The production of cement occurs on a huge scale, and the resulting byproducts have a large effect on the environment. Cement is used to make sidewalks, buildings, bridges, dams, pipes, roads, canals, storm drains and numerous other products. Approximately 1 ton of cement is produced for every person on the planet each year.[1] Approximately 5% of global carbon emissions are produced in the manufacture of cement.
Cement production requires large quantities of raw materials and energy. The main component of cement is called clinker. Clinker consists of cinder lumps formed by heating limestone, bauxite, and iron ore sand to 2,770o Fahrenheit. The production of clinker is very resource intensive. Producing one ton of clinker requires an energy input of between 3000-6000MJ and approximately 1.6 tons of raw material.[2]
Cement Manufacturing Process
The cement manufacturing process is diagramed in the flowchart in Figure 1. Processes requiring energy inputs are outlined in yellow and processes requiring heat are outlined in red. The pollutants emitted during manufacturing are particulates and gases such as CO2, SO2 and NOX. Coal fly ash slag or pozzolans may be blended with the raw material. The addition of these optional materials will result in lower emissions.
Traditional Cement Manufacturing
Cement kiln dust (CKD)is a by-product of cement manufacturing. CKD is collected using particulate control measures such as filtering and scrubbing. For every ton of clinker produced, approximately 300-400 pounds of CKD are landfilled. Some alternative cement manufacturing processes use CKD for carbon sequestering or recycling into the raw material input stream.[1]
Portland vs. Blended Cement
In many modern cement manufacturing plants, waste from other industrial processes is blended with the raw cement materials to change the properties of the cement. This practice can reduce the use of raw materials in cement production.
Carbon Sequestration
Carbon Sequestration is the capture and permanent, safe storage of the greenhouse gas CO2. Most sequestration efforts have been focused on the storage of CO2 in oceans, deep geologic formations, and the terrestrial biosphere. However, carbon sequestration can be achieved on a smaller scale during the production of cement. When CKD is combined with CO2, the CO2 is stabilized. The main advantage of mineral carbonation is the formation of carbonate minerals such as calcite (CaCO3) and magnesite (MgCO3), end-products which are known to be stable over geologic time scales.[3]
Life Cycle Assessments
Life cycle assessments (LCAs) evaluate the environmental impacts of a product or a process. Most LCAs evaluate environmental impact over the entire life-cycle (from cradle to grave) of a product or process.
Scope
Cradle to grave assessments are complex. For cement, a cradle to grave assessment is especially difficult because cement has so many end uses, and each use has a unique, often complex life-cycle. Therefore, the assessments reviewed here are "cradle-to-gate" studies. These cradle-to-gate assessments evaluate impacts of producing cement from the raw material extraction process to the packaging and shipping process. Thus, the end use and disposal of cement are not included in these assessments.[1][4]
Methodology
All life cycle assessments share the objective of evaluating the impacts that a product or process has on the environment and identifying the resources required to produce, use, and retire that product or process. Despite this common goal, there are a multitude of diverse methodologies used to achieve this objective.
Huntzinger et al. performed a cradle to gate LCA of cement and compared traditional processes with alternative manufacturing processes. The LCA used the following methodology:[1]
- The scope of the cement production process to be evaluated (the control boundaries) was determined.
- The inventory of outputs and inputs were determined.
- The environmental impact data found in literature were assessed.
- The results were interpreted and suggestions were made for improvements.
Josa et al. performed a comparative analysis of the environmental impacts caused by the production of 16 cement products in the European Union. The CML 1992 methodology was used in their cradle to gate assessment. CML 1992 is a problem-oriented method described in a publication by the Centre for Environmental Studies of the University of Leiden. The stages of this methodology were:[4]
- A life-cycle inventory (LCI) was compiled. An LCI quantifies the materials used, the pollutants emitted, and the energy used during the production of a product.
- A life-cycle impact assessment (LCIA) was developed for each cement product.
- The LCIAs were compared and analyzed.[4]
Results
Table 1 shows the emitted pollutants from the production of cement. Again, the portion of the life-cycle under consideration is from raw material extraction to packaging and shipping. The results for the 16 cement products evaluated were considered to be inconsistent by the authors. These inconsistencies were hypothesized to be the result of a variation in control boundary definitions for each product. For the purpose of this wiki, the 5 most consistent product results were averaged. These averages represent 5 type-I cement products from the European Union (EU)..[4]
The main constituents emitted from the production of cement are shown in Table 1. These constituents are CO2, SO2, and NOX. The total greenhouse gas emissions are primarily CO2. The emissions that contribute to acidification are primarily from NOX. Eutrophication emissions are in units of grams of equivalent phosphatic compound (PO4) per kg of cement. Dust and Winter smog have units of grams of suspended particulate matter per kg of cement.
| Pollutants Emitted from Cradle to Gate for 1 kg of Cement | |
|---|---|
| CO2 (g/kg-cement) | 812 |
| SO2 (g/kg-cement) | 0.63 |
| NOX (g/kg-cement) | 2.5 |
| Dust (g-SPM/kg-cement) | 1.5 |
| Total Greenhouse Effect (g/kg-cement) | 817a |
| Total Acidification (g/kg-cement) | 2.4b |
| Total Eutrophication (g-PO4/kg-cement) | 0.33c |
| Total Winter Smog (g-SPM/kg-cement) | 2.1d |
| a Includes minor contributions from CH4, N2O or HF. |
| b Includes minor contributions from HCl and NH3. |
| c Includes minor contributions from NH3, N-tot and COD. |
| d Includes minor contributions from soot. |
The results of Huntzinger et al.[1] are shown in Table 2. Table 2 shows the environmental impact scores for four cement products. These scores are used to compare the different technologies; they are an index of the environmental factors affected by cement production. The use of pozzolans as a substitute for clinker (Blended Cement) reduces the greenhouse score from 0.088 to 0.069. The recycling of cement kiln dust (CKD) and the use of CKD for CO2 sequestering both had more minor affects on the environmental impacts scores.
| Environmental Impact Category | Traditional | Blended | Recycled CKD | CO2 sequestration |
|---|---|---|---|---|
| Greenhouse | 0.088 | 0.069 | 0.088 | 0.084 |
| Acidification | 0.03 | 0.034 | 0.043 | 0.043 |
| Eutrophication | 0.006 | 0.005 | 0.066 | 0.006 |
| Heavy Metals | 0.204 | 0.161 | 0.204 | 0.204 |
| Carcinogens | 0.003 | 0.003 | 0.002 | 0.003 |
| Winter Smog | 0.039 | 0.031 | 0.039 | 0.039 |
| Summer Smog | 0.009 | 0.007 | 0.009 | 0.009 |
| Energy Resources | 0.05 | 0.04 | 0.05 | 0.05 |
Opportunities for Future Life Cycle Assessment
These cradle to gate life cycle assessments may be useful in additional studies. These results could be used in cradle to grave assessments of specific end uses of cement, e.g., an LCA of concrete pavements. Understanding the impacts of cement at each stage in it's life-cycle is important for putting the future production of cement products into a global perspective. These cradle to gate studies will hopefully help to provide such an understanding, and may be useful in determining new mechanisms to decrease the environmental impacts of producing cement.
References
- ↑ 1.0 1.1 1.2 1.3 1.4 1.5 1.6 Huntzinger, Deborah N., and Eatmon, Thomas D., "A life-cycle assessment of Portland cement manufacturing: comparing the traditional process with alternative technologies" Journal of Cleaner Productions 17 (2009) 668-675
- ↑ Boesch, Michael Elias, and Koehler, Annette, and Hellweg, Stephanie, "Model for Cradle-to-Gate Life Cycle Assessment of Clinker Production" Environmental Science Technology 43 (2009) 7578-7583
- ↑ Huntzinger, Deborah N. "Carbon Dioxide Sequestration in Cement Kiln Dust Through Mineral Carbonation" Michigan Technological University Dissertation (2006)
- ↑ 4.0 4.1 4.2 4.3 4.4 Josa, Alejandro, and Aguadoa, Antonio, and Cardimb, Arnaldo, and Byars, Ewan, (2007) "Comparative analysis of the life cycle impact assessment of available cement inventories in the EU" Cement and Concrete Research Volume 37, Issue 5, 781-788
