Environmental Impact of Concrete
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"Sustainable development is the development that meets the needs of the present generation without compromising the ability of future generations to meet their own needs. It requires the reconciliation of environmental, social equity and economic demands. It is the capacity to endure" - Dawei Han
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Preamble
All over the world by now, it is an accepted fact that - "Concrete is the most commonly used construction material in the world, and after water is the second most consumed product on the planet". "Each year worldwide the concrete industry uses 1.6 billion tons of cement, 10 billion tons of rock and sand and 1 billion tons of water. Every ton of cement produced requires 1.5 tons of limestone and fossil fuel energy inputs"- (Mehta 2002). And its use is expected to double in the next 30 years (Eco-Smart Concrete). Concrete's popularity is due to the many advantages the material offers. It can be durable and high strength with the proper mix of cementitious and pozzolonic materials, admixtures, aggregates, and water. A high reflectance value can be achieved to aid in heat island reduction. It is generally locally available. It can be used without finishes, and, with the right mix, is resistant to weathering. It can be made porous to aid in storm water infiltration and groundwater recharge. And recycled materials can be incorporated into the mix, reducing consumption of raw materials and disposal of waste products.
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Popularity of concrete has its own demerit in terms of its constituents and manufacturing process. It also has an impact on the environment. The most harmful of which is the high energy consumption and CO2 release during the production of Portland cement.
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There can be several measures to minimize the environmental and human health impacts of concrete and some can result in improved performance and durability of the concrete as well. One of the most important strategy being minimizing the use of Portland cement by substituting industrial by-products like fly ash, ground granulated blast furnace slag, or silica fume or other cementitious materials for a portion of the mix. Recycled materials substituted for natural aggregates will also help to minimize the use of nonrenewable materials and the environmental impacts due to extraction process of these materials. Porous concrete can contribute to the sustainable function of a site by allowing for storm water infiltration, and light-colored concrete can minimize a pavement’s contribution to the urban heat island (UHI) effect.
This article discusses the environmental impacts of commonly used concrete components and the possible remedies.
Portland Cement
The Indian cement industry is the 2nd largest market after China accounting for about 8% of the total global production. It had a total capacity of about 347 m tonnes (MT) as of financial year ended 2012-13. Cement is a cyclical commodity with a high correlation with GDP. The housing sector is the biggest demand driver of cement, accounting for about 67% of the total consumption. The other major consumers of cement include infrastructure (13%), commercial construction (11%) and industrial construction (9%).
According to PCA, Portland Cement Association, the manufacture of Portland Cement is a four stepped process.
- The virgin raw materials, including limestone and small amounts of sand and clay.
- The materials are carefully analyzed, combined, and blended, and then ground for further processing.
- The materials are heated in a very large kiln, which reaches temperatures of 1,870°C (3,400°F). The heat causes the materials to turn into a new, marble-sized substance called clinker.
- Red-hot clinker is cooled and ground with a small amount of gypsum. The end result is a fine, gray powder called Portland cement. This cement is so fine that one pound of cement powder contains 150 billion grains.
Portland cement is manufactured with one of the following processes: wet process, long dry process, dry process with preheater, or dry process with precalciner. The wet process is the oldest and most energy consumptive. Newly constructed plants and some that are retrofitted use the more energy-efficient dry processes of preheater or precalciner.
In addition to the release of carbon dioxide and energy use, mining of limestone, the major raw material in cement, causes habitat destruction, increased runoff, and pollutant releases to air and water. Some limestone mining operations are abandoning open pit mining techniques in favor of underground mining. This technique may reduce some habitat and pollution impacts yet may increase cost.
Energy Use - The production of cement is an energy-intensive process using primarily fossil fuel sources. Cement composes about 10% of a typical concrete mix but accounts for 92% of its energy demand. Cement production requires the pyro-processing of large quantities of raw materials in large kilns at high and sustained temperatures to produce clinker. Coal is the primary energy source here for cement production, followed by petroleum coke and electricity, a high percentage of which is produced from coal.
Emissions - Emissions from Portland cement manufacturing include carbon dioxide (CO2), particulate matter, carbon monoxide (CO), sulfur oxides (SOx), nitrogen oxides (NOx), total hydrocarbons, and hydrogen chloride (HCl). Emissions vary by type of cement, compressive strength, and blended constituents. Worldwide, the cement sector is responsible for about 5% of all man-made emissions of CO2, the primary greenhouse gas that drives global climate change (Humphreys and Mahasenan 2002).
Some methods that cement producers use to reduce CO2 emissions are as follows:
- Use of the dry process, which uses as little as 830 kWh/ton of clinker to produce. The less efficient wet process uses 1,400–1,700 kWh/ton of clinker (Humphreys and Mahasenan 2002). In the United States, new plants use the dry process and some older plants have converted from wet to dry.
- Increasing use of blended cements that include materials such as fly ash or slag which do not need processing in the cement kiln.
- Use of alternative fuels and fuel-efficient processes.
Waste Production - The major waste material from cement manufacturing is cement kiln dust (CKD). An industry average of 38.6 kg of CKD is generated per metric ton of cement. Seventy-nine percent of this is land filled and 21% is recycled (Medgar et al. 2006).
Water - Water is used in cement production to suppress dust, to condition or cool kiln exhaust gases, to finish mills, and for noncontact cooling. About one ton of water is discharged in the production of one ton of cement. Effluents result from quarry dewatering, storm water runoff of facilities, CKD pile runoff, and landfill wells. Discharged water contains suspended solids, aluminum, phenolics, oil and grease, nitrates, dissolved organic compounds, chlorides, sulfates, ammonia, zinc, and pH (Medgar et al. 2006).
Aggregates
Extraction and Manufacturing
Aggregates, coarse and fine put together in concrete make up 80% to 90% of the concrete volume. Aggregates are either extracted from the quarry or manufactured. Some are even by-products of industrial processes or post-consumer waste products. Natural fine aggregates are usually quarried natural sand and coarse aggregates are either quarried or manufactured from crushed stone. Sand and gravel are typically dug or dredged from a pit, river, or lake bottom. They usually require minimal processing. Crushed rock, a manufactured aggregate, is produced by crushing and screening quarry rock or larger-size gravel (Lippiatt 2007).
The primary impacts of aggregate extraction and processing can be pointed to two important aspects namely, habitat alteration and fugitive dust. It is difficult to capture dust in operations of mining and blasting, quarry roads, loading and unloading, crushing, screening, and storage piles. Primary impacts of crushed rock, aside from mining impacts, stem from fugitive dust released during crushing and screening operations. Processing of aggregates, particularly the commonly used silica sand, releases particulates into the air that can cause eye and respiratory tract irritations in humans.
Mining, dredging, and extraction of sand and gravel alter plant and animal habitats and contribute to soil erosion and air and water pollution. Mining for sand and gravel near or in water bodies causes sedimentation and pollution in water and disrupts aquatic habitats. The operation of mining equipment consumes energy and releases emissions from internal combustion engines. Impacts from mining and quarrying aggregates are discussed in greater detail in the stone and aggregates chapter.
Energy Requirement
Energy to produce coarse and fine aggregates from crushed rock is estimated by the PCA’s Life Cycle Inventory to be 35,440 kJ/metric ton. The energy to produce coarse and fine aggregate from uncrushed aggregate is 23,190 kJ/metric ton (Medgar, Nisbet, and Van Geem 2007). Energy sources are split evenly between diesel oil and electricity. Fuel consumption and environmental impacts of fuel combustion for transportation of aggregates can be significant, as they are heavy and bulky materials. Using local or on-site materials for aggregate can minimize fuel use, resource consumption, and emissions.
Embodied Energy
Production of concrete through Ready Mix Concrete plants in India still not caught up with rural areas. Majority of Rural area construction is still using conventional mixers. However, in case of concrete production through ready mix concrete plants energy use and emissions of ready mix concrete vary widely by cement type and use of pozzolonic constituents such as fly ash, silica fume or slag. Mixes with lower cement content and higher percentages of other pozzolonic constituents have lower embodied energy and lower emissions. A Life-cycle Inventory by the Portland Cement Association of three different ready mixes supports this idea. One mix studied was a standard 28-day compressive strength, 3,000 psi ready mix with 100% Portland cement. The second mix replaced 25% of the cement with fly ash, and the third replaced 50% of the Portland cement with slag cement. Key findings are (Medgar et al. 2007):
- Embodied energy for the standard PCC mix is highest at 1.13 GJ/m3 of concrete and is lowest for the 50% slag cement mix at 0.73 GJ/m3.
- CO2 emissions are highest for mix 1 at 211 kg/m3 and lowest for mix 3 at 112 kg/m3. CO2 reductions are even more substantial for mixes 2 and 3 because of the additional savings of CO2 release from calcination of limestone, which accounts for an average of 60% of CO2 emissions from the production of cement.
- Particulate emissions from cement production account for 70% of the total and aggregate production for 30% of total particulate emissions for concrete. The use of fly ash and slag lowers total particulate emissions.
Transportation
Transportation of materials throughout the life cycle of concrete varies. Portland cement is manufactured at different locations involves transportation causing enormous fuel usage. Admixtures, slag, fly ash, and silica fume may have longer transport distances, but their quantities in the mix are limited. Obtaining local aggregates will likely have the largest impact on reducing transport energy use and related emissions.
Recommended Remedies
Build Structures for durability
Some sources claim that many exposed exterior concrete structures and pavements are not built to last their 30–40 year design life and are, on average, only in place for half that time (Mehta 1998). Premature failure can result in a great deal of resource use for structures that must be replaced before the end of their design life. The reduced durability may be the result of a variety of factors, including improper mix design, improper placement or curing, or overuse of deicing chemicals (Mehta 1998). Mixes tailored to specific installations; use of pozzolonic or cementitious industrial by-products such as fly ash, silica fume, or ground granulated blast furnace slag; or use of high-performance concrete can extend the life of concrete structures.
The development and increasing use of high performance concrete (HPC) mixes can reduce the amount of energy-intensive cement, water, and/or aggregate used in concrete and result in a stronger, more durable structure. HPC is concrete that has a low water/cement (W/C) or water/binder (W/B) ratio, often made possible through the use of super plasticizers. It results in concrete of higher compressive strength (6,000 to 7,200 psi as opposed to the typical concrete mix’s 2,200 to 3,600 psi). It is considered to be economical as structures can be smaller or thinner, use less concrete and reinforcing steel, and require less formwork. High-performance concrete has a low porosity which makes it more resistant to freezing and thawing, sulfate and chloride-ion penetration, and other chemical attack. The life cycle of high-performance concrete has been estimated to be two to three times longer than that of usual concrete, and it can be recycled two to three times before it is transformed into road base aggregate (Aïtcin 2000).
Build Small but Effective
Designing smaller structures and thinner concrete sections can reduce the total amount of materials and resources used to make concrete. However, thinner sections of walls and paving may require increases in the amount and size of reinforcing needed, potentially negating any resource savings. In cold climates, use of modular unit retaining wall systems set on a sand base can eliminate the need for extensive concrete footings for a cantilever retaining wall extending below the frost line. Use of pier foundation systems may use less concrete than spread footing foundation systems, and they are often formed with recycled cardboard sonotubes rather than the typical plywood formwork. This makes it more resistant to freezing and thawing, sulfate and chloride-ion penetration, and other chemical attack. The life cycle of high-performance concrete
has been estimated to be two to three times longer than that of usual concrete, and it can be recycled two to three times before it is transformed into road base aggregate (Aïtcin 2000).
Use Cement Substitutes
Reduction in cement use in a concrete mix is easily achieved through the substitution of other pozzolonic or hydraulic materials for Portland cement. The most common Supplementary Cementitious Materials (SCMs) are industrial by-products used individually or in some combination in a concrete mix. These include fly ash (both Class C and Class F), ground granulated blast furnace slag (GGBFS), and silica fume. Other SCMs are natural pozzolans such as calcined clay, calcined shale, and metakaolin. While substitution amounts vary by design requirements and substituting materials, it is estimated that a 30% reduction of Portland cement use in mixes worldwide could reverse the rise in CO2 emissions (Mehta 1998). Replacing 50% of cement with ground-granulated blast furnace slag (GGBFS) in a typical ready mix is estimated to save 34% of embodied energy (560,000 btu) and 46% of embodied CO2 emissions (248 lb) per cubic yard of concrete (SCA 2006).
Other benefits of substituting some portion of SCMs for Portland cement are reduced air emissions of concrete mixes, the reuse of industrial waste products, and improved performance of concrete. SCMs’ basic chemical components - silica, alumina, calcium, and iron are similar to those of Portland cement and work in two sometimes combined ways (Federal Highway Administration [FHWA] 2006). Hydraulic SCMs such as GGBF slags and some Class C fly ashes set and harden like Portland cement when mixed with water. Pozzolonic materials, such as Class F fly ash or silica fume, require a source of calcium hydroxide, which is usually supplied by Portland cement in the mix to react.
Pozzolans can produce stronger and more durable concrete in the end; however, they take longer to gain strength than concrete with Portland cement. ASTM standard C618 defines a pozzolan as "a siliceous or siliceous and aluminous material, which in itself possesses little or no cementitious value but which will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties" (ASTM 2005b). In addition to industrial by-products, other pozzolans are mined (e.g., diatomaceous earth or volcanic tuffs) and manufactured (e.g., metakaolin from calcined clay). ASTM C618 defines these pozzolans as Class N. Fly ash and ground granulated blast furnace slag are sometimes blended with cement during the cement manufacturing process, resulting in reduced CO2 emissions, a reduction in energy consumption, and increased production capacity. Blended cements are discussed following this section on cement substitutes added during concrete mixing.
References
1. Materials for Sustainable Sites-Meg Calkins
2. Our built and natural environments A Technical Review of the Interactions between Land Use, Transportation, and Environmental Quality
3 Concise Environmental Engineering by Dawei Han
Acknowledgement:
Contents of this article are not the original thoughts of the author. Substantial part of the article is essentially the compilation of relevant information for the benefit of the readers. The author acknowledges the above references.
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World Business Council for Sustainable Development
The Cement Sustainability Initiative (CSI) is a serious international effort by leading cement companies to reduce environmental and human health impacts of cement production while "increasing the business case for the pursuit of sustainable development" (WBCSD). The group of eighteen cement producers, accounting for 40% of global cement production, is organized under the World Business Council for Sustainable Development. The stated purpose of the initiative is to explore what sustainable development means for the cement industry and identify actions and facilitate steps companies can take, individually and as a group, to accelerate progress toward sustainable development (WBCSD).