Design 40A / Christina Cogdell
March 14, 2016
Raw Materials for Cement
As people all walk on sidewalks, walk into buildings, go to their homes and drive over roads every day the value of cement, being the second most consumed substance in the world right after water, is highly overlooked. On average, each year, “three tons of concrete are consumed by every person on the planet,” according to Madeline Rubenstein in an article known as “Emissions from the Cement Industry.” Obviously it is not an everyday thing that we walk into a bank and stop to look around or stop on the side of the highway to acknowledge how important cement is in our ever day lives. Although many roads and structures could be damaged and people complain, cement is an indispensable product which is tightly linked to the global economy and continues to rapidly increase its production rates by 2.5% annually. It is thanks to cement that we have firm buildings that can mostly withstand heavy storms and earthquakes and bridges and roads to make traveling easier. Kids just love the sight of big cement trucks with the huge roller on the back and adults probably think it’s a simple process of just a matter of adding water to the powder but many people are not fully aware of how cement is created or where it comes from. This essay is a study of all the raw materials that go into cement from the initial extraction of raw materials, the process of making cement and to the recycling of the product.
The raw materials needed to create cement are extracted from huge quarries of limestone, rock, chalk and clay. In the “Manufacturing Process” section for cement in the website entitled Lafarge.com, it states that cement generally consists of calcium carbonate, silica, iron ore and alumina. In order to obtain these materials blasting is required, traditionally using black powder and dynamite but modern types of explosives use ANFO (ammonium nitrate/fuel oil), slurries, and emulsions. Under the article “Blasting,” by Maurie Phifer and Priyadarshi Hem, certified people who have gone under at least three years of training known as blasters follow specific guidelines in order to have safe detonations and controlled ground vibrations to reduce a massive amount of over break. To give more room to these detonations and vibrations “pre-splitting” is a useful technique which consists of a Splitter inserting two metal rods in a V-shape inside the hole where the explosive will be inserted, then another metal rod being pushed down in between the other two rods to break apart the ground. Blasters also follow designs such as bench blasting, which is, “short-hole blasting which is usually limited to drilling rounds of 1.2 m to 5.0m length and hole diameters of up to 43 mm…Holes are generally drilled in a square pattern” (Phifer and Hem 1). Being that blasting is one of the most hazardous factors of mining, as reported on the National Institute for Occupational Safety and Health website, the selection of explosives varies between critical diameter, hydrostatic pressure, temperature, minimum primer weight, density weight strength, , gap sensitivity, water resistance, coupling or decoupled properties, shelf life, reliability for bulk operations and overall drilling. The level of danger with this is definitely extremely high so many precautions need to be taken to have a fully successful blasting.
After the blasting is done the raw materials are then removed by pit loader tractors and dumped into large haul trucks. They are then transported out of the pit and to the plant where they will continue to be processed. The next step is breaking down the big rocks and in order to do that the raw materials are put into a primary crusher where they are broken down to smaller pieces. Then it passes through a very fine grinder to finally have a fine powder consisting of about 85% limestone and 15% shale which, according to the “Manufacturing Process” article, is known as raw meal. The roller mill grinder is used for more than 85% of the time today for all the raw material grinding compared to the traditional ball mill grinder. After adding a few more materials and chemicals such as gypsum the raw meal is stored in large silos and awaits until the next step.
The raw meal is then transported to the kiln to either a wet process kiln or a dry process kiln. In the wet process kiln the raw meal is moved in a slurry form which is a semiliquid state, typically from particles of the cement in water. In the article “Manufacturing – the cement kiln” it explains how the raw meal passes through a rotary kiln which, “…is a long cylinder rotating about its axis once every minute or two. The axis is inclined at a slight angle, the end with the burner being lower” (“The Cement Kiln” 1) and also states how this type of kiln was introduced in the 1890’s. This type of kiln was quickly was being used during the early periods of the 20th century because it was “…giving continuous production and a more uniform product in larger quantities” (“The Cement Kiln” 1). The cylinder itself can be up to 200 meters long and 6 meters in diameter, which has to be this long in size because a lot of water is evaporated so the process of the heat transfer is not very efficient. The slurry may contain up to 40% water so it does take a lot of energy under the sintering portion to evaporate it all, even being heated at 1400-1500 degrees Celsius. “The wet process has survived for over a century because many raw materials are suited to blending as a slurry. Also, for many years, it was technically difficult to get dry powders to blend adequately” (“The Cement Kiln” 1) although today many cement kilns have adopted the dry process. Compared to the wet process, a dry process kiln is substantially smaller measuring around 70 meters long and that is thanks to an additional burner known as precalciner. The precalciner decarbonizes 85-95% of the raw meal before it enters the kiln, heating it up to temperatures around 900 degrees Celsius. The dry process is much more efficient than the wet process due to the fact that since the dry process kiln is smaller and uses less heat energy it reduces the capital costs of a new cement plant and it produces the same amount of the next substance of the raw meal known as clinker.
Once the clinker is made it is dropped into the clinker cooler. The reason for this is to cool the clinker, hence its name, but it has several important and beneficial factors to it. According to the “The Cement Kiln” article, cooling the clinker, “…is necessary to prevent damage to clinker handling equipment such as conveyors…is beneficial to minimize clinker temperature as it enters the cement mill. The milling process generates heat and excessive mill temperatures are undesirable…the cooler reduces energy consumption by extracting heat from the clinker, enabling it to be used to heat the raw materials…faster cooling of the clinker enhances silicate reactivity” (“The Cement Kiln” 1). After the cooling the clinker is finely grounded once more to make what is known as “pure cement” which is then ready to be sent out in bulks either by cargo ships or trains or by bags that are shipped out to stores and markets by trucks. The cement is then ready to be used by simply adding water and sand to make concrete.
Fortunately a lot of these raw materials and the cement can still be recycled after they have been used. “According to a 2004 Federal Highway Administration study, 38 states recycle concrete as an aggregate base; 11 recycle it into new Portland cement concrete” (“Recycled Aggregates” 1) and the states that do use the recycled concrete aggregate report that it works equally as virgin cement without any recycled additives. It is crushed down once again in order to be mixed into new cement which can be used for pavements, shoulder roads, sidewalks, curbs, gutters, bridges, and those are just a few things that the recycled cement can be used for. In order to mix the recycled cement into the new cement, “It is recommended that recycled cement aggregate be batched in a pre-wetted and close to a saturated surface dry condition, like lightweight aggregates. To achieve the same workability, slump, and water-cement ratio as in conventional concrete, the paste content or amount of water reducer generally have to be increased” (“Recycled Aggregates” 1). The fact that the recycled cement still contains some of the original aggregates and hydrated cement paste still allows it to have the same durability as virgin cement. With much more cement being recycled it reduces the amount of materials that need to be landfilled thus reducing the economic impact of the cement. Not only does it reduce the landfill it reduces the need for raw materials for virgin cement which also reduces the environmental impact from blasting and mining. Lastly, another important aspect about recycled cement is that it absorbs large amounts of carbon dioxide from the surrounding environment.
Although it is a major process in order to create cement it is definitely worth it and really valuable. Cement is just as equally important to the human life as water and food is. It is interesting and amazing to know how it all begins with the extraction of the raw materials from the big quarries. Although a lot of work goes into tearing down enormous parts of the ground places rehabilitate the quarries to preserve ecosystem sites such as Haller Park in Mombasa, Kenya. Recycling is such an important aspect for cement as well. As natural resources continue to be used up it is necessary to preserve the raw materials in order to help out the grinding, burning, and cooling processes become more efficient. There are always new ways coming up to reduce the amounts of energy and waste that go into making cement which become extremely useful as much more product is being produced for less work. The amount of work that goes into making this substance is truly significant and cannot be underestimated once it becomes concrete since it basically makes the human life much easier.
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"Manufacturing Process." Lafarge, 2016. Web. <http://www.lafarge.com.ng/wps/portal/ng/2_2_1-Manufacturing_process>.
Phifer, Maurie, and Priyadarshi Hem. "Blasting." Mine Blasting & Explosives Technology, and Safety Regulations. InfoMine Inc., Mar. 2012. Web. <http://technology.infomine.com/reviews/Blasting/welcome.asp?view=full>.
"Recycled Aggregates." Recycled Aggregates. Portland Cement Association, 2015. Web. <http://www.cement.org/for-concrete-books-learning/concrete-technology/concrete-design-production/recycled-aggregates>.
Rubenstein, Madaleine. "Emissions from the Cement Industry." State of the Planet Emissions from the Cement Industry Comments. Earth Institute, Columbia University, 9 May 2012. Web. <http://blogs.ei.columbia.edu/2012/05/09/emissions-from-the-cement-industry/>.
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Wastes and Emissions
Cement is a building material that was first developed by the Greeks and Romans, and is still used in a different variation today ("Cement"). Commonly confused, it is a binding agent, while concrete is a composite building material made from cement mixed with aggregates like crushed rock. The life cycle of cement can be broken down into three main steps: production, implementation, and post-usage. There are wastes and emissions to consider in every part of this system, and Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by mass of calcium silicates (3 CaO·SiO2 and 2 CaO·SiO2), the remainder consisting of aluminium- and iron-containing clinker phases and other compounds. As per ASTM standards, “The ratio of CaO to SiO2 shall not be less than 2.0. The magnesium oxide content (MgO) shall not exceed 5.0% by mass.” ("Cement")
The basic elements of modern (Portland) cement are iron, silicon, aluminum, and calcium. There is also usually some sulfates and other trace substances added too. These basic elements are found in nature in limestone, clay, and sand. All three major substances are extracted from earth, a process that it is both very energy and time consuming, and leads to a significant amount of wastes and emissions.
To begin with, calcium compounds, like limestone, are extracted most commonly via mining. Limestone mining consists of blasting rock faces. When a limestone reservoir is found, workers dig up earth around it to expose the rock face. It would seem simple enough to just take chunks of the rock to a factory, but limestone is a very tough material to just use blunt force and dig at. To combat this, miners drill strategically calculated holes in a 40x25 ft rock face and fill them with dynamite to detonate. A series of carefully placed explosions can bring down a significant amount of rock than compared to simply excavating. It is not without faults, as dynamite mining is very loud and the shockwaves produced by the blasts affect nearby ecosystems. In addition, these explosions cause ultra-fine bits of limestone (a few microns wide) to go into the air, which can lead to respiratory problems when breathed in (Nisbet). The process of digging earth around the mines and moving the limestone produces many emissions too. Diggers and trucks burn large amounts of fossil fuels to transport all the rock. Typically, these machines use diesel engines that release large amounts of CO2 and NOx gases. Particulate filters and technologies like advanced engine management help drastically reduce emissions compared to decades ago, but given the fact that the diesel engines that power diggers and mine trucks are well over 40x times more massive than the average car engine, it is still a significant amount of pollution.
The mining itself, however, has the biggest impact. Limestone is calcium-based mineral. Therefore, when mixed with water, it will form a basic solution. During mining, there is often water involved such as when groundwater or streams are diverted. This can destabilize land when an underground water table is affected, or ruin ecosystems that depend on a river (Nisbet). Such drastic changes are often irreversible and have lasting impacts. In addition mixing of the limestone with this water, whether accidental or intentional can affect the pH balance of the ecosystem. Even a slight increase in pH causes all plant and animal life to be dysfunctional, as several key body processes including cell structure depend on safe pH levels around 7. Along with limestone, mining chemicals like lubricants, and cleaners are often mixed in with water, poisoning wildlife.
Similar environmental effects are evident for the extraction of silicon, aluminum and iron. Once all these materials are obtained, they are shipped to a processing facility to be mixed in set proportions. From there, they are sent to be processed in these specific ratios. In summary, this amalgam of materials is heated to 2700 degrees Fahrenheit to produce marble sized balls called clinker (PCA). These balls are then crushed, which at that point is powdered cement.
As with mining, shipping plays a considerable part in terms of emissions produced by the industry. The premix is transported most commonly via truck, train, or ship. Once at the factory, workers load materials into crushers that make the rocks and minerals sand grade. Usually, there are two sets of crushers: one for to grind into a coarse, gravel like consistency, and one that is sequentially after to further grind it into a sand-like coarseness. This system is usually consistent of a series of conveyor belts and chutes to help transport materials in and out of the crushers. Although seemingly harmless, having the powdered raw meal, also known as premix, be in open contact with the atmosphere is not ideal. This is because the powder is so fine, even a small gust can make a cloud. In the factory, the premix is tossed and thrown around by conveyors, allowing billions of micro-particles to enter the air. This is hazardous to lung tissue, as the mix is caustic, and workers must wear respiratory protection like a respirator. This dust can be carried through the wind and can settle down on nearby land or water, affecting the ecological chemistry there (Csanyi). Airborne premix is such a problem that modern cement facilities have designed ways to combat it, with techniques ranging from water misting to strategically designed transfer systems. Water mist traps the microscopic particles in droplets, which can then be collected by an air filter. An idealized transfer system is uses conveyor belts that churn the material as little as possible and therefore cause less dust to go flying. In addition, the number of chutes is lowered, while the height at which the premix drops is made to be as little as possible.
Once the material is moved through the transport system, it reaches the initial stages of the furnace. First, the mixture passes through a series of tubes and containers that circulate hot air to form a vortex. This is to “preheat” the mix for the official furnace. The hot air is actually just waste air coming from the furnace, and using it to preheat the mixture. Once it is preheated, it goes into a rotary kiln. Here the minerals melt together to form the active compounds in cement like calcium silicate while being rotated in a giant oven that is slightly angled downwards to promote flow. During this process and the preheat, CO2 is released from the breakdown of calcium carbonate. This breakdown leads to the purification of lime, but also churns out large amount of CO2; it is the most CO2 heavy step in the cement lifecycle along with the kiln fire. Unlike the meal dust, there is no viable method for capturing all this CO2, as it is extremely hot and economically inefficient to store chemically or pressure-wise. To heat the kiln, a gas or coal furnace is used to bring internal temperatures nearing 1600 degrees Fahrenheit. As one would expect, the burning of such hydrocarbon fuels leads to the production of NOx and CO2. However, due to the presence of minerals in the meal, noxious gases like SO2 are formed too. Such gases are less in quantity than CO2 or NOx, but are more damaging to the environment. For example, SO2, when mixed with water, can form H2SO4, more commonly known as sulfuric acid. This reaction happens in clouds and causes a phenomenon called acid rain, which ruins manmade and natural materials. These gases are funneled into the preheating chambers where they transfer heat into the meal, helping the factory conserve energy. After this heat exchange, they pass through a particulate filter and are piped into the atmosphere.
After the kiln, the melted mix starts to form into small marbles called clinker (from the sound they make). Clinker is cooled down by passing through a grate with that is ventilated with cool air. This heat exchange takes the heat out of the partially molten clinker (PCA) . The hot air is directed back into the preheating and kiln to help conserve energy. This is the “cleanest” step in the lifecycle, as there is no waste or emissions. After the cool down, the clinker is ground up into a fine powder, which is cement. Typically, the clinker is fed into a rotating cylinder that has thousands of steel balls and small quantities of gypsum in it (PCA). As the device rotates, the steel balls and clinker collide, pulverizing the clinker. This process is very loud, and it creates airborne dust. Some of this is leaked when pouring out the newly formed cement.
Cement’s wastes and emissions are mostly concentrated in the production phase. The rest of the lifecycle, although much less burdensome on the environment, still has an impact. When construction companies use cement for building structures, they usually have some runoff water that has cement mixed in. In addition, random gusts of wind can blow away cement dust. With a pH of 13.5, cement is very basic and caustic enough that workers must use protection when handling it. This runoff and airborne material can cause respiratory problems and disrupt local ecosystems by altering the pH levels of the water (EPA). Once cement is mixed with water/stones to make concrete and set, it has almost no environmental effects. Over years, a little erosion may occur, but the ratio of water/air to is negligible. In some batches of concrete, a significant amount of radiation is released due to it containing traces of potassium, uranium, thorium, and radon-releasing minerals. This is only poses a problem if the air contained by the cement structure is stagnant.
Cement is one of the most valuable materials used in society; its consumption is only upped by water. Nearly every modern construction, whether it be a sidewalk, statue, or skyscraper has a majority of its structure due to concrete. Due to its massive amount of production, cement production is responsible for 5% of global CO2 and requires mining for its key elements (Rubenstein). With that said, research into its formation has allowed for the production of recycled cement and even new forms that are made from organic materials. With newer technologies, cement can be produced with far less wastes and emissions, enabling the world to have one less burden on its environment.
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