Professor Christina Cogdell
13 March 2014
Portland Cement Stucco Materials
In December 1923, The Washington Post asserted, “Few people…realize the great possibilities in Portland cement stucco. This adaptable material lends itself admirably to any style of architecture, any color scheme, any ideal which the builder desires to express.” Beyond the great potential of stucco, it is also waterproof, fire resistant, rot, insect, and fungus resistant, durable, impact resistant, low-maintenance, and low cost (By). This impressive capability of stucco is proven by its rich history that has extended far beyond 1923. Mesopotamian structures from 3000 to 3500 BC are the earliest use of stucco discovered thus far. From then onward, nearly every construction method throughout history has exploited some amount of stuccowork. Examples include Aztec architecture in Mexico, Islamic architecture in North Africa and Spain, ancient Greek temples, and ancient Roman monuments. The stucco, regardless of location and era, was a plaster combination of an aggregate, a binder, and water; the aggregate being loosely compacted mass of particles and the binder being some material that promotes cohesion amongst the aggregate, which was very often lime. Then in 1843, Joseph Aspin invented Portland cement stucco after experimenting with the heating of limestone. This new variation of lime stucco was exceedingly stronger, effective, and versatile (de Turenne). Portland cement stucco became the most widespread used type of Stucco and its great popularity remains today, particularly as an exterior finish for buildings. Though application of Portland cement stucco and its constitution may differ slightly, the primary composition of Portland cement stucco is a mixture of Portland cement, Type S hydrated lime, silica sand, and potable water.
Portland cement, the ingredient that distinguishes Portland cement stucco from other variations of stucco, is a fine gray powder that serves as the binder in the plaster mixture. To produce Portland cement, more than thirty raw calcareous, siliceous, argillaceous, and ferriferous components are obtained through mining and then chemically combined through a process of grinding and heating (United States Environmental Protection Agency, “Portland Cement Manufacturing,” 1). Calcium, the highest concentrated element in Portland cement, is acquired from the mining of a variety of materials, including limestone, chalk, marl, sea shells, and aragonite. The calcareous constitution of the materials determines which and how much of them are utilized, as a precise blend is required to obtain the correct chemical composition of Portland cement. The other elements in the raw mix of the cement, including silicon, aluminum, and iron, are obtained from ores and minerals such as sand, shale, clay, and iron ore. These raw materials are blended together, sized to appropriate chemical and physical properties, crushed, and then fed into a kiln. Dependent on the Portland cement facility and the raw materials it exploits, either a dry process or a wet process is employed. For the dry process, the mixture of materials is grinded dry in a mill. If upon grinding the desired dryness has not been met, the materials are transported by various conveyers to either impact dryers, drum dryers, paddle-equipped rapid dryers, air separators, or autogenous mills and dried with the gas heat exiting from the pyroprocessing system. For the wet process, water is added into the mill during the grinding of the raw materials. After either the wet or dry process, the wet or dry mixture is fed into a long, slightly declined rotary kiln. The raw blend enters the elevated side, and is slowly and continuously moved down the kiln while being heated with a progressively higher temperature. During this pyroprocessing, the raw mix undergoes chemical reactions and is transformed into gray, glass-hard, spherical nodules called “clinker” (EPA, “Portland Cement Manufacturing,” 3). Upon completion of the pyroprocessing, the clinker is fed into either reciprocating grate, planetary, or rotary coolers and is cooled from approximately 1100°C to 93°C. Once the clinker has been cooled enough to be maneuvered by the conveyer equipment, the material is once again blended and grinded to transform the clinker to a finer powder. During the final grinding, mined gypsum or natural anhydrite and other minor materials are added to yield the final properties of Portland cement. The Portland cement is then shipped either in bulk or in paper sacks, available for a variety of construction purposes.
Hydrated lime, a dry flocculent powder, serves as the binder in Portland cement stucco that allows the plaster to be spreadable. To begin the process of manufacturing hydrated lime, lime, a sedimentary rock consisting mainly of calcium and magnesium carbonate, is extracted from the earth. The rock is then crushed to a particular size, sized, screened, and washed. Next the particles are transported by conveyer belt to rotary kilns similar to those used in the manufacturing of Portland cement. In the rotary kilns, a gradually increasing heat calcinates the limestone and chemically alters it to quicklime. Once the quicklime has cooled, it is crushed and then placed into closed containers with water and subjected to high pressure. This high pressure causes a chemical reaction, hydrating the magnesium oxide in the lime and transforming the quicklime into fine particles of hydrated lime (Graymont Limited). The product is then milled to produce Type S hydrated lime as defined by ASTM C206 and C207, the variation of hydrated lime necessary for stuccowork. Milled to be a consistent fine particle size with high surface area, the hydrated lime is ideal for the plasticity and water retention needed in Portland cement stucco. Like Portland cement, Type S Hydrated lime is then shipped in bulk or in paper sacks, available for all construction and industrial purposes.
Sand serves as the aggregate in Portland cement stucco. Though stuccowork is not limited to variety of sand, the sand must have certain qualities to be appropriate for such a purpose. Industrial plaster sand serves this purpose, as it is the appropriate quality to be spreadable yet to not induce cracking in the stucco. Industrial plaster sand is high purity silica sand composed purely of silicon and oxygen typically obtained from quartz crystal (National Industrial Sand Association). Found in nearly every variation of rock and in river streams and banks from the weathering of rocks, the raw material is mined or excavated and then subjected to assorted processing. To first obtain the desired size of the raw material, multiple stages of crushing is conducted utilizing a variation of crushers that include gyratory crushers, jaw crushers, roll crushers, and impact mills (EPA, “Sand and Gravel Processing,” 3) However, to obtain the fineness of the sand necessary for plasterwork, the crushed raw material is then grinded to particles of fifty micrometers or smaller with the use of either smooth rolls, media mills, autogenous mills, hammer mills, or jet mills. The now minute particles are classified by being either wet screened, dry screened, or air classified, depending on the facility. After having been determined to be of correct size, the sand is washed to remove unwanted dust and debris, screened and classified again, and then subjected to an attrition scrubbing procedure. This scrubbing procedure employs an agitated, high-density blend that is rubbed against the sand—effectively removing all surface stains. To ensure the industrial sand is at least 95% Silicon Dioxide, a subsequent series of procedures remove any more possible impurities. This procedure involves pumping the sand and added water through cyclones for “desliming” (EPA, “Sand and Gravel Processing,” 3). If the sand was determined to contain mica, iron, or feldspar minerals, sodium silicate and sulfuric acid is added to the mixture, and the mixture is subjected to a froth flotation process. It is then sent through spiral classifiers that allow the impurities to float in the solution and be separated from the now pure whole grain silica. The sand is now dried to a moisture content of less than .05 percent using heat in rotary or fluidized bed dryers, and then cooled, screened and classified for a final time. The final product is packaged in either paper sacks or sandbags and shipped to be used for primarily flooring compounds, mortars, specialty cements, stucco, roofing shingles, skid resistant surfaces, and asphalt mixtures (NISA).
Clean potable water must be used in Portland stucco cement to ensure purity of the plaster so that it may perform to its highest capability. If unclean water is utilized, the impurities will prevent the formation of necessary air entrapment during mixing of the stucco, proper setting of the plaster, and strength development (Technical Services Information Bureau). Thus water must be processed and treated before applied in stuccowork. The purifying of the water can be accomplished with a wide variety of treatment processes. The choice of treatment depends on the initial quality of the water, the desired final quality of the water, and the community the water treatment plant is located in (as regions have different standards and requirements). Ground water normally requires less processing than water from streams, rivers, or lakes (EPA). However though there are many variations of water treatment, the majority shares similar processes. These processes are: coagulation, sedimentation, filtration, and disinfection. Once the water is delivered to the treatment plant, it is run through screens to remove large debris and then allowed to settle for a time so that some sedimentation of sand and gravel may occur. At this time, chemicals, most commonly Copper Sulfate, may also be added to prevent the growth of algae (Knappe 56). After this preliminary treatment, the water undergoes a coagulation process, in which it is vigorously mixed in a large basin with chemicals such as alum to cause small particles still present in the water to coagulate. The water then slowly moves through another tank, allowing the sedimentation of solid residue to accumulate at the tank’s bottom to be pumped or scraped out. At this point the water is tested for hardness with a USEPA-approved Hach titration procedure and treated accordingly. If too much calcium, magnesium, and other minerals are present, they are removed with a great variety of methods, including adding sodium and employing a magnetic water conditioner. If the water does not contain enough of the minerals, they are added. Next, the water undergoes filtration, passing through layers of sand, gravel, charcoal, and other similar substances and effectively having minute suspended particles be filtered out from the water. After filtration, fluoride is often added to reduce tooth decay. Before disinfection, the water is transported to closed tanks or reservoirs. Once this occurs, a disinfectant, most commonly chlorine, is added to eliminate disease-causing entities. The now potable water is then either directly transported from community holding tanks through pipes to faucets or is stored in other various containers and sold. Thus for stuccowork, though purchasing bottles of water is idyllic, water may perhaps be also obtained directly from the to-be-stuccoed building’s faucets if that area’s public water is reported to be safe enough to drink.
A mixture of Portland cement, Type S hydrated lime, silica sand, and potable water produces Portland cement stucco. The materials may either be purchased with the dry ingredient pre-mixed and only the addition of water being necessary, or the materials may be purchased separately at a cheaper cost and be mixed to one’s desire. The typical ratio of the materials is one part Portland Cement, one part hydrated lime, 1.5 to 5 parts sand, and a varied amount of water. The amount of sand and water added to the mixture depends on the desired appearance, the type of sand used, and the number of layers of the plaster being applied. The materials are mixed in a wheelbarrow or other comparable container until a homogenous thick paste results. Water must be added continuously throughout the application to maintain the desired consistency and prevent the mixture from drying out pre-application. The paste is then applied as a plaster in multiple layers to obtain a properly thick surface of stucco. The number of layers varies between one and three, with the most common and accepted being three. The three layers, the scratch coat, the brown coat, and the finish coat are mixed with different amounts of sand and are applied in a slightly different manner. Pre-application, the walls to which the stucco will be applied to are cleaned with detergent and water. Generally, a wire mesh lath is then nailed to the surface. The lath provides an even, solid, clean, stable surface for the plaster. However, if the wall is sturdy and rough enough for stucco to be directly applied, a lath is not necessary. The scratch coat stucco mixture of 1 part Portland cement, 1 part hydrated lime, 2.5 to 4 part sand, and water is then applied to the entire surface. This layer is applied thick enough to cover the lath, which is about 3/8 of an inch. Once the layer has been applied, it is scratched with a rake or an alternative to allow a rough based for the next layer. After the scratch coat has been allowed to harden for 36 to 48 hours, the brown coat is applied. The brown coat is a mixture of 1 part Portland cement, 1 part hydrated lime, 3.5 to 5 parts sand, and water. This layer is approximately 3/8 of an inch thick and is allowed to harden for 7 to 14 days. The final coat is then applied to a thickness of approximately 1/8 of an inch with the with a mixture of 1:1 Portland cement and hydrated lime and 1.5 to 2 parts sand and allowed to completely harden. During the hardening of each layer, the stucco must kept slightly moist be being consistently sprayed with clean water. This prevents cracks and uneven hardening of the stucco. Once the stucco has completely hardened, the stucco application is complete.
As proven by the ancient stuccowork that is still present today, Portland cement stucco may last upwards of a century without much maintenance. When maintenance is needed, it is usually to treat cracking in stucco occurs as the building experiences settling and movement. To treat cracking or some other destruction, stucco mixture may either be reapplied over just the particular area where the destruction is or over the entire stuccoed surface to ensure consistency. A variety of processed and synthetic sealers, caulk, and patching compounds may also be purchased and employed to fill the cracks. To further maintain the appearance of the stucco, it may be washed with warm water, a soft bristled brush, and a diluted solution of trisodium phosphate and bleach. If this maintenance is consistent and conscientious, the life of the stuccoed exterior will only end once the building is demolished or the siding has been decided to be removed. Unfortunately, once this has occurred, Portland cement stucco has little—if any— life beyond. If the broken stucco has been separated from the surface it was applied to, it may be recycled by being sent to crushing companies. These companies break down the solid pieces into smaller particles that may be used as aggregate for road base, subbase material, backfill, and other construction uses (Nelson and Roberts 12). If the stucco is not recycled or reused for another construction purpose, the life of Portland cement stucco ends at a landfill.
In conclusion, Portland cement stucco, a long-lasting, low-maintenance, long-used, resistant-to-many architectural plaster, is a mixture of processed Portland cement, hydrated lime, sand, and water. This material begins its life in Earth among a great variety of rocks, water sources, and earth. After considerable treatment and chemical processes at four different specialized facilities, the four primary materials are then either mixed at another facility or sold separately and then mixed at construction site. Post-application, the stuccoed surface may live for well over a century with little maintenance. Once it has been decided this stuccoed surface may no longer live as it has, the stucco’s life is either continued as an aggregate in another construction surface or ended at a landfill.
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13 March 2014
Embodied Energy of Portland Cement Stucco
When determining the net energy that goes into the production of portland cement stucco, it is critically important to consider the embodied energy of each of the component materials. This is because compared to the entire production process, the mixing of the final stucco mixture represents a practically negligible portion of the product’s total embodied energy.
Rather, huge sums of energy are expended during the production of stucco’s component materials—those being portland cement, hydrated lime, and sand. The predominant primary energy through which production is carried out is obtained by fossil fuels, each process typically involving a combination of oil, coal and natural gas usage. The extraction from the earth of the necessary raw material to produce stucco, the transportation and distribution of both the raw and the refined material, and above all the physical and chemical processing of the material, each necessitate tremendous expenditures of energy.
Fossil fuels, primarily oil, provide by far the greatest primary energy input throughout the production of Portland cement, but the energy that physically converts the raw material into cement is a combination of mechanical and thermal. Gas or diesel-powered tractors are the primary excavators of limestone and clay from quarries and clay pits (Cutler). The extracted limestone is first run several times through diesel-powered impact crushers, each operating at roughly 300-400 horsepower. This reduces the rocks to pebbles via mechanical energy. The pebbles are washed via diesel-powered washing machines and then, along with the clay, are mechanically combined in an either electric- or diesel-powered industrial mixer. With the addition of small amounts of iron ore or fly ash, the mixture fed into a rotary kiln. Through the burning of coal, oil, or natural gas, the material is heated to temperatures of up to 2,700 degrees Fahrenheit until trapped gases in the mixture are released; what remains are tiny balls of crude cement called clinker (National Institute). The clinker is transported by truck, train, or boat to cement plants, which expends diesel and/or coal energy. An electric-motor ball mill then reduces the clinker into a fine powder (Portland Cement). Where this electricity comes from depends largely on the grid powering the plant, but in the United States comes more often than not from the burning of petroleum or coal. Gypsum or raw limestone may next be added to resultant powder, at which point the mixture is ran once again through the ball mill. The cement mixture is lastly transported for packaging and distribution by truck or train, further consuming diesel and, occasionally, coal.
The production of hydrated lime involves many of the same steps as cement production; the embodied energy of the process is thus mostly similar. As in cement production, lime production begins with the quarrying and crushing of limestone. And also in lime production, the crushed rock is ran through a diesel-powered washer, which removes unwanted debris via mechanical energy. Next, the processed limestone is added to a rotary or vertical kiln for calcination. The kiln generates thermal energy by the burning of coal, diesel fuel, or various natural gases. This intense heat, combined with the injection of combustion gases, causes the chemical reaction that turns the limestone into lime. Moisture is then added to the lime by an atmospheric or pressure hydrator powered by one or two electric motors. Finally, the hydrated mixture is transported to distributors, typically by truck or train—but typically not overseas. Since limestone is a relatively abundant material and quarries are globally established, lime production is most often carried out domestically.
The silicate used for Portland cement stucco is variously called “builder’s sand”, “plaster sand” or “mason’s sand” (Imasco). It is extracted by tractor from quarries, much the same as Of the three component materials, it is sand that remains closest to its natural state, although it must first be run through a washer to separate the sand particles from mud and other debris. Sand is transported from quarries to packagers by either truck or by train, which expends fossil fuel energy.
Each of the component materials of portland cement stucco undergoes processing from its natural state; the raw materials for portland cement and lime in particular go through extensive physical and chemical processing. Every step along this process is, at least from an energy standpoint, very costly.
That such an energy-intensive process relies so heavily on the burning of fossil fuels is undoubtedly alarming. Perhaps of even greater concern, however, is how easily—and how totally—the typical plasterer or stucco mason might overlook this process. After all, stucco isn’t generally available premixed—it is the buyer’s job to combine the portland cement, lime, sand, and water to produce the stucco mixture used on commercial and residential building exteriors (Discovery). For this reason, it is not unfathomable that the typical stucco worker might merely include their own interaction with the materials when considering the embodied energy of the product. The component materials, sold separately, give little indication of the costly processes that went into their production; the names “cement,” “lime,” and “sand” even sound relatively close to the earth, as if the products are sold in a near-natural state.
In reality, of course, this couldn’t be farther from the truth. The Athena Sustainable Materials Institute estimates that 8.3681 megajoules goes into every square meter of finished stucco—that’s almost one megajoule per square foot (Athena 15). For perspective, 1 megajoule is the approximate kinetic energy required to keep a car traveling at freeway speed. Of that embodied energy, an astounding 97% is due to processing, and only 3% is due to extraction and transport.
With ratios as blaringly unbalanced as that, these manufacturers must be either extremely efficient at excavation and shipping, or extremely inefficient at processing their raw materials. Given the tremendous input of fossil fuel energy at every stage of the production process, one might be inclined to suspect the latter.
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Wastes and Emissions from the Production and Use of Stucco
Walking down the street one will notice that many buildings are coated with a plaster that has a concrete like appearance. This plaster is stucco and it is used in the construction of numerous buildings every year. Stucco can be used as a decorative material or as a way to cover up less appealing materials. However most people do not know what it is made of or how it is produced. The materials must first be mined, after that they are processed into the basic ingredients that are then mixed together. After stucco is applied and utilized it is ether put into a landfill or it can be recycled into other products. The ingredients that make up modern stucco include: Portland cement, Type S Hydrated Lime, sand, and water. The majority of the emissions and waste produced when manufacturing and maintaining stucco come from the processing of the raw materials into the ingredients for the stucco mix.
Portland cement is the most common type of cement. This is because it is the, “basic ingredient of concrete, mortar, stucco, and most non-specialty grout” and its ingredients are fairly inexpensive (Princeton, 1). It consists of ground Portland cement clinker mixed with small amounts of gypsum and limestone. The raw mix is primarily made of calcareous materials such as limestone, chalk or seashells. Other secondary raw materials that are sometimes included in the raw mix are clay, iron ore, or coal ash when the kiln is fueled by coal (Portland Cement Wikipedia). Portland cement clinker is produced by heating the raw mix in a kiln to a temperature of approximately 1450 °C (Princeton). This is called pyroprocessing, it causes a physical or chemical change to the materials and it is the most energy intensive part of the process (Portland Cement Manufacturing Section 11.6, 1). The mix is then cooled and mixed with the gypsum and limestone (How Cement is Made)(Reference Figure 1).
There are several emissions that are released during the production of Portland cement. They can be grouped to ether particulate matter (PM) emissions or gaseous emissions. During the first steps of the production, quarrying for the calcareous materials and grinding of the materials, the primary emissions are PM emissions and CO2 from the combustion engines used to move the materials. The follow on steps include the pyroprocessing in the kiln. Pyroprocessing produces PM emissions as well as nitrogen oxides (NOx), sulfur dioxide (SO2), carbon monoxide (CO), and carbon dioxide (CO2). Nitrogen oxides are generated solely from the fuel. The type of fuel used effects the quantity and type of nitrogen oxide produced. Recently plants have “switched to coal, which generates less NOx than does oil or gas”, some plants have even, “switched to systems that burn a combination of coal and waste fuel” but the effect on emissions is not clear yet (Portland Cement Manufacturing Section 11.6). Sulfur dioxide is generated from the sulfur compounds in the fuel and in the raw materials. The sulfur content in the materials varies with geographic location; pockets of pyrite in the limestone increase the emissions of SO2 (Portland Cement Manufacturing Section 11.6). Carbon dioxide is primarily produced from the combustion of fuel but is also produced through the reaction of calcareous materials with heat. Approximately 500kg of carbon dioxide per Mg of cement is produced from this reaction. This brings the total carbon dioxide produced from pyroprocessing to, “0.85 to 1.35 Mg of CO2 per Mg of clinker” this is, “approximately one third of the mass” of the primary material being lost as carbon dioxide (Portland Cement Manufacturing Section 11.6). Other small emissions such as carbon monoxide and volatile organic pollutants are also released from the incomplete combustion of fuel.
Controls for these emissions are in place as an attempt to lower them. Fabric filters are used to collect the particulate matter, once collected the dust is then added back into the kiln. Cement kilns also have a high alkaline internal environment, which allows the concrete to absorb 95% of the potential sulfer dioxdes. (Portland Cement Manufacturing Section 11.6). Once the Portland cement is made the remaining ingredients for stucco are hydrated lime and sand.
Hydrated Lime is made by the hydration of lime with water. Lime is, “the high-temperature product of the calcination of limestone”, this process is similar to the pyroprocessing seen when making Portland cement (Lime Manufacturing). Limestone is an abundant material and can be, “found in every state”, however, “only a small portion is pure enough for industrial lime manufacturing” (Lime Manufacturing). First the raw limestone is quarried and then crushed. Some plants will sell the ground limestone as a byproduct for agricultural purposes (EET Manual for Lime). The rest of the ground limestone is fed into the kiln where the heat chemically changes its compositing and releases a carbon dioxide molecule. At this point the new material, lime, can be sold as is or it can be hydrated with water to make hydrated lime, which is used in stucco (Lime (material)). Lime is hydrated using an atmospheric method or by pressure hydrators. The atmospheric method is more common and continuously processes the lime. Pressure hydration can process high quality lime but only work in batches (Reference Figure 2)
The primary emissions created in the manufacturing of lime are particulate matter (PM) emissions and gaseous pollutants. Particulate matter makes up the majority of the emissions and is considered, “the only dominant pollutant” (Lime Manufacturing). The largest contributor to particulate matter is the kiln, using a vertical kiln instead of a rotary kiln can reduce the emission. But rotary kilns require more fuel than the vertical kiln (EET Manual for Lime). Fabric filters are also used in an attempt to collect the particulate matter. The PM emissions are in the form of dust and are non-toxic, however the gaseous pollutants are all greenhouse gasses. The gasses released include carbon monoxide (CO), carbon dioxide (CO2), sulfur dioxide (SO2), and nitrogen oxides (NOx). All of these gasses are produced in the kiln while the limestone is being calcenated. Sulfur Dioxide emission is foremost influenced by the sulfur content in the fuel. Other influences include the sulfur content in the stone, quality of the stone, and the type of kiln. Carbon dioxide is discharged from the kiln as a result of combusting fuel and as an outcome of calcination limestone. Theoretically there is, “two moles of CO2 for each mole of limestone produced” (EET Manual for Lime). Interestingly a portion of carbon dioxide is recovered and reused in sugar refining (Lime Manufacturing).
Now only one more ingredient is left to make stucco, sand. Common sources of sand include beaches and dunes, but it can also be dredged from ocean and riverbeds. Mining and dredging for sand causes degradation to the local environment and wildlife often ruining specific habitats (EET Manual for Mining). Once the sand has been acquired from the source it is transported to a processing plant where it is filtered through vibrating screens to remove large rocks. Sometimes these large rocks are sold separately or they are crushed and added into the sand. Next the sand is washed and screened several times then dried before being classified for sale (Sand and Gravel)(Refer to Figure 3).
Throughout sand processing particle matter emissions are emitted from every step. Water is often added to the sand to try and lower the spread of the particulate matter and often a, “substantial portion of the emissions”, will, “settle out within the plant” (EET Manual for Mining). Some plants will also take extra steps to lower PM emissions by reducing the free-fall height and adding wind blocks for the storage piles (Sand and Gravel). Other emissions include the typical combustion products such as CO, CO2, and NOx. These are all discharged during transportation, use of machinery, and drying of the sand. Additionally the dryers may also be sources of, “volatile organic compounds (VOC) or sulfur oxides (SO2) emissions, depending on the type of fuel used to fire the dryer” (EET Manual for Mining). After completing the manufacturing of the basic ingredients for stucco all that remains is for them to be shipped directly to a distribution company. Transporting these material will result in carbon gas emission through the combustion of fuel. If we assume that the materials have to be shipped 30 miles and that we are burning diesel fuel to do this approximately .0037 metric tons of carbon equivalent per ton of virgin material will be released (USEPA Background Document). The gasses included as a carbon equivalent are methane (CH4) and carbon dioxide (CO2) (USEPA Background Document). Once the virgin materials reach their destination the distribution company will then assemble the mix for sale or apply the stucco for the consumer.
The application and use of stucco is a simple process. It can be mixed on site in a, “ratio of 4:12:1 (cement to sand to lime)”, with the addition of water until it reaches a, “peanut butter”, consistency (Stucco). The stucco can now be spread over the wall in any pattern or texture desired. However if the stucco is going on to a wooden wall metal grating must first be placed onto the wall. Three layers of stucco are usually applied to the whole surface. There are no emissions from this process; the only waste created may include used wash water. Maintenance of stucco is also very easy, if a crack forms or a section of stucco breaks off new stucco is mixed and then applied to the damaged spot. This produces no new emissions besides the ones made manufacturing stuccos basic ingredients
When stucco reached its end of life usefulness it is torn down then ether put into a landfill or recycled as aggregate. Aggregate is, “crushed stone, gravel, and sand”, it can be used, “as road base, fill, and as an ingredient in concrete and asphalt pavement”, approximately, “2 billion tons of aggregates are consumed each year in the US” (USEPA Background Document). Using crushed stucco instead of virgin aggregate results in avoiding the emissions from mining and processing new aggregate. Additionally recycling old stucco means that it does not have to take up space in a landfill. But because of the huge demand for aggregate only 5% of the aggregate produced comes from recycled materials (USEPA Background Document). This form of recycling is an “open loop” system because the stucco is recycles into a product other than itself.
The recycling method does produce some emissions, these occur during the transportation and processing of the old stucco. The machines used to transport and grind the stucco require the use of combustion engines and fuel. In total recycling stucco makes .0025 MTCE/ton (metric tons of carbon equivalent per ton) of stucco recycled (USEPA Background Document). This was calculated assuming a average of 30miles for transportation. If we compare this number to the total emissions produced from making virgin aggregate we find that we are saving .0021 MTCE/ton of emissions from being produced.
The wastes and emissions produced throughout stuccos life cycle occur most heavily in the beginning and then appear again during the recycling process. The production of the ingredients results in particulate matter pollution and emission of CO, CO2, SO2, and nitrogen oxides. These gasses are primarily emitted during the burning of fuel for kilns or dryers but can also come from chemical reactions within the raw materials themselves. It is not surprising that these gases are released when burning fuel, but it was interesting to discover that a portion of raw materials themselves are converted to CO2 gas when exposed to heat. The most shocking part what that the portion converted to CO2 was 1/3 of the raw material. Once the stucco was created the application, use, and maintenance of it does not contribute any emissions. It is only once the stucco is recycled that emissions are once again produced. This makes sense considering that the old stucco would need to be transported and crushed by machines that ran on fossil fuel. The emissions at this step include carbon-based gases like CH4 and CO2. Stucco may have the illusion that it is not harmful to the environment since the average consumer will not witness any emissions being released, however it is important to remember that there are pollutants being produced at other steps.
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