8.+Material+Balances+and+Flow+Analysis

Authors: Michael Vrtis, Sam Bell, Jeremy Gregory, Alec Van Huele

=Objectives=
 * Introduce material balances and flow based on the law of the conservation of mass.
 * Explain the significance of the origin and disposal of infrastructure materials on the overall sustainability of a project
 * Provide a detailed example of a complete material balance and flow analysis using Portland Cement as a model

=Application to Costa Rica= A better understanding of material balances can only help Costa Rica to continue to be a global leader in ecologic and environmental preservation. Costa Rica has successfully been able to market its natural resources and has since become a mecca of "eco-tourism". Knowing the full implications of a material selection gives the project coordinator more power and responsibility over environmental impacts than previously available. With this additional environmental consideration, Costa Rica will be able to further protect it's valuable natural resources even when working in urban areas by simply recognizing which materials have greater environmental impacts.

Introduction
All infrastructure components require the use of building materials. Common infrastructure materials are concrete, steel, and timber. In the past, materials were primary selected based on cost and availability. However, there are many more factors that must be considered when selecting the proper material for a sustainable project. A thorough understanding of the origin, application, and disposal of a specific building material is required to evaluate the sustainability of the material. A material balance can be more simply described with the “Law of Conservation of Mass.” The “Law of Conservation of Mass” states: the mass of the products of a reaction equal the mass of the reactants. Or in other words “the amount of mass coming into a system is equal to the amount of mass coming out”. This law was developed by a French scientist, Antoine Laurent Lavoisier, prior to the French Revolution. Although he is now credited as being the father of modern chemistry, he was executed shortly after the French Revolution for his political beliefs. [1] The same logic must be applied to infrastructure materials. Materials are not created at the local hardware store where they are sold; they do not disappear once the dump truck leaves the demolition site. It is necessary to consider the entire process of a material’s natural origination through its complete decomposition. Figure 1 shows the important characteristics of a material as it flows from the earth, to its application, and eventually back into the earth. == Figure 1- Material Balance Flow Chart

Origin
There are both financial and environmental costs associated with bringing a material to a job site. Material availability is the first concern when examining the materials origin. A material cannot be used in any project if it is not available in the quantity needed. The availability directly affects the cost and effort necessary to obtain the material. Extraction is the next concern, for example lumber needs to be cut from a tree and metals, aggregates, minerals, and hydrocarbons need to mined from the earth. All of these extractions require at least some amount of energy. The required energy and the resulting emission from exploration and extraction must be included in the material balance. Once the material has been removed from the earth it often needs to be improved by a manufacturing process before it can be used in modern infrastructure. Some manufacturing processes are more obvious than others. Iron ore must be heated to high temperatures and carefully proportioned with other metals in order to make steel. Crude oil must be refined before it is used in asphalt. Lumber is dried and often treated before it is ready for use. Even aggregates require crushing and screening prior to their arrival at the project site. The required energy and the resulting emissions of the manufacturing process need to be included in a complete material balance. No matter what type of material is used, the material must be transported from the extraction site to the manufacturing site and finally to the project site. Again, the required energy and the resulting emissions of transporting materials need to be included in a complete material balance.

Infrastructure Application
The actual application of a material has historically been the primary concern when selecting materials. The major material characteristics when choosing a suitable material are performance, life span, and aesthetics. The performance of a material is vital to the success of the infrastructure. A material cannot be used if it does not have the ability handle its application. The life span of a material is the amount of time the material can successfully handle the given application. Aesthetic appeal can only considered after a material meets desired performance and life span requirements.

Disposal
Inevitably, an infrastructure component will need to be demolished for a variety of reasons such as failure or becoming unnecessary. The demolition and removal process requires energy and will produce emissions. All aspects of demolition and removal, from fuel usage in transportation to dust generation, must be included in the material balance. Some materials can be reused in future infrastructure products. The ability to reuse a material eliminates most of the concerns associated with the origin of the material for a given project because the origin would be previously addressed in the material balance on the original application. Even when small portions of a material can be reused it reduces the environmental burden of future projects and reduces the amount of material that must be wasted in a landfill. Building materials vary in their natural decomposition rates but eventually all materials will be broken down naturally. The complete decomposition of timber can be measured in years, steel measured in centuries, and plastics measured in millennia.

=Portland Cement = Portland cement (PC) is the most common cement agent used in modern concrete construction. The primary raw materials needed for the creation of PC are limestone, marlstone, shale, sand, and clay. The chemical constituents of these raw materials necessary for PC creation are calcium oxide, silicon dioxide, aluminum dioxide, and iron oxide. Figure 2 shows the percent of each raw component required for 1 kg of cement. [2] It is important to note that the total weight of the constituents is greater than the amount of cement is produced because of the release of large amounts of carbon dioxide.

Figure 2- Portland Cement Raw Constituents [2]

Raw Material Extraction Energy, Environmental Impact, and Emission
As seen in Figure 2 limestone is the primary constituent in PC and therefore will be the focus of the raw material extraction. Limestone is a sedimentary bedrock material that is available in large deposits worldwide. It is primarily composed of crystalline calcium carbonates (CaCO3). The traditional extraction method for limestone has been an open mining technique in which limestone is removed in large surface quarries. This type of extraction has negative impacts on the surrounding environments including habitat destruction, pollutant release, and increased runoff. [2] An alternative to surface mining is underground mining. Underground mining has less of an environmental impact but is more cost intensive and presents more dangers to the required labor force.

Manufacturing Energy and Emissions
Once all of the raw constituents have been obtained and transported to the plant, a chemical analysis of each constituent is determined. The actual chemical composition of each constituent material varies depending on the source. PC requires a very specific blend of each chemical component; therefore, it is necessary to quantify the amount of required chemical components brought in by the constituents in order to create the desired blend. After the proper batch ratios are known, the constituents are ground and blended together. At this point in cement production, there are four common processes utilized for the manufacturing of PC. The wet process is the oldest and most energy-intensive process. The other three processes are derivations of a dry process: the long dry process, dry process with pre-heater, and dry process with pre-calciner. [2] Regardless of which process is selected the blended materials must be heated at 3,400° F and held at the temperature for long periods of time. This heating occurs in a large rotary kiln and the final product is a marble sized mater ial known as clinker. The clinker is ground with small amounts of gypsum to produce a fine grey powder that is known as PC. The production of clinker is the major source of negative environmental impacts of cement production. Both the energy required to heat the kiln as well as the emissions released by the constituents create large amounts of carbon dioxide.

Application
Portland Cement Concrete (PCC) continues to have negative environmental impacts even after it is in place. The hydration reaction that occurs as concrete sets emits large amount of carbon dioxide. The color and solar reflectivity of the concrete contributes to the urban heat island effect. Lighter colored concretes absorb and radiate less heat than darker concretes and should be used when possible. The type and source of pazzalons used contributes to the color. Increased runoff and poor drainage designs are also problems with large concrete surfaces such as parking lots. Pavements cover the natural soil and replace vegetation that once stabilized the ground. With increased pavement surface areas there is an increase in the amount of runoff water generated by a storm because the ground is now unable to absorb moisture. High storm-water runoff necessitates drainage systems to control the water. Even carefully-designed drainage systems are sometimes overwhelmed by runoff water. Permeable concrete mixes allow water to drain through the pavement and have a promising impact on urban runoff.

Disposal
Concrete is easily recycled by removing any reinforcing steel and then crushing it down to a desired aggregate size. Most recycled concrete is used for the base and sub-base of a pavement. There are some states that use reclaimed concrete aggregate (RCA) for both coarse and fine aggregates in new concrete mixes. [2] Used concrete is readily available throughout urban areas. The reuse of concrete not only eliminates the transportation and landfills cost on the disposal but it reduces the cost on the future project as well. Figure 3 shows the simplicity of a typical concrete recycling. The crusher can be brought directly to the demolition site, where it breaks large chunks of concrete into smaller size pieces that are ready to use as aggregate. Figure 3 - Concrete Recycler = = =References=
 * P. G. Larson, "Antoine Laurent Lavoisier," University of Virginia, [Online]. Available: http://cti.itc.virginia.edu/~meg3c/classes/tcc313/200Rprojs/lavoisier2/home.html#conservation. [Accessed 25 May 2012]. ||
 * M. Calkins, Materials for Sustainable Sites, John Wiley & Sons, Inc., 2009. ||
 * "Industry Direct," [Online]. Available: http://news.directindustry.com/press/zhengzhou-dingsheng-engineering-technology-co-ltd/stone-crusher-rock-crusher-investment-trend-analysis-85283-363995.html. [Accessed 12 June 2012]. ||