows - Portland cement. Engineers are familiar with its mechanical properties. And of course, Portland cement is cheap. It may guzzle energy like there is no tomorrow, but a couple of dollars will buy you as much of the stuff as you can carry away from a hardware store. The market for Portland cement is so vast that it is difficult to see magnesium cements making much of an in road in the next 10 years. Of course, if the world fries, Portland cement could prove to be very, very expensive.
We hear a lot these days about getting rid of incandescent lights and using fluorescent lights instead. There is even legislation proposed to require these changes. We need to do the same for Portland cement and magnesium-based cements.
|
