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Clinker substitute (slag, natural pozzolans, synthetic pozzolans)

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. One possible way to reduce energy and process emissions in cement production isto blend cements with increased proportions of alternative (non-clinker) feedstocks,such as volcanic ash, granulated blast furnace slag from iron production, or fly ashfrom coal-fired power generation.

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 clicker. As a result, cement manufacturing is the third largest cause of man-made CO2 emissions due to the production of lime, the key ingredient in cement. Therefore, energy savings during cement production could lead to lower environmental impact.

The largest opportunities for improving energy efficiency and reducing CO2 emissions can be achieved by improving the cement manufacturing process. In the cement industry pyroprocessing (processing the raw material into cement under a high temperature, e.g., above 8000C) is a very common technological procedure, which accounts for 74% of the energy consumption in global cement/concrete industries. Since the thermal efficiency through the use of this conventional technology of pyroprocessing is slightly higher than 30% on average (Mersmann, 2007), there could be considerable scope for improvements. Grinding and milling account for 5.8% of cement/concrete energy consumption (Choate, 2003). These operations have an energy efficiency ranging from 6 to 25% and also offer a large opportunity for energy saving.

The world average clinker/cement ratio is about 0.81, with the balance comprising gypsum and additives such as blast furnace slag, fly ash, and natural pozzolana. As clinker production is the most energy-intensive and CO2-emitting step of the cement-making process, reductions in the clinker/cement ratio (through use of clinker substitutes) reduce energy use and process CO2 emissions.One possible way to reduce energy and process emissions in cement production isto blend cements with increased proportions of alternative (non-clinker) feedstocks, such as volcanic ash, granulated blast furnace slag from iron production, or fly ashfrom coal-fired power generation (IEA 2008, Gielen et al. 2008). Some basic approaches are:

  • Blast-furnace slag that has been cooled with water, rather than air.: Blast furnace slag (BFS) 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.
  • Fly ash from coal-fired power plants, including also pyrite ash and phosphogypsum (from flue gas desulfurization and phosphoric acid production)
  • Steel slag has been used in several different cementing systems, and it provides some advantages over conventional cements. In many countries, most steel slag is currently being used as unbound aggregate for asphalt concrete pavement.

Materials that can be added to cement to extend its volume without a significant loss of properties are known as ‘pozzolans’, after the ash deposits adjacent to the Pozzol volcano that the Romans used as cement. The Pantheon in Rome is a testament to the high strength and durability of pozzolan cement. Its hemispherical dome, 43 meters in diameter, is made completely from pozzollan cement and does not contain any reinforcing bars. It has been in use since 135 AD. The addition of pozzolan will modify the characteristics of cement. Depending on the type of pozzolan chosen, the density and compressive strength of the formed concrete may be increased and porosity reduced. Pozzolanic materials can be combined with uncarbonated lime (calcium hydroxide) to form stable compounds, thus reducing the risk of early leaching or frost damage and increasing the potential durability of the mortar.

Pozzolan materials, in general, do not require pyroprocessing and, therefore, can save very significant quantities of energy and lower COemissions when supplementing regular cement. Concrete research is now calling for increased usage and high-volume usage of pozzolans, especially fly ash. Some sources suggest that all concrete should contain fly ash (Scalon, 1992). The economic and environmental advantages of adding pozzolan would seem to indicate that their regular and high-volume use will become standard practice in the concrete industry. The US Environmental Protection Agency requires fly ash content in concrete to be used in buildings that receive federal funding (US Department of Energy, 2003).

Coal-fired power fly ash is sometimes used as a source of silica in cement manufacturing, but is more commonly used in concrete production as a substitute for a portion of the cement. This is beneficial in two ways: it reduces solid waste and overall energy use since it does not require pyroprocessing. Fly ash can readily substitute 15 to 35% of the cement in concrete mixes and in some applications fly ash content can be up to 70%. Of the 68 million tonnes of coal fly ash produced in 2001, 12.4 million tonnes were used in cement and concrete products (ACAA, 2001). However, it should be noted that fly ash can contain elements (e.g. carbon), compounds (e.g. ammonia) and other constituents that are detrimental to the quality of concrete.

Nonetheless, fly ash and slags react with any free lime left after the hydration to form calcium silicate hydrate, which is similar to the tricalcium and dicalcium silicates formed in cement curing. This process increases strength, improves sulfate resistance, decreases permeability, reduces the water ratio required, and improves the pumpability and workability of the concrete. EU and US-based coal-fired power plants produce better fly ash for concrete than other plants in the world, because of the lower sulfur and lower carbon content in the ash. Fly ash from waste incinerators cannot be used.

Other materials that could be used to a greater extent as clinker substitutes include volcanic ash, ground limestone and broken glass. Such approaches could alleviate clinker substitute availability problems, and possibly pave the way to a 50% reduction of energy use and CO2 emissions. The possible contribution of Best Available Technologies for the cement sector are displayed in the Figure below.

illustration © climatetechwiki.org

Figure 1: Energy savings for cement (Source: IEA, 2010)

Feasibility of technology and operational necessities top

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 carbon content of fly ash can affect the concrete setting time, which determines the quality of the cement. To be used as clinker substitute, high-carbon fly ash must be upgraded, through froth flotation, triboelectrostatic separation, or carbon burn-out in a fluidized bed. Technologies for this are just emerging. Special grinding methods are also being studied as a way to increase the reaction rate of fly ash, allowing the fly ash content of cement to increase to 70% compared with a maximum of 30% today (Justnes et al., 2005). China and India have the potential to significantly increase the use of fly ash. The CemStar process, which uses a 15% charge of air-cooled steel slag pebbles in the rotary kiln feedstock mix, has been developed and successfully applied 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 priority for technical, economic, and environmental reasons.

The use of such blended cements varies widely from country to country. It is high in continental Europe, but low in the United States and the United Kingdom. Inthe United States and in China, other clinker substitutes are added directly at the concrete-making stage. 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 (IEA 2008). A key parameter determining the suitability of a material as a clinker substitute is the compressive strength, in addition to tensile strength and water absorption (Gielen et al. 2008).

Further reductions in cement-to-clinker ratios will require additional R&D to assess substitution materials and to evaluate regional availability. Changing the composition of the cement output through the new materials could have an impact on the quality of the product. Changes in cement product formulations require significant time before they are incorporated into international standards and accepted in the market. The development and implementation of international standards for blended cements would also supportgreater use of clinker substitutes (IEA 2010), which is a slow process that limits thepotential in the short and medium term.

Status of the technology and its future market potential top

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. At the same time, the Government of China encourages the use of coal ash powder to produce low quanlity cement through tax refunds. China merits special attention because of its high share of world production and its production technology. Thus far, inefficient vertical shaft kilns dominate in China, but the country’s capacity base is changing quickly. In developed countries, such as the U.S., the generally declining trend in the U.S. output of iron and steel implies future overall supply constraints from domestic sources, especially as existing stockpiles get drawn down. This is especially true for the long-term availability of air-cooled slag, given the continuing decline in the number of operating blast furnaces

In the long term, new cement types may be developed that do not use limestone as a primary resource. These new types are called synthetic pozzolans. The technological feasibility, economics and energy effects of such alternative cementsremain speculative (IEA 2008). Admixtures help create “high-performance cements” basedon mechanochemical activation of certain ratios of clinker, gypsum,admixture, and optionally, a mineral additive of industrialor natural origin that imparts a high strength and extreme durabilityto the concrete or mortar made from it. A wide range ofnatural pozzolanic materials, sand, limestone, granulated blastfurnace slag, fly ash, and broken glass can be used as mineral additives in these cements (Gielen et al. 2008).

How the technology could contribute to socio-economic development and environmental protection 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).

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.

illustration © climatetechwiki.org

Figure 2: Energy intensity in the cement production

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

The availability of clinker substitutes is sufficient to allow the cement-to-clinker ratio to be reduced to 0.7 globally, theoretically enabling a saving of a further 15 Mtoe of thermal energy. Taking into account all these potentials, the global intensity of cement production could be reduced by 0.9 GJ/t of cement produced, with significantly higher savings possible in many countries and regions (IEA 2010). As calculated by the IEA (2008), in total, the savings potential for blended cements amounts to 300 Mt CO2 to 450 Mt CO2 by 2050.

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. In addition, the limestone used to produce cement has “self-scrubbing” properties, which enable the industry to handle high-sulfur fuels. By using several products for clinker substitution, such as for instance coal ash power, reduced coal ash powder waste and reduced land are needed to dispose of coal ash powder.

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). Based on the IEA (2008) analysis for blended cements, in total, the savings potential in this case amounts to 300 Mt CO2 to 450 Mt CO2 by 2050. If all blast-furnace slag were used, this would yield a CO2 reduction of approximately100 Mt CO2. If the 50% of all fly ash that currently goes to landfill could be used, this would yield a CO2 reduction of approximately 75 Mt.  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.

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: http://cdm.unfccc.int/methodologies/PAmethodologies/approved.html

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.

Anecdotal prices in the U.S. (EPA 2008) revealed that in 2004, average sales prices for GBFS were $55.79 per short ton, with a reported range of $20.00 per ton for un-ground GBFS to $64.99 per ton for GGBFS, and average sales prices of BFSA were $5.90 per short ton, with a range of $1.40 to $15.74 per short ton (EPA 2008). The additional investment needed to achieve the CO2 reduction outlined in the IEA scenarios is in the range of USD 350 billion to USD 840 billion. Much of the additional investment will be needed in developing countries where CO2 policies are now emerging (IEA 2010).

Clean Development Mechanism market status top

[this information is kindly provided by the UNEP Risoe Centre Carbon Markets Group]

Project developers of clinker substitute projects in the CDM pipeline mainly apply the following methodologies:

For optimizing the use of clinker:
ACM0005 “Consolidated Baseline Methodology for Increasing the Blend in Cement Production”
For Use of non-carbonated calcium sources:
AM0033 “Use of non-carbonated calcium sources in the raw mix for cement processing”
Further information on these metholodogies can be found here.

There are only 33 cement projects in the CDM pipeline. One of the reasons for the small number of these projects is that the expected emissions reductions are relatively low, and do not compensate for the high transaction costs (i.e. research and development coupled with communication strategy and success uncertainties).

illustration © climatetechwiki.org

Figure 3: Overview of clinker substitute projects in the CDM (Source: UNEP Risoe CDM/JI Pipeline Analysis and Database, February 1st 2010)

Example CDM project:
Title: Optimum utilisation of clinker by production of Pozzolana Cement at UltraTech Cement Ltd. (UTCL), Andhra Pradesh (CDM Ref. No. 438)
UltraTech Cement Limited is one of India’s largest cement producers with state of-the-art dry process plants incorporating pre-calcination technology,
advanced instrumentation systems, computerized process control and online quality control by X-ray - ensuring consistent production of high quality cement. The project activity entails a reduction of the clinker content by producing the Portland Pozzolana cement (PPC), thereby replacing an equivalent amount of clinker at UTCL’s cement manufacturing units at Tadipatri, Andhra Pradesh & Arakkonam, Tamilnadu.
Project investment: N.A.
Project’s CO2 reductions over a crediting period of 7 years: 418,382 tCO2e
Expected CERs revenue (assuming a 10 USD/CER): USD 4,183,820
References top

American Coal Ash Association (ACAA), 2001. Coal Combustion Product Survey. 

Choate, W., 2003. Energy and Emission reduction opportunities for the cement industry. US Department of Energy. 

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.

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

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

Justnes, H., Elfgren, L. and Ronin, V., 2005. Mechanism for Performance of Energetically Modified Cement Versus Corresponding Blended Cement, Cement and Concrete Research, 35 (2), pp. 315-323.

Mersmann, M., 2007. Pyro-process Technology. Cement Industry Technical Conference Record, IEEE, pp. 90-102.

Scalon, J., 1992. Mineral Admixturer, ACI Compilation 22.

United States Environmental Protection Agency (EPA), 2008. Study on Increasing the Usage of Recovered Mineral Components in Federally Funded Projects Involving Procurement of Cement or Concrete to Address the Safe, Accountable, Flexible, Efficient Transportation Equity Act: A Legacy for Users, pp. 2-5 and 2-8.

U.S. Department of Energy, 2003. Energy and Emission Reduction Opportunities for the Cement Industry, Washington, D.C., USA.

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