Lafarge recently completed its second successful industrial trial for Aether®, its new-generation clinker for lower carbon cements. These trials have confirmed the feasibility of industrial-scale production of Aether cements, in existing installations and using conventional raw materials, for a lower overall environmental footprint. Aether cements open up a new avenue for CO2 mitigation in the cement sector.
For many years Lafarge has been working to reduce its carbon footprint. In 2001, as part of its partnership with environmental NGO WWF International, the group made a voluntary public commitment to reduce CO2 emissions per tonne of cement.
After reaching its initial 20% reduction target ahead of schedule, the group announced its 'second generation' commitments in 2011. These see Lafarge aiming to reduce its CO2 emissions per tonne of cement by 33% by 2020 compared to 1990 but also to develop new products and contribute to construction projects to improve energy efficiency and reduce carbon footprint.
By the end of 2012, Lafarge had reduced emissions by 24.7% compared to 1990, representing a 192kg reduction in CO2 emitted per tonne of cement and around 30Mt of avoided carbon emissions in 2012.
Lafarge's actions over the past decade have focused on the three traditional levers for CO2 mitigation in the cement sector. These are:
1. Reducing specific heat consumption of cement kilns;
2. Substituting fossil fuels with other energy sources and;
3. Using cementitious additions to produce lower carbon blended cements for specific applications.
However, Lafarge research teams have also been investigating a number of new, non-conventional solutions because the combination of these traditional levers will ultimately be insufficient to reduce CO2 emissions in the cement sector beyond a certain level.
Exploring new avenues for CO2 mitigation
Carbon Capture and Storage (CCS) is one avenue that Lafarge teams have been exploring, through partnerships with other industrial groups. Carbon Capture and Transformation (CCT) is another potential solution and Lafarge researchers have studied the possible use of micro-algae to absorb the CO2 released by cement kilns and their reuse as biofuel.
Lafarge teams ran a pilot project at Lafarge's Val d'Azergues plant in France in 2009 in partnership with Salata GmbH, a German biotech company that specialises in micro-algae cultivation. This pilot demonstrated the feasibility of CCT, but new advances in processes and biotechnologies will be necessary to improve the overall economic and environmental results.
Lafarge researchers have therefore been focusing their attention on the most promising avenue so far: the development of a new class of lower-carbon clinkers, or 'Aether' clinkers. Back in 2003, they started looking at belite-rich clinker containing calcium sulphoaluminate (ye'elimite, also known as Klein's compound) and calcium aluminoferrite as the other major phases. Cements based on these clinkers had the potential to be significantly less CO2-intensive than cements based on Portland cement clinker, as they required significantly less calcium carbonate in the kiln feed.1 A number of calcium sulphoaluminate (CSA)-based cements have been developed for a wide range of possible applications over the past few years,2-7 although only China has so far normalised these for use in construction.
Two main classes are currently produced in significant volumes, based either on sulpho-aluminate clinker (SAC) containing 55-75% ye'elimite (C4A3$) (where $ = SO3), 15-30% belite (C2S) and 3-6% ferrite (C2(A,F)) or on ferro-aluminate clinker (FAC) containing 45-65% ye'elimite, 15-35% belite and 10-25% ferrite.8-9 With different types and dosages of mineral admixtures, such as gypsum, anhydrite, limestone and others, cements with a very wide range of performance characteristics can be made, including those that incorporate rapid-hardening, self-stressing or expansive properties.
However, these cements are adapted for specific niche applications rather than general concrete construction and therefore do not represent a genuine alternative to OPC. Belite-rich cements are also well-known in the literature and many CSA-based cements also contain significant amounts of belite. However, the low reactivity of the belite remains an issue, as efficient belite hydration is necessary for good long-term strength development.
The challenge for Lafarge researchers was therefore to combine the benefits of both CSA and belite into a single clinker that could be produced efficiently and at an acceptable cost. The aim was to develop a new type of clinker with lower CO2 emissions compared to OPC but that could also be produced in existing OPC plants, using mainly conventional raw materials and that could be used by customers for the same applications as OPC.
Aether®, a new-generation clinker for lower carbon cements
The result of this research was Aether®, a new, patented class of clinker, based not on alite (the major phase in OPC), but instead based on dicalcium silicate (belite) as the major phase, with calcium sulphoaluminate (CSA or ye'elimite) and calcium alumino-ferrite (ferrite) as two other principal phases.
In terms of chemistry, the concept of Aether clinker is based on the replacement of two phases, C3S and C3A, that generate significant emissions of CO2, by a single, very reactive ye'elimite phase: C4A3$.
This C4A3$ phase, which is not present in Portland clinker, fulfils the normal functions of C3S and C3A in OPC by itself because it generates rheological characteristics and mechanical strength at an early age. Many of the properties of Aether clinker are therefore based on C4A3$, which ensures the reactivity of the binder between zero and seven days before the C2S and the C4AF phases take over.
Aether raw meal contains less CaO, meaning a reduction in CO2 emissions. It also contains much higher levels of SO3 and much higher Al2O3 (related to the C4A3$ phase). This type of chemistry can be obtained using conventional raw materials: limestone, marl, clay, iron oxide, etc. However, it generally requires supplies of alumina (bauxite or other sources of Al) and a source of sulphates in greater quantities than raw meal for OPC. The formulation of the raw meal has to be adapted on a case-by-case basis according to the locally available raw materials.
Validating industrial feasibility
Research carried out by the Lafarge Research Centre since 2007 resulted in the identification of different clinker compositions offering similar levels of reactivity to those of Portland clinker. Several patent applications were filed for these. Based on the results obtained from this research work, Lafarge decided to launch a development phase to evaluate the compatibility between the production of this new type of clinker and existing industrial installations. At the same time, research is still ongoing to optimise the composition of Aether clinker and the cements produced from it.
For this new development phase in the project, Lafarge received support in 2010 from the European Union's LIFE+ environmental funding programme. The objective was to demonstrate the industrial feasibility of Aether cements. The LIFE+ support has covered a period of three years, 2010 - 2013, and contributed to the funding of pilot tests at Lafarge's Polish partner, the Institute of Ceramics and Building Materials (ICiMB), two industrial trials at Lafarge facilities and independent testing of concretes and mortars made with Aether cements at UK-based BRE.
ICiMB first ran pilot trials at its specialist cement-testing facilities in Krakow, Poland, in order to test certain parameters and predict conditions for Aether clinker production in real industrial installations. Selected cements made from semi-industrial clinker, with different finenesses and sulphate content, were analysed for setting time, water demand, soundness, compressive and flexural strength, heat of hydration, workability and consistency.
A first industrial feasibility trial of Aether clinker was carried out in February 2011. The Lafarge plant chosen for the trial used the grid Lepol (semi-dry) process, which offers production conditions that are very similar to those of the laboratory tests. Around 5000t of Aether clinker were produced during an eight-day production period, confirming that it was possible to produce reactive Aether clinker in an industrial kiln.
The final step to validate industrial feasibility was a second trial carried out at Lafarge's Le Teil plant in southern France, in December 2012, this time using a dry process kiln. The trial was successful and involved the production of around 10,000t of Aether clinker over an eight-day production period.
A specific organisation was set up for each trial to accompany the plant teams during the production phase. Technical assistance was provided by experts from Lafarge Technical Centres, to aid the plant teams in operating the kiln and other equipment. Technicians from the Lafarge Research Centre were also on hand to carry out analyses throughout the trial, some of these requiring additional laboratory equipment that was provided to the plant. Teams worked around the clock in shifts to ensure real-time monitoring of production.
Each team included a member of the plant's management staff, a Technical Centre expert acting as 'kiln coach' to advise the kiln operators, several technicians to take regular samples of Aether raw mix and clinker and carry out analyses of these, an analyst in charge of compiling and interpreting data on clinker quality and the plant's normal production staff.
Key learning points on process conditions
Aether production requires no additional infrastructure to that already used for OPC production. Prehomogenisation, grinding, burning and cooling can all take place in existing installations. Besides an XRF machine for chemical analysis, only an additional XRD machine is required, with accompanying software that quantifies the different mineralogical phases. This ensures constant monitoring of product quality.
As the chemical composition of Aether clinker is fairly different to Portland clinker, it is nevertheless necessary to clean equipment before switching from one sort of production to the other, to meet regulatory quality requirements.
While Aether clinker production is similar to Portland clinker production, the industrial trials nevertheless showed that a higher level of control is needed for each process step. In particular, a very narrow temperature range is required in the clinkering zone of the kiln, to ensure that the clinker is neither under- nor over-burnt, as in either case this reduces the overall amount of ye'elimite and therefore the early strength gain of Aether cements.
Furthermore, under-burnt clinker leaves an uncompleted combination of the different elements present in the raw meal, while over-burnt clinker generates higher SOx emissions, due to a decomposition of the ye'elimite phase (C4A3$). Too high a temperature in the clinkering zone also leads to a risk of ring formation or melting that can force a kiln stop, while the over-burnt clinker is also harder to grind.
Nitrogen oxide (NOx) emissions are lower than with OPC production, due to the lower temperature at which Aether clinker is produced. If the raw mix is correctly designed and the clinkering temperature well monitored, Aether clinker production generates the same level of SOx emissions as OPC.
Confirming environmental performance
The two trials run in Lafarge plants confirmed not only industrial feasibility of Aether cement production, but also the environmental performance of Aether cements. Overall, Aether cement production allows a reduction in CO2 emissions of 25-30% compared to OPC.
10-15% of CO2 savings are related to lower emissions from decarbonation, due to a lower quantity of limestone in the raw meal. 5-10% of savings are fuel-related, as burning temperatures are lower than for OPC. Finally, CO2 savings of 5-10% are obtained during the cement grinding process. This is thanks to the excellent grinding characteristics of Aether clinker compared to Portland clinker, which result in lower energy consumption during this phase in production. Comparative data indicate that energy savings of up to 40% can be obtained during grinding (See Figure 2).
Testing reactivity and strength of Aether cements
Testing is being carried out by Lafarge research teams and UK-based BRE to assess the mechanical properties and durability of mortars and concretes made with Aether cements. So far, results have been very encouraging. Concretes produced with Aether cements show high early compressive strength gain of up to 20MPa at six hours and compressive strength at 28 days equivalent to that achieved with a standard cement (CEM I 52.5R).
Furthermore, Aether cements show better dimensional stability than OPC for certain applications. Testing has demonstrated 50% less shrinkage in concretes made with Aether cements compared to concretes made with OPC (tested in air at 20°C and 50% Relative Humidity, after one day of curing).
Testing the durability of concretes made with Aether cements
A comprehensive durability testing programme has also been developed by Lafarge and BRE to test the resistance of Aether cements to a number of well-identified risks. In particular, tests are being run to evaluate the risk of corrosion of the steel rebars used in structural concretes and the risk of degradation of the concrete itself. In-situ and accelerated testing is being carried out and the first results of this testing programme will be available later in 2013.
Three concrete mix designs have been developed for durability testing, representing a variety of different concrete applications:
- Typical C20/25 concrete: Cement = 240kg/m3; water to cement ratio of 0.6. For use without steel reinforcements in non-structural applications.
- Typical C25/30 concrete: Cement = 300kg/m3; water to cement ratio of 0.5. For use with embedded steel rebars in structural applications such as slabs, walls and columns in non-aggressive environments.
- Typical C35/45 concrete: Cement = 360kg/m3; water to cement ratio of 0.45. For use with embedded steel rebars in structural applications in chemically aggressive environments such as the presence of acid in soils, seawater chloride or in industrial applications.
The durability programme includes tests on carbonation ingress. These measure the oxygen permeability (durability index) of the three concrete mix designs compared to OPC-based concrete and thus their resistance to atmospheric carbonation, which reduces the pH of concrete and increases the risk of corrosion to the embedded steel bars. The second two concrete mixes are being tested for their resistance to chloride ingress, important for concretes exposed to seawater or de-icing salt. These two mixes are also being tested for their resistance to chemically aggressive attacks such as sulphate or organic acid, particularly important for concretes used in aggressive soils or industrial applications.
Developing Aether cements for specific applications
Lafarge is now looking at a wide range of potential applications for Aether cements, for example in the precast segment, where their high early-strength gain could be a particular advantage. Up to 20MPa is obtained at six hours without heating, whereas conventional concrete has a much lower compressive strength under these conditions. The launch of the first Aether products is planned for 2014. For more information: www.aether-cement.eu.
References
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