ecological profile of concrete & steel

AUTHOR: AU YONG THEAN SENG

www.madisonvelocity.blogspot.com

TOPICS HIGHLIGHTED OVERVIEW What is the difference between the greenhouse effect, global warming and climate change? Does only CO2 affect climate change? How does life cycle affect climate change?

STEEL Facts and figures on steel Energy used for steelmaking CO2 emission from steelmaking The real culprit: Blast furnace New Zealand - Undertaking world's best practice study

CONCRETE Energy used for concrete-making CO2 emissions from concrete-making What is the truth behind CO2 and the cement industry? Putting CO2 emissions into perspective Other air emissions Water pollution Solid waste Concrete reabsorbs CO2



OVERVIEW What is the difference between the greenhouse effect, global warming and climate change? Global warming is generally defined as an increase in the average temperature of the earth's atmosphere, especially a sustained increase sufficient to cause climatic change. Climate change is generally defined as any long-term significant change in the weather patterns. Climate change can be natural or caused by changes people have made to the land or atmosphere. “Greenhouse effect” is used to describe a scenario of how various gases cause global warming or climate change. Carbon dioxide (CO2) and other gases exist naturally in the atmosphere. These gases retain the sun’s heat and create the atmosphere that sustains life on earth. Burning fossil fuels such as natural gas, gasoline, coal, and oil adds unnatural amounts of CO2 and other gases into the air. These have the potential to trap heat, raise air temperatures, and change the balance of life on earth. These gases, in the form of pollution (emissions to air), have increased 30% in the past century. The primary source of CO2 emissions is fossil fuel power plants, which in the US, contribute to 35% of all CO2 emissions. Cars, sport-utility vehicles and other light trucks account for another 20%. Energy efficient buildings and vehicles worldwide can have a significant affect on climate change. Does only CO2 affect climate change? Although carbon dioxide produced by burning oil and coal is often singled out as the contributor to climate change, a number of other emissions to air (pollutants) as a result of human activities contribute to global warming. They include: methane (agriculture and burning natural gas), ground level ozone (car exhaust and power plants), water vapor (naturally occurring), nitrous oxide (fertilizer use and a pollutant) and chlorofluorocarbons (refrigerants and aerosol). Pound for pound, these other emissions to air have a much greater effect on global warming than CO2, as shown in the table 1 below. In terms of global warming potential, one pound of methane is 21 times more potent than one pound of CO2, and one pound of N2O is 310 times more potent than one pound of CO2. Similarly, the listed refrigerants are highly potent.



Table 1: Global Warming Potentials (GWP) and Atmospheric Lifetimes (years)



How does life cycle affect climate change? Concrete is a locally produced material shipped only short distances – another environmental and energy saving plus. Its primary components, sand and gravel or crushed stone, are among the most universally available materials. Accordingly, as wood and steel become scarce materials, developing nations are relying more on concrete. Concrete can last a lifetime or longer, unlike wood, it does not rot or burn; unlike steel, it does not rust. Concrete’s low maintenance needs and long service life requires less repairs and rebuilding, and, as a result, conserves additional energy and materials. When comparing construction alternatives, a life cycle assessment (LCA) provides a level playing field. An LCA is based on a consistent methodology applied across all products and at all stages of their production, transport, energy use, maintenance, and disposal or recycling at end of life. A number of published articles espouse the sustainability of one building product over another based on a few selected metrics instead of a full life cycle assessment (LCA). For instance, some articles representing themselves as LCA studies use only the metrics of embodied energy or embodied CO2 emissions. These comparisons are flawed because they only consider limited metrics and do not cover a full life cycle assessment of the product or building. A full LCA includes the impacts of energy use and associated emissions over the life of the product or structure, such as climate change, acidification, materials acquisition, and human health effects. Studies show that the most significant environmental impacts are not from construction products but from the production and use of natural gas and electricity to heat, cool, and operate the buildings. For concrete houses compared to wood frame houses, the CO2 emissions from the production of the cement used in the house is more than offset by the savings in CO2 emissions from energy savings during the life of the house.



STEEL Facts and figures on steel

Figure 1: Steel Consumption’s Figures & Distribution by Country

Source: World Coal Institute, Coal & Steel Fact (2006)

Figure 2: Steel’s Production & Distribution by Country



From the figure 2, it can be seen that China accounted for most of the increase in world steel production, with production increasing by 25% in 2005. China’s share of the world total rose to 31%. Energy used for steelmaking Currently, energy represents about 20% of the total cost of producing steel and is rising. The increasing cost of energy and even its current and future availability have led to the need to refocus attention on energy intensity in steel production. To address this issue long-term, American Iron and Steel Institute (AISI) members are proposing the “Saving One Barrel of Oil per Ton”, or SOBOT, Research Program. The steel industry as a sector reported 12.6 million BTUs per ton of steel shipped in 2003. This means it takes the equivalent of 2.07 barrels of oil to produce a ton of steel [using 6.09 MMBTU per barrel of oil].

Table 2: Steel Industry Energy Usage (2003)

The goal is to develop new steelmaking technologies which, when in commercial use, will take steel production from 2.07 barrels of oil/ton [2003] to 1.2 barrels of oil/ton in 2025, approximately one barrel of oil less to produce a ton of steel.

Table 3: SOBOT Energy Use Goal (projected 2025)

Greenhouse gas emissions in the steel sector are primarily the result of burning fossil fuels during the production of iron and steel. Currently there are two main routes for the production of steel: production of primary steel using iron ores and scrap and production of secondary steel using scrap only. A wide variety of steel products are produced by the industry, ranging from slabs and ingots to thin sheets, which are used in turn by a large number of other manufacturing industries. Table 4 provides information on typical primary energy intensities of the key iron and steelmaking processes.

Iron making During the iron making process, palletized iron ore is reduced using coke (produced in coke ovens) in combination with injected coal or oil to produce pig iron in a blast furnace. Limestone is added as a fluxing agent. Reduction of the iron ore is the largest energy-consuming process in the production of primary steel. In 1994, this process was responsible for over 45% of the CO2 emissions from US integrated steelmaking and had a primary energy intensity of 18.6 GJ/ton of steel produced (including the energy used for ore preparation and



coke making). Other countries, such as Finland and Luxembourg, it use significantly less energy for iron making, consuming 12.7 and 12.9 GJ/ton, respectively.

Table 4: Ranges of primary energy intensities of key iron and steelmaking processes

Smelt reduction processes are the latest development in pig iron production, omitting coke production by combining the gasification of coal with the melt reduction of iron ore. Direct reduced iron (DRI) is an alternative iron making processes. DRI is produced by reduction of the ores below the melting point in small-scale plants (<1 Mt/year) and has different properties than pig iron. Production of DRI typically requires between 10.9 and 16.9 GJ/ton of steel, including the energy used for ore preparation. DRI production is growing and nearly 4% of the iron in the world is produced by direct reduction, of which over 90% uses natural gas as a fuel. DRI serves as a high-quality alternative for scrap in secondary steelmaking. Primary steel It is produced by two processes: open hearth furnace (OHF) and basic oxygen furnace (BOF). Steelmaking using a BOF has a relatively low energy intensity (0.7–1.0 GJ/ton) compared to the 3.9–5.0 GJ/ton energy intensity of OHF, which are much more common in developing countries. The OHF is still used in Eastern Europe and some developing countries. While the OHF uses more energy, this process can also use more scrap than the BOF process. However, the BOF process is rapidly replacing the OHF worldwide, because of its greater productivity and lower capital costs. In addition, this process needs no net input of energy and can even be a net energy exporter in the form of BOF-gas and steam. The process operates through the injection of oxygen,



oxidizing the carbon in the hot metal. Several configurations exist depending on the way the oxygen is injected. Secondary steel It is produced in an electric arc furnace (EAF) using scrap. In this process, the coke production, pig iron production, and steel production steps are omitted, resulting in much lower energy consumption and a primary energy intensity of 4.0–6.5 GJ/ton. To produce secondary steel, scrap is melted and refined, using a strong electric current. DRI is used to enhance steel quality, or if high-quality scrap is scarce or expensive. Several process variants exist, using both AC or DC currents, and fuels can be injected to reduce electricity use. Casting Casting can be a batch (ingots) or a continuous process (thin slabs). Ingot casting is the classical process and is rapidly being replaced by continuous casting machines (CCM). Continuous casting is a significantly more energy-efficient process for casting steel than the older ingot casting process. Continuous casting uses 0.1 – 0.34 GJ/ton of steel, significantly less than the 1.2 – 3.2 GJ/ton required for ingot casting. Rolling The process of rolling the cast steel begins in the hot rolling mill where the steel is heated and passed through heavy roller sections reducing the thickness of the steel. Hot rolling typically consumes between 2.3 and 5.4 GJ/t of steel. The sheets may be further reduced in thickness by cold rolling. Finishing is the final production step, and may include different processes such as annealing, pickling, and surface treatment. Cold rolling and finishing add 1.6–2.8 GJ/ton to the rolling energy use. Thin slab or near net shape casting are more advanced casting techniques which reduce the need for hot rolling because products are initially cast closer to their final shape. Primary energy used for casting and rolling using thin slab casting is 0.6–0.9 GJ/ton.



CO2 emission from steelmaking Steelmaking generates greenhouse gas emissions, mainly carbon dioxide, both directly when making iron and steel, and indirectly through the use of electricity and gas in a range of steel manufacturing processes. The majority of greenhouse gas emissions occur during the iron making process, where coal and coke is used as a chemical reductant to extract iron from iron ore in the blast furnace. An increased concentration of greenhouse gases in the earth's atmosphere is believed by many experts to contribute to the phenomenon of global warming, leading to climate change. We accept that climate change is occurring, although the nature and timeframe of its impacts are uncertain. We view climate change as a serious global issue and one that needs a global response. Australia's States and Territories are currently proposing a national emissions trading scheme, based on a 'cap and trade' mechanism. As nations develop their economies, demand for steel products increases. Meeting growing demand while improving the world steel industry's greenhouse intensity is an enormous challenge. It is a challenge that requires a global response. Global demand for steel products is driving large increases in production, especially in developing countries such as China and India. Regulation of greenhouse gas emissions in developed countries only is likely to lead to the relocation of steel production to the developing world. Relocating steel production to the developing world is not likely to reduce worldwide greenhouse gas intensity, and may actually increase intensity if less efficient technology is used. That is why measures to cut greenhouse gas emissions must be comprehensive and global, covering both developed and developing countries. Such measures must also take account of the legitimate aspirations of all countries for economic development. This includes technology that reduces emissions from energy generation, and technology that cuts emissions from steelmaking itself. Research currently being undertaken in Europe - the Ultra Low CO2 Steelmaking (ULCOS) project - aims to find new ways of producing steel that result in significant reductions in greenhouse gas emissions. This is a long-term project to develop breakthrough technologies that could result in a paradigm shift in steel manufacturing processes. A wide range of technologies for reducing greenhouse emissions are being examined, including carbon-lean technologies combined with carbon capture or sequestration, and innovative use of natural gas, hydrogen, biomass and electricity.

China’s former Minister of Metals and President of the Chinese Society for Metals once quipped, “The Europeans should do Emission Trading while we will make Steel”



According to the International Iron and Steel Institute (IISI), 1999, U.S. pig iron production was 48 million metric tons (Mt) and 52 Mt in l998 and l999, respectively. Thus, the iron slag production was about 11 and 12 Mt in 1998 and 1999, respectively. Similarly, U.S. steel production for l998 and l999 was reported to be 99.0 and 97.3 Mt, respectively. The expected steel slag production would be about 17 Mt in each year. The IISI reported that world pig iron output was about 536 Mt and crude steel production was 786 Mt in 1999. The estimated combined iron and steel slag production from this output was approximately 200 Mt. Producing molten iron and steel products requires a mix of carbon-intensive energy sources: coal and electricity. Because of this use, over half of the energy-related carbon dioxide emitted by the primary metals sub-sector is due to production at iron and steel plants. In 2002, the primary metals sub-sector emitted 212.8 million metric tons of emissions, of which nearly 60 percent were from Iron and Steel Plants. At 75.4 million metric tons per quadrillion British thermal units consumed, producing iron and steel ranks as one of the top carbon-rich outputs in manufacturing for use by consumers, largely because of its use of coal-based resources to reduce iron ores in blast furnaces or heat metal in electric arc furnaces.



Figure 3: Specific CO2 emissions improvement from German steel industry

100%

1990

96.2% 93.9% 91.5% 88.6% 85.9% 85.9%

1992 1994 1996 1998 2000 2001

100% CO2 emissions of blast furnace route = 1.85 ton CO2/ton crude steel



The real culprit: Blast furnace

What you don’t know about Blast Furnace Process? o Main producer of CO2 in steel industry (75 %) o World-wide biggest chemical reactor (up to 5000 m³) o Process chemically reduces iron ore to hot metal o Use of carbon (mainly coke) as reduction agent unavoidable, no other

technology known o Substitution of coke only partly possible by other carbon reductants

(coal/oil/gas) o Process close to the theoretical limits of carbon demand to reduce iron

ore

Problems of blast furnace process o 75 % of CO2 is generated by

the Blast Furnace (BF) process o 98 % of CO2 is generated in

accordance to BF o 24 % of CO2 is directly emitted

by BF (Recuperator) o 63 % of CO2 is emitted by

installations of the gas- and heat combination network, mainly power plants

o Power plants in network are not able to influence their CO2 emissions



Figure 4: Gas and heat combination network (from iron ore to crude steel)

coal

limestone dolomite

iron ore

Pellet plant

Sinter plant scrap

Electric arc furnace

Blast furnace

Basic oxygen

furnace

Continous caster

oxygen

coal

gas

hot metal

crude steel

Coking plant



8.3 % steelmaking shops

1.9 % hot rolling mills

0.1 % cold rolling mills

0.1 % further processing

0.1 % power plant

11.5 % sinter plant

2.3 % coke oven plant

75.5 % blast furnace

100 % = 20.2 million ton CO2 *

CO2 emissions indirectly caused by blast furnace process

* Amount of CO2 in 2001 from a steel plant at Duisburg (production of 11 mill tons of crude steel)

Figure 5: CO2 emissions (all production stages)



New Zealand - Undertaking world's best practice study Since 1968, New Zealand Steel's fully integrated steel plant has produced flat steel products for both the New Zealand and export markets in the Pacific region. The steel plant employs 1200 people to produce over 600,000 tons of steel slabs per year. New Zealand Steel's operations emit some 2 million tons of CO2-emission per annum (including an allowance for electricity purchased from the grid). In August 2005, after extensive correspondence, the New Zealand Government advised that New Zealand Steel was eligible to negotiate a Negotiated Greenhouse Agreement in relation to the Glenbrook and Waikato North Head business units. The Government has introduced Negotiated Greenhouse Agreements for firms or industries that, as a result of a carbon tax, face significant risk to their competitiveness relative to producers in countries with less stringent climate change policies. When agreed, a Negotiated Greenhouse Agreement will provide a firm with either partial or full relief from a carbon tax which is due to come into force on 1st April 2007 at a rate of NZ$15/ton CO2-emission. The Negotiated Greenhouse Agreement process calls for New Zealand Steel to develop a World's Best Practice Study, which consists of two parts - an audited CO2-emissions inventory for the 2004 calendar year plus a determination of what New Zealand Steel's CO2 emissions intensity should be at 31st December 2012 if the company were operating at world's best practice levels. The methodology to be used during the study has to be submitted to the Government's Validator and approved prior to the Study being performed. New Zealand Steel's iron making process is quite unique on a worldwide basis and the Terms of Reference for the Study produced by the Ministry for the Environment advises how to proceed in situations like this. Advantages of using integrated plants o have highest energy-efficiency o all installations are located on one site o coupling gases from coking plant, blast furnace and basic oxygen furnace

and waste heat are completely used in i. heating processes and ii. power plants (generation of electrical power)



Figure 6: Gas and heat combination network at an integrated plant

Sinter plant

blast furnace gas

Power plant

Blast

furnace Basic oxygen

furnace

f.e. district heating grid

coke oven gas

basic

oxygen gas Coking plant

other costumer, flares

waste heat

Recuperator



CONCRETE Energy used for concrete-making Energy consumption is the biggest environmental concern with cement and concrete production. Cement production is one of the most energy intensive of all industrial manufacturing processes. Including direct fuel use for mining and transporting raw materials, cement production takes about six million Btus for every ton of cement (Table 5). The average fuel mix for cement production in the United States is shown in Table 6. The industry’s heavy reliance on coal leads to especially high emission levels of CO2, nitrous oxide, and sulphur, among other pollutants. A sizeable portion of the electricity used is also generated from coal. The vast majority of the energy consumed in cement production is used for operating the rotary cement kilns. Newer dry-process kilns are more energy efficient than older wet-process kilns, because energy is not required for driving off moisture.

Table 5: Embodied Energy for Cement and Concrete Production



1. Waste fuel includes used motor oil, waste solvents, scrap tires, etc. 2. Electricity figure includes primary energy used to generate the electricity.

Source: Portland Cement Association, U.S. Cement Industry Fact Sheet (1990)

Table 6: Fuel Use for Cement Production

In a modern dry-process kiln, a pre-heater is often used to heat the ingredients using waste heat from the exhaust gases of the kiln burners. A dry-process kiln so adapted can use up to 50% less energy than a wet-process kiln, according to UBC researchers. Some other dry-process kilns use a separate combustion vessel in which the calcining process begins before the ingredients move into the rotary kiln—a technique that can have even higher overall efficiency than a kiln with pre-heater. In the United States, producing the roughly 80 million tons of cement used in 1992 required about 0.5 quadrillion Btus or quads (1 quad = 1015

Btus). This is roughly 0.6% of total U.S. energy use, a remarkable amount given the fact that in dollar value, cement represents only about 0.06% of the gross national product. In some Third World countries, cement production accounts for as much as two-thirds of total energy use, according to the Worldwatch Institute. While cement manufacturing is extremely energy intensive, the very high temperatures used in a cement kiln have at least one advantage: the potential for burning hazardous waste as a fuel. Waste fuels that can be used in cement kilns include used motor oil, spent solvents, printing inks, paint residues, cleaning fluids, and scrap tires. These can be burned relatively safely because the



extremely high temperatures result in very complete combustion with very low pollution emissions. Indeed, for some chemicals thermal destruction in a cement kiln is the safest method of disposal. A single cement kiln can burn more than a million tires a year, according to the Portland Cement Association. Pound for pound, these tires have higher fuel content than coal, and iron from the steel belts can be used as an ingredient in the cement manufacturing. Waste fuels comprise a significant (and growing) part of the energy mix for cement plants (see Table 6), and the Canadian Portland Cement Association estimates that waste fuel could eventually supply up to 50% of the energy. Energy use for concrete production looks considerably better than it does for cement. That’s because the other components of concrete—sand, crushed stone, and water—are much less energy intensive. Including energy for hauling, sand and crushed stone have embodied energy values of about 40,000 and 100,000 Btus per ton, respectively. The cement, representing about 12% of concrete, accounts for 92% of the embodied energy, with sand representing a little under 2% and crushed stone just under 6% (see Table 5). Use of fly ash in concrete already saves about 44 trillion Btus (0.04 quads) of energy annually in the U.S. Increasing the rate of fly ash substitution from 9% to 25% would save an additional 75 trillion Btus.



CO2 emissions from concrete-making There are two very different sources of carbon dioxide emissions during cement production. Combustion of fossil fuels to operate the rotary kiln is the largest source: approximately 3/4 tons of CO2 per ton of cement. But the chemical process of calcining limestone into lime in the cement kiln also produces CO2. This chemical process is responsible for roughly 1/2 ton of CO2 per ton of cement, according to researchers at Oak Ridge National Laboratory. Combining these two sources, for every ton of cement produced, 1.25 tons of CO2 is released into the atmosphere (Table 7). In the United States, cement production accounts for approximately 100 million tons of CO2 emissions, or just fewer than 2% of our total human-generated CO2.

Table 7: CO2 emission from cement and Concrete Production

Worldwide, cement production now accounts for more than 1.6 billion tons of CO2 over 8% of total CO2 emissions from all human activities. The most significant way to reduce CO2 emissions is improving the energy efficiency of the cement kiln operation. Indeed, dramatic reductions in energy use have been realized in recent decades, as discussed above. Switching to lower- CO2 fuels such as natural gas and agricultural waste (peanut hulls, etc.) can also reduce emissions. Another strategy, which addresses the CO2 emissions from calcining limestone, is to use waste lime from other industries in the kiln. Substitution of fly ash for some of the cement in concrete can have a very large effect. What is the truth behind CO2 and the cement industry? According to the World Business Council for Sustainable Development (WBCSD), “Concrete is the most widely used material on earth apart from water, with nearly three tons used annually for each man, woman, and child.” Carbon dioxide emissions from a cement plant are divided into two source categories: combustion and calcinations. Combustion accounts for approximately 40% and calcinations 50% of the total CO2 emissions from a cement manufacturing facility.



The combustion-generated CO2 emissions are related to fuel usage. The CO2 emissions due to calcinations are formed when the raw materials (mostly limestone and clay) are heated to over 2500°F and CO2 is liberated from the decomposed limestone. As concrete ages, it carbonates and reabsorbs the CO2 released during calcinations. Calcinations are a necessary key to cement production. Therefore, the focus of reductions in CO2 emissions during cement manufacturing is on energy use.

Figure 7: Typical carbon dioxide emissions from a cement plant

In the US, cement manufacturing accounts for approximately 1.5 to 2% of CO2 emissions attributable to human activities. Worldwide, cement manufacturing accounts for approximately 5% of CO2 emissions. When all greenhouse gas emissions generated by human activities are considered, the cement industry is responsible for approximately 3% of global emissions. Using the same ratio of CO2 emissions to greenhouses gases in the U.S., 1% of the greenhouse gases are attributed to cement manufacturing. In the US and elsewhere, the industry strives to further reduce that contribution. China produces 37% of the world’s cement, followed by India with 6% and the U.S. with 5%. Most facilities in China rely on inefficient and outdated technologies; these plants contribute to 6 to 8% of the CO2 emissions in China.



The cement industry has made progress towards reducing energy associated with cement manufacturing and associated emissions. Since 1972, the cement industry has improved energy efficiencies by 33%. According to the U.S. Department of Energy, U.S. cement production accounts for only 0.33% of U.S. energy consumption. According to the WBCSD, over the decade of the 1990s, global cement production increased approximately 20% while unit-based cement industry CO2 emissions decreased by approximately 1.5%. Unit-based emissions vary across worldwide regions from 0.73 to 0.99 lb of CO2 per lb of cement. Putting CO2 emissions into perspective The manufacture of cement produces about 0.9 pounds of CO2 for every pound of cement. Since cement is only a fraction of the constituents in concrete, manufacturing a cubic yard of concrete (about 3900 lbs) is responsible for emitting about 400 lbs of CO2. The release of 400 lbs of CO2 is about equivalent to: o The CO2 associated with using 16 gallons of gas in a vehicle o The CO2 associated with using a home computer for a year o The CO2 associated with using a microwave oven in a home for a year o The CO2 saved each year by replacing 9 light bulbs in an average house

with compact fluorescent light bulbs Other sources responsible for CO2 emissions include: o 28,400 lbs for an average U.S. house in a year o 26,500 lbs for two family vehicles in the U.S. in a year o 880,000 lbs for a 747 passenger jet traveling from New York to London The reason concrete is responsible for 1.5 to 2% of the U.S. anthropogenic CO2 (that is, due to humans) is due to the vast quantities of concrete used in the world around us. Other air emissions Besides CO2, both cement and concrete production generate considerable quantities of air-pollutant emissions. Dust is usually the most visible of these pollutants. The U.S. EPA estimates total dust emissions of 360 pounds per ton of cement produced, the majority of which is from the cement kiln. Other sources of dust from cement production are handling raw materials, grinding cement clinker, and packaging or loading finished cement, which is ground to a very fine powder. The best way to deal with the dust generated in cement manufacturing would be to collect it and put it back into the process. This is done to some extent, using mechanical collectors, electric precipitators, and fabric filters. Dust emissions can be controlled through water sprays, enclosures, hoods, curtains, and covered chutes.



Other air pollution emissions from cement and concrete production result from fossil fuel burning for process and transportation uses. Air pollutants commonly emitted from cement manufacturing plants include sulfur dioxide (SO2) and nitrous oxides (NOx). SO2 emissions resulted from sulfur content of both the raw materials and the fuel (especially coal). Strategies to reduce sulfur emissions include use of low-sulfur raw materials, burning low-sulfur coal or other fuels, and collecting the sulfur emissions through state-of-the-art pollution control equipment. Interestingly, lime in the cement kiln acts as a scrubber and absorbs some sulfur. Nitrous oxide emissions are influenced by fuel type and combustion conditions (including flame temperature, burner type, and material/exhaust gas retention in the burning zone of the kiln). Strategies to reduce nitrogen emissions include altering the burner design, modifying kiln and pre-calciner operation, using alternate fuels, and adding ammonia or urea to the process. Water pollution Another environmental issue with cement and concrete production is water pollution. The concern is the greatest at the concrete production phase. “Wash-out water with high pH is the number one environmental issue for the ready-mix concrete industry,” according to Richard Morris of the National Ready Mix Concrete Association. Water use varies greatly at different plants, but Environment Canada estimates water use at batching plants at about 500 gallons per truck per day, and the alkalinity levels of wash-water can be as high as pH 12. Highly alkaline water is toxic to fish and other aquatic life. Environment Canada has found that rainbow trout exposed to Portland cement concentrations of 300, 500, and 1,000 milligrams/liter have 50% mortality times (the time required for 50% of the population in test samples to be killed) of 68, 45, and 29 minutes, respectively. Solid waste While the cement and concrete industries can help reduce some of our solid waste problems, one cannot overlook the fact that concrete is the largest and most visible component of construction and demolition (C&D) waste. According to estimates presented in the AIA Environmental Resource Guide, concrete accounts for up to 67% by weight of C&D waste (53% by volume), with only 5% currently recycled. Of the concrete that is recycled, most is used as a highway substrate or as clean fill around buildings. As more landfills close, including specialized C&D facilities, concrete disposal costs will increase and more concrete demolition debris will be reprocessed into roadbed aggregate and other such uses. Concrete waste is also created in new construction. Partial truckloads of concrete have long been a disposal problem. Ready mix plants have come up with many innovative solutions through the years to avoid creating



waste such as using return loads to produce concrete retaining wall blocks or highway dividers, or washing the unset concrete to recover the coarse aggregate for reuse. But recently, there have been some dramatic advances in concrete technology that are greatly reducing this waste. Concrete admixtures are available that retard the setting of concrete so effectively that a partial load can be brought back to the ready mix plant and held overnight or even over a weekend then reactivated for use. When it is possible to use pre-cast concrete components instead of poured concrete, doing so may offer advantages in terms of waste generation. Material quantities can be estimated more precisely and excess material can be utilized. Plus, by carefully controlling conditions during manufacture of pre-cast concrete products, higher strengths can be achieved using less material. Waste water run-off can also be more carefully controlled at centralized pre-cast concrete facilities than on jobsites. Another interesting trend that relates to waste minimization is the idea of producing reusable concrete masonry units. Concrete reabsorbs CO2 During the life of a concrete structure, the concrete carbonates and absorbs the CO2 released by calcinations during the cement manufacturing process. Once concrete has returned to fine particles, full carbonation occurs, and all the CO2 released by calcinations is reabsorbed. A recent study indicates that in countries with the most favorable recycling practices, it is realistic to assume that approximately 86% of the concrete is carbonated after 100 years. During this time, the concrete will absorb approximately 57% of the CO2 emitted during the original calcinations. About 50% of the CO2 is absorbed within a short time after concrete is crushed during recycling operations

ecological profile of concrete & steel

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