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Blast furnace slag granulation

Cement is a global commodity, manufactured at thousands of plants. The industry is consolidating globally, but large international firms account for only 30% of the worldwide market. The principal and most visible market for cement is the construction industry in a multitude of applications where it is combined with water to make concrete. Most modern civil engineering projects, office buildings, apartments and domestic housing projects use concrete, often in association with steel reinforcement systems. In many developed countries, market growth is very slow, with cement used in bulk primarily for infrastructure construction, based on UNEPTIE. In developing country markets (e.g. China), growth rates are more rapid. Because it is both global and local, the cement industry faces a unique set of issues, which attract attention from both local and international level.

Cement accounts for 83% of total energy use in the production of non-metallic minerals and 94% of CO2 emissions. Energy represents 20% to 40% of the total cost of cement production. The production of cement clinker from limestone and chalk by heating limestone to temperatures above 950°C is the main energy consuming process. Portland cement, the most widely used cement type, contains 95% cement clinker. Large amounts of electricity are used grinding the raw materials and finished cement.

Introduction top

Cement is a global commodity, manufactured at thousands of plants. The industry is consolidating globally, but large international firms account for only 30% of the worldwide market. The principal and most visible market for cement is the construction industry in a multitude of applications where it is combined with water to make concrete. Manufacturing industries in general account for one-third of global energy use. Direct industrial energy and process CO2 emissions amount to 6.7 gigatonnes (Gt), about 25% of total worldwide emissions, of which 30% comes from the iron and steel industry, 27% from non-metallic minerals (mainly cement) and 16% from chemicals and petrochemicals production (IEA, 2008). Cement production involves the heating, calcining and sintering of blended and ground materials to form clinker (see Wikipedia clinker (cement)). As a result, cement manufacturing is the third largest cause of man-made CO2 emissions due to the production of lime (see Wikipedia lime mortar), the key ingredient in cement. Therefore, energy savings during cement production could lead to lower environmental impact. In the cement/concrete industry improvement of energy efficiency and reduction of CO2 emissions could be mainly achieved through two procedures: (i) by changes in the manufacturing and production processes, and (ii) by adjusting the chemical composition of cement. Manufacturing and production processes can be improved by changing energy management and by investing in new equipment and/or upgrades. Changes in the chemical formulation of cement have been demonstrated to save energy and reduce CO2 emissions, but their widespread adoption has thus far been hampered by the fact that developing a new industrial standard is complex and requires time. This holds in particular for the cement industry which is a highly capital intensive and competitive sector with long economic lifetimes of existing facilities so that changes in the existing capital stock cannot easily be made.

One way to reduce energy and process emissions in cement production is to blend cements with increased proportions of alternative (non-clinker) feedstocks, such as volcanic ash, granulated blast furnace slag from iron production, or fly ash from coal-fired power generation. Blast furnace slag (BFS, see Wikipedia ground granulated blast-furnace slag) is a nonmetallic byproduct of the manufacture of pig iron in a blast furnace. BFS consists primarily of silicates, aluminosilicates, and calcium-alumina-silicates. BFS forms when slagging agents (e.g., iron ore, coke ash, and limestone) are added to the iron ore to remove impurities. In the process of reducing iron ore to iron, a molten slag forms as a non-metallic liquid (consisting primarily of silicates and aluminosilicates of calcium and other bases) that floats on top of the molten iron. The molten slag is then separated from the liquid metal and cooled (Gielen et al., 2008). If the molten slag is cooled and solidified by rapid water quenching to a glassy state, little or no crystallization occurs. This process results in the formation of sand size (or frit-like) fragments, usually with some friable clinkerlike material. The physical structure and gradation of granulated slag depend on the chemical composition of the slag, its temperature at the time of water quenching, and the method of production. When crushed or milled to very fine cement-sized particles, ground granulated blast furnace slag (GGBFS) has cementitious properties, which make a suitable partial replacement for or additive to Portland cement. The process of GGBFS is presented below

illustration © climatetechwiki.org

Figure 1: Production of ground granulated blastfurnace slag (Source: Sustainable Concrete)

The possible contribution of Best Available Technologies for the cement sector are displayed in the Figure below.

illustration © climatetechwiki.org

Figure 2: Energy savings for cement (Source: IEA, 2010)

Feasibility of technology and operational necessities top

In the cement pyroprocessing process it is important to keep in mind that waste materials combust and burn at different temperatures under different conditions. Therefore, solid waste fuels need to be introduced into the kiln in such a manner that they do not significantly change the temperature profile and chemical reactions in the overall pyroprocessing. Sometimes it is necessary to add solid waste through a hatch or valve structure in the kiln shell, which implies a technical challenge and which partly offsets the efficiency gains and CO2 emission reductions. Finally, receiving and handling of alternate or waste fuels can raise technical liability and political concerns. Cement manufacturing companies do not desire to be labeled as handlers of hazardous wastes and surrounding communities may have concerns about hazardous waste transport and handling in a nearby cement plant.

Cements containing ground granulated blast furnace slag are available from many producers of Portland cement or directly from ground granulated blast furnace slag cement manufacturers. Standard AASHTO M240 describes three types of blended cements containing slag. They include Portland blast furnace slag cement (type IS), slag modified Portland cement (type I (SM)), and slag cement (type S). The primary distinction among the three types is the percentage of slag they contain. Slag cement may contain portland cement (see on Wikipedia) or hydrated lime (or both) while the other two are blends of Portland cement and slag only. Blast furnace slag materials are generally available from slag processors located near iron production centres. Furthermore, blended cements offer a major opportunity for energy conservation and emission reductions, but their use would in many cases require revisions to construction standards, codes and practices.

Ground granulated blast furnace slag is a cementitious material and can be substituted for cement on a 1:1 basis. In the absence of special circumstances or mix specific data, the substitution of ground granulated blast furnace slag should be limited to 50 percent for areas not exposed to deicing salts and to 25 percent for concretes which will be exposed to deicing salts. While substitution of ground granulated blast furnace slag for up to 70 percent of the portland cement in a mix has been used, there appears to be an optimum substitution percentage which produces the greatest 28 day strength. This is typically 50 percent of the total cementitious material but depends on the grade of ground granulated blast furnace slag used. Also, research has shown that the scaling resistance of concretes decreases with ground granulated blast furnace slag substitution rates greater than 25 percent (see GGBFS on the website of the US Department of Transport). In the absence of special circumstances, the use of ground granulated blast furnace slag as a cement replacement should be limited to grades 100 and 120 ground granulated blast furnace slag and in the absence of special circumstances or mix specific data, the substitution of ground granulated blast furnace slag should be limited to 50 percent for areas not exposed to deicing salts and to 25 percent for concretes which will be exposed to deicing salts.

Status of the technology and its future market potential top

The main potential in reducing energy consumption and CO2 emissions from cement/concrete production is in improvement of cement pyroprocessing. Pyroprocessing transforms the raw mix into clinkers. At present, about 78% of Europe's cement production is from dry process kilns, a further 16% of production is accounted for by semi-dry and semi-wet process kilns, with the remainder of European production, about 6%, coming from wet process kilns. The wet process kilns operating in Europe are generally expected to be converted to dry process kiln systems when renewed, as are semi-dry and semi-wet processes kiln systems. On average, pyroprocessing (see Wikipedia) systems in the EU and US operate at below 35% thermal efficiency, which is rather low. The percentage is even lower for developing countries. These process improvements will come from better energy management, upgrading existing equipment (e.g. replacing wet kilns, upgrading to preheater and precalciners), adopting new pyroprocessing technologies (e.g. fluidised bed systems) and, in the longer term, performing the R&D necessary to develop new concepts for the cement manufacturing processes.

About half of all blast-furnace slag is already used for cement-making where the slag is water-cooled and where transport distances and costs are acceptable. The CemStar process, which uses a 15% charge of air-cooled steelslag pebbles in the rotary kiln feedstock mix, has been developed and successfullyapplied in the United States, resulting in a CO2 reduction of approximately 0.47 t/t steel slag added (Yates et al., 2004). In China, there are about 30 steel slag cement plants with a combined annual output of 4.8 Mt. However, steel slag quality varies and it is difficult to process, which limits its use. If the total worldwide BOF and EAF steel slag resource of 100 Mt to 200 Mt per yearwas used this way, the CO2 reduction potential would be 50 Mt to 100 Mt per year (IEA, 2008). The use of steel slag as a cementing component should be given a priorityfor technical, economic, and environmental reasons.

While slag cement use is miniscule in contrast with portland cement use, it has been around for a while. In fact, it was used in the building of both the Paris underground system and the Empire State Building. According to the Slag Cement Association, slag cement can replace up to 50% of portland cement in most common concrete mixtures, and up to 80% "in massive concrete elements and other specialized structures." Not only does its use cut down CO2 emissions, it also helps save energy. In many countries, slag is being used for cement production, such as in China, where nowadays, steel slag is a useful resource rather than a “waste”, since extensive applications of slag have been developed. In general, the estimated annual production level is 200 million tonnes and is dependent on the future iron and steel production volumes (IEA, 2009). 

Contribution of the technology to economic development (including energy market support) top

An important benefit of enhancing energy efficiency in the cement industry would be the reduction in energy costs. Broadly speaking, in the EU cement industry the energy bill represents about 40% of total production costs, while European cement production techniques are amongst the most energy efficient in the world. Since the 1970s, in Europe the energy required for producing cement has fallen by about 30% and the scope for further improvements has became rather small.

In cement manufacturing, cost-effective efficiency gains in the order of 10% to 20% are already possible using commercially available technologies. The energy intensity of most industrial processes is at least 50% higher than the theoretical minimum determined by the basic laws of thermodynamics. Energy efficiency tends to be lower in regions with low energy prices. Cross-cutting technologies for motor and steam systems would yield efficiency improvements in all industries, with typical energy savings in the range of 15% to 30%. The payback period can be as short as two years, and in the best cases, the financial savings over the operating life of improved systems can run as high as 30% to 50%. In those processes where efficiency is close to the practical maximum, innovations in materials and processes would enable even further gains (IEA, 2008).

According to ConstructIreland, the energy consumption per tonne of Portland cement produced equals 4,000 MJ (1,100 KW.hrs). In contrast, the manufacture of GGBS cement only involves the transport, drying and grinding of an industrial by-product, and is a low energy operation. In addition, it is a recycling operation and has downstream benefits in that it eliminates the need for landfill disposal. The energy consumption per tonne of GGBS produced equals 307 MJ (85 KW.hrs). Thus the energy saved by replacing Portland cement with GGBS cement equals 3,693 MJ (1,015 KW.hrs) per tonne. 

The cement industry, together with other related construction industries, succeeded in raising the awareness of the relevance of energy efficiency in buildings through the European Construction Forum (ECF) of which CEMBUREAU (the European Cement Association) is an active member. In its EU strategy on energy efficiency, ECF advocated that “the building sector offers one of the largest single potentials for energy efficiency and should thus be a major focus for action.” As a result of this strategy, the European Parliament and Council Directive 2002/91/EC on Energy Performance of Buildings was adopted on 4 January 2003. It affects all new residential and non-residential buildings, including renovation of large existing buildings. It is intended to lead to substantial increases in investment in energy efficiency measures within these buildings. According to the EU Directive, this will be mainly achieved by imposing energy performance standards, promote renewable energy sources and establish a system of regular inspections. The cement industry does not have a so critical role throughout the process. However, the modernisation of existing procedures of cement production can lead to considerable improvements, in the sense that it will reduce heating and lighting losses.

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Figure 3: Energy intensity in the cement production

Contribution of the technology to protection of the environment top

SO2 emissions from cement plants result from the combustion of sulfur-bearing compounds in coal, oil, and petroleum coke, and from the processing of pyrite and sulfur in raw materials. To mitigate these emissions, cement plants typically install air pollution control technologies called “scrubbers” to trap such pollutants in their exhaust gases. The use of slag or slag cements usually improves workability and decreases the water demand due to the increase in paste volume caused by the lower relative density of slag (Hinczak, 1990). The higher strength potential of Grade 120 slag may allow for a reduction of total cementitious material. In such cases, further reductions in water demand may be possible (Admixtures and ground slag, 1990). According to the Cementitious Slag Makers Association (CSMA) in the UK, by comparison with Portland cement, manufacture of GGBS requires less than a fifth the energy and produces less than a fifteenth of the carbon dioxide emissions. Further 'green' benefits are that manufacture of GGBS does not require the quarrying of virgin materials, and if the slag was not used as cement it might have to be disposed of to tip. In the production of Portland cement, 1.6 tonnes of clay and limestone are removed from the landscape for every tonne of Portland cement produced. However, there is zero depletion of natural resources associated with the manufacture of GGBS.

illustration © climatetechwiki.org

Figure 4: CO2 emissions from GGBS (Source: ConstructIreland)

Climate top

Cement manufacturing produces CO2 as it requires very high temperatures to burn raw materials and give the clinker its unique properties. CO2 is generated from three independent sources: de-carbonation of limestone in the kiln (about 525 kg COper tonne of clinker), combustion of fuel in the kiln (about 335 kg CO2 per tonne of cement) and use of electricity (about 50 kg CO2 per tonne of cement). If the total worldwide BOF and EAF steel slag resource of 100 Mt to 200 Mt per year was used this way, the CO2 reduction potential would be 50 Mt to 100 Mt per year. Because GGBS is derived as a by-product from another industry its use is an example of industrial ecology. It can be used un-ground as a coarse aggregate or as a supplementary cementitious material, where it can replace up to 70% of cement in a concrete mix and reduce carbon dioxide emissions per tonne of concrete by up to 60 or 70%. GGBFS typically replaces 35% to 65% portland cement in concrete. Thus, a 50% replacement of each ton of portland cement would result in a reduction of approximately 0.5 ton of CO2.

For calculation of these GHG emission reductions, it is recommended to apply the approved methodology for consolidated methodology for increasing the blend in cement production project (large scale activities) which has been developed under the Clean Development Mechanism of the UNFCCC Kyoto Protocol (CDM). This methodology helps to determine a baseline for GHG emissions in the absence of the project (i.e. business-as-usual circumstances), how emission reductions below this baseline can be calculated, and how these reductions can be monitored. General information about how to apply CDM methodologies for GHG accounting can be found at the CDM website: Approved Baseline and Monitoring Methodologies for Large Scale CDM Project Activities

Financial requirements and costs top

Global demand for cement is forecast to grow by 4.7% annually to 2.8 billion metric tons in 2010. China, which is already by far the largest market for cement in the world, will show the largest increase in total amount of cement sold. Other developing parts of the Asia/Pacific region and Eastern Europe, as well as a number of nations in the Africa/Middle-East and Latin American regions will also record above-average cement market gains, fueled by a robust construction outlook. Vietnam, Thailand, Ukraine, Turkey and Indonesia are also expected to record strong increases in percentage terms. Market advances will be less robust in the developed areas of the USA, Japan and Western Europe, with maintenance and repair construction accounting for most of the growth in cement demand through 2010. However, a pickup a construction spending in Germany and Japan following an extended period of decline will help bolster overall developed world market growth.

Cement industry has devoted substantial effort to introducing innovative procedures in cement production. Considerable resources have been spent in recent years to investigate emerging and hopefully non-controversial and non-polluting technologies. Unfortunately, many such technologies have low capacities (some are still under development), are technically sophisticated, and currently not affordable by many developing countries. When comparing the state of the art technologies in terms of sustainability, suitability, performance, robustness, cost-efficiency, patent restrictions (availability), and competence requirements it can be concluding that at least in the short term cement industries are going to be based on pyroprocessing and grinding mills. The price of GGBS varies within the countries of production and an average price ranges from 4-6 $ per tonne of final product. 

References top

Admixtures and ground slag, 1990. Transportation research circular no. 365 (December). Washington: Transportation Research Board, National Research Council.

Gielen, D., Newman, J., and Patel, M., 2008. Reducing industrial energy use and CO2 emissions: The role of materials science. MRS Bulletin 33, pp. 471-477.

Hinczak, I., 1990. Alternative cements—The blue circle experience. In Onoda Pacific Conference 1:1-21.

IEA, 2008. Energy Technology Perspectives - Scenarios and Strategies to 2050. International Energy Agency, Paris, France.

IEA, 2009. Cement Technology Roadmap - Carbon Emissions reductions up to 2050. World Business Council for Sustainable Development/ International Energy Agency, Paris, France.

IEA, 2010. Energy Technology Perspectives - Scenarios and Strategies to 2050. International Energy Agency, Paris, France.

Yates, J.R., Perkins, D. and Sankaranarayanan, R., 2004. Cemstar Process and Technology for Lowering Greenhouse Gases and Other Emissions While Increasing Cement Production, Hatch, Canada. Available at: http://hatch.ca/Environment_Community/Sustainable_Development/Projects/Copy%20of%20CemStar-Process-final4-30-03.pdf