There are already many examples of the cement industry attempting to reduce total CO2 emissions. To date, these reductions in CO2 have been financially as well as environmentally led and have included using alternative fuels, such as those from substances traditionally viewed as waste; scrap tyre-chips, waste oil, bone meal and processed sewage pellets. This saves on fossil-fuel costs, thus making the production process cheaper for producers. Where the fuels are bio-mass based, it also reduces CO2 emissions because less fossil fuel needs to be sourced and it also diverts the waste materials from landfill.
Another common approach is to substitute a percentage of OPC clinker for pozzolanic materials such as fly-ash, metakaolin, silica fume or (ground) granulated blast-furnace slag to produce different CEM products. Producing 1t of CEMII–CEMV therefore emits less CO2 than producing 1t of OPC (CEMI). These types of cement also displace a percentage of the OPC that would have been produced in their absence.
Such additives are often sourced from wastes. Recently however, producers of such materials have realised the value of their 'waste' and are now beginning to charge for them, especially in situations where there are emissions tariffs associated with their prodution. If unchecked, this trend has the potential to raise the costs of alternative materials and actually deter manufacturers from including these based on cost savings.
Cement producers are also looking at in-house efficiency measures to reduce CO2 emissions and outgoings. As reported in Global Cement Magazine in 2010, various producers including Lafarge, Holcim and Pretoria Portland Cement have all reported hitting their long-term (20 to 25 year) targets regarding improvements to clinker production efficiency and hence have reported respectable reductions in CO2 output.
'You can't change the chemistry'
But however efficient the above practices may become, most manufacturers do not consider (or do not want to consider) the source of around 50% of cement CO2 emissions, namely the decarbonation step in which calcium carbonate (CaCO3) is converted to calcium oxide (CaO). If somehow this step could be changed or even removed altogether, it may then be possible to achieve significant further reductions in CO2 emissions. This may involve taking a fresh look at the definition of cement (or cementitious materials), from the point-of-view of their chemistry and the entire production process. Ruling-out new types of cementitious materials would be to pass-up on the opportunity for the potentially massive reductions in CO2 emissions, and potentially a lucrative market share of the existing OPC market.
One viewpoint on this challenge was summed up in 2007 by Dimitri Papalexopoulos, managing director of Titan Cement, when he said, "No matter what you do, cement production will always release CO2. You can't change the chemistry, so we can't achieve spectacular cuts in emissions." This statement is correct from the perspective of OPC, but looking outside OPC, perhaps it is possible to change the chemistry. There are some exciting prospective products emerging, many of which have the potential to replace significant portions of the OPC market given favourable conditions.
Requirements for alternatives
OPC is cheap to produce and incredibly strong, with well-established production plants, supply chains and markets. Any low-CO2 alternative would need to be at least a like-for-like replacement in terms of ease of use, properties and cost to be able to replace an appreciable amount of OPC. In other words, any low-CO2 properties or other novel properties of a new cementitious material are likely to be a relatively minor concern compared to their ability to match OPC for mechanical strength.
Alternative cementitious materials
Alkali activated binders (AAB): These differ from defined CEM products because the pozzolanic materials and latent hydraulic materials present in them are not activated by cement clinker. They can be split into two groups, i) those that are high in CaO and are activated by small quantities of activator and ii) those formed by strong alkali activation of CaO-free aluminosilicates. In the second case they form complex 3D poly-aluminosilicate structures upon activation, termed 'geopolymers.'
Both types of AAB are multi-component systems, in which the activator may be powdered and activated upon addition of water or may be dissolved or suspended in the mixing water.
In general the aluminosilicates, such as metakaolin (Al2Si2O5(OH)4) dissolve in the aqueous alkaline environment, enabling the formation of dimers then oligomers and finally polymers. The polymers have a high degree of cross-linking, leading to high compressive strength, which has been shown to increase over a period of many years for some systems.
Other advantages of alkali-activated binders include rapid initial hardening, high resistance to chemical attack, good resistance to a range of temperatures and low CO2 emissions. These can be lower than OPC by up to 80% in some cases. An additional property is that they have the ability to absorb harmful organic and inorganic materials for long-term storage.
Calera - Mineralisation via aqueous precipitation:Calera is a Californian company looking to directly convert CO2 into cementitious materials. In its radically different, bio-mimetic system, waste CO2 is passed through water with a high proportion of calcium and magnesium salts. This encourages the formation of insoluble salts, which precipitate out of solution. The process essentially mimics 'marine cement,' which is produced by corals for the production of shell materials and reefs. Such organisms take the calcium and magnesium ions present in sea-water salts and use them to form carbonates at ambient temperatures and pressures.
The Calera process thus has two key environmental advantages. Firstly, it makes use of waste CO2, which could be sourced from a conventional power station. The water used could also be sourced from a large number of industrial processes. Indeed there are practically no limitations on the raw materials of the Calera cement. Sea water containing billions of tons of calcium and magnesium covers 70% of the planet and the 2775 power plants in the US alone pumped out 2.5bnt of CO2 in 2006.
The process results in sea water that is stripped of calcium and magnesium, ideal for desalinisation use for making clean water, but also safe enough to be dumped back into the ocean. Attaching the Calera process to the 600 coal-fired power plants in the US alone and to steel mills and other industrial sources is very attractive because burning coal results in flue gas with CO2 concentrations as high as 15%.
While Calera's process of making calcium carbonate cement wouldn't eliminate all CO2 emissions, it would reverse the direction of the equation. "For every ton of cement we make, we are sequestering half a ton of CO2," explained crystallographer Brent Constantz, the founder of Calera. "We probably have the best carbon capture and storage technique there is by a long shot. It's just a little better than carbon neutral - That alone is a huge step forward."
Calix Technology: Calix is a company based in Sydney, Australia, which in 2005 patented its 'Horley Flash Calciner,' a novel type of calciner, the rights to manufacture it and the downstream materials processed from it.
The Horley Flash Calciner, invented by the late Connor Horley, converts lime, dolomite or magnesite into their respective oxides in a process far more efficient than traditional calcining. There are several commercial advantages associated with flash calcining, including the unique reactivity of its products, a lower capital cost for the necessary processing plant and reduced production costs.
In the system, fine dolomite particles are dropped down a tall vertical tube containing superheated steam at 400°C. Horley found that this process converts the material to its oxides before they reach the bottom of the dropping tube (a process taking only three seconds) and that the particles also had a sponge-like structure with effective surface areas in the region of 100,000m2/kg. Thanks to these large surface areas, chemical reactions can occur far more rapidly than with traditional ground clinkers.
For example, the sponge-like semidolime binds so well that the Calix product sets to the same strength as normal concrete in 20 minutes and continues to strengthen over the following 24hr to give a product with two to three times the strength of normal concrete.
In addition, the products made from semidolime retain the CO2 bound to calcium as CaCO3. The CO2 released from the MgCO3 calcination is captured as a pure gas stream, which can be sequestered to eliminate emissions and the CO2 produced from the combustion process can be scrubbed by using <3% of the product stream. This combination of features is unique and the products have a very low carbon footprint, being at least an order of magnitude less than that of OPC.
Calix has a flash calcining plant in Queensland, which became operational in December 2007. It currently produces samples of Calix's various calcined products to demonstrate its wide range of applications, not only in the building materials industry, but also in CO2 sequestration and the production of fertilisers.
Celitement: Celitement is the flagship product of Celitement GmbH, a company founded in 2009 by the Schwenk Group at Karlsruhe Institute of Technology (KIT) in Baden-Württemberg, Germany.
It relies on the traditional ingredients of cement, but uses significantly less limestone than OPC, which enables savings in terms of energy, associated costs and CO2 emissions. The process by which Celitement is made dates back to 1994, when the company started its research into calcium silicate hydrates. In 2007, sufficient development had occured that a series of patents was granted for a new class of binders, the basis of Celitement.
Celitement is made by taking calcium oxide and a number of different silicates in a 1:4 ratio and heating them in an autoclave at a temperature between 150°C and 300°C. This process leads to calcium silicate-hydrates, which when mixed with further silicates and milled in a reactive mill, produce Celitements; ie: hydraulically active calcium hydrosilicates. Pozzolans and/or OPC clinker can also be added to the Celitement, which gives a product that has properties similar to OPC and hence one that is easily handled by established methods. Clearly the calcium oxide (CaO) has to be formed from limestone, (CaCO3), but a lower proportion of limestone is used than for conventional OPC and so CO2 emissions are lower. The chemical process from formation of calcium oxide to the production of the finished cementitious product is described below:
Step 1: CaCO3 + Heat --> CaO + CO2
Step 2: CaO + SiO2 +1/2 H2O --> Celitement
Step 3: Celitement + 1/2 H2O --> CaO-SiO2-H2O
Celitement has a number of advantages over OPC. It is a low temperature process and because it lowers the proportion of calcium carbonate used it has a lower CO2 output. It can also be utilised much like OPC and has low porosity, making it durable and resistant to chemical attack. Celitement began construction of a pilot plant in Germany in August 2010.
Cenin: Cenin is a UK company that manufactures 'superior' cement replacement materials from industrial waste streams after 15 years of academic research. It currently delivers two different products to end-users, namely Cenin Semi Dry Product (SDP) and Cenin Wet Cast Product (WCP), from its production site at Stormy Down, Bridgend, Wales, where production began in 2008.
Cenin SDP is formed of calcaerous fly ash, a pozzolanic material that conforms to the European Standard 197-1:2000 Part 1. Calcareous fly ash is a powder that can be hydraulic and/or pozzolanic consisting of calcium oxide (CaO), silicon dioxide (SiO2) and aluminium oxide (Al2O3). It also contains small amounts of iron oxide (Fe2O3) and other components.It is primarily sold for use in masonary applications such as block paving because it has high early strength. Its production is a low CO2 process.
Cenin WCP is a latent hydraulic material that also falls into the category of calcareous fly ash. It should be noted that neither of the products requires calcining and hence both have low CO2 outputs.
The production process for both products is patent-protected. Typically the process modifies the pozzolanic material, which although not cementitious in itself, can be chemically engineered to create a chemical reaction with Portland cement during the hydration process to improve the properties of the product. This therefore creates more cement per ton of clinker with the obvious associated CO2 savings. In fact, once all of the process is taken into account, Cenin's 'replaCements' can have CO2 emissions as low as 43kg/t.
Cenin's products can be tuned to the individual requirements of the client, to give, for example, low heat of hydration in a hot climate, or increase it for harsh winter environments.
CemStar: CemStar is a patented production process licensed by Texas Industries Inc. (TXI), which produces cementitious materials with the same chemical composition as OPC. Patented in 1995, the process incorporates a percentage of blast furnace slag but is unlike ground granulated blast furnace slag cement because it is made by introducing the slag to the kiln, not to ground OPC clinker. The process therefore produces more OPC than a conventional OPC plant.
As slag consists of silicates, aluminosilicates, and calcium-alumina-silicates, its chemical composition closely matches that of OPC clinker. In addition, by the time it leaves the steel-production process, it has already undergone many thermo-chemical changes that make it suitable for turning into cement. This factor, in addition to its low melting point, means that the inclusion of slag in the kiln does not require significantly more fuel. TXI claims that every ton of slag introduced to the kiln incrementally produces an additional ton of OPC and hence saves around a ton of CO2 from entering the atmosphere.
Several plants in the US have been converted to use slag in their kilns since the 1990s, with several CemStar licences issued to other US manufacturers. Typically the plants record production increases of 8-15%, reductions of 5-10% in CO2 output and fuel cost savings of up to 15%. From a US perspective, CemStar says that it can help to raise the level of US cement production and hence reduce the reliance on imports.
C-Fix: C-Fix is not a mineral-based product, but may still find use in applications where OPC may have been used and hence could reduce CO2 emissions by displacing a portion of the OPC market. It is produced by C-Fix bv, a joint venture between Royal Dutch/Shell Group and UKM Ltd. C-Fix is a thermoplastic binder, which when mixed with aggregates, sand and filler forms 'carbon concrete.'
It is formed from the sludge left over after oil refining, a product often seen as having no value. Traditionally, sludge has simply been burnt at refineries in order to get around waste-disposal legislation. This is a process that produces CO2 with no benefits.
Instead of burning this oil, C-Fix bv has developed a process in which it is heated to around 200ºC and mixed with aggregates and sand. The company claims that using 1t of C-Fix prevents the emission of 2.5t of CO2 when replacing OPC as a binder in concrete mixes. The preparation of C-Fix does not use any water, allowing this to be diverted to other uses, which represents secondary environmental benefit.
C-Fix is currently sold mainly as an industrial flooring material, which has properties between those of concrete and asphalt. It has also found use in sea-wall defences at Ijmuiden, near Amsterdam, as roof-tiles and sewage pipes. It is less suitable for hot climates because it is thermoplastic and cannot currently be used for load-bearing applications.
Calcium sulpho-aluminate (CSA) cements: CSA cements have been produced commercially in China, notably by Oreworld Trade (Tangshan) Co Ltd, for around 20 years. They are generally made by sintering industrial wastes such as fly ash and gypsum with limestone at a temperature between 1200°C and 1250°C in a rotary kiln. After calcining, the clinker is ground and mixed with 35% to 70% belite (calcium disilicate, Ca2SiO4) and between 10% and 30% ferrite (calcium ferroaluminate, Ca2(Al,Fe)2O5). These types of mixture, CSA-belite cements, are fast-setting cements with specific surface areas in the region of 3500m2/kg.
In established Chinese plants, CO2 emissions are around 80% of those for OPC. The product has similar setting characteristics to OPC, but adjustments can be made according to the preferences of the client. CSA cements are still a small-scale operation, with production a mere 1Mt in 2009. They are also expensive at present and are unlikely to be able to substitute for OPC in bulk at the current time.
Eco-Cement: Eco-Cement is a trade-marked product being developed by the Tasmanian firm TecEco Pty. Ltd. It uses a partially traditional approach to cement chemistry. However, by using magnesium carbonate (MgCO3) as well as traditional calcium carbonate (CaCO3) the company has built in two distinct advantages over OPC. Firstly, the decarbonation reaction that converts magnesium carbonate to magnesium oxide (MgO) requires significantly less energy than the analogous calcium reaction. This means that the kiln may be run at temperatures in the region of 650-750ºC, contributing to a massive reduction in fuel requirements and hence both cost and CO2 emissions. TecEco says that reducing the firing temperature makes it far easier to use alternative fuel sources such as solar condensers in combination with fossil-fuels.
Secondly Eco-Cement is more porous than OPC, which enables it to absorb atmospheric CO2 very rapidly once it is set. This enables the material to have a negative net carbon output over a period of months instead of millennia, as is the case with OPC.
The finished product is comparable in many respects to OPC in terms of strength and chemical resistance. TecEco also claims higher resistance to chlorides and sulphates, as well as other beneficial properties such as lower shrinkage than OPC. In addition to these benefits, Eco-Cement could be made in existing plants, because the equipment required is identical to that currently used.
There are also large reserves of magnesium carbonate (MgCO3) in the form of various hydrates across the world. Unfortunately though, magnesium carbonate has traditionally been viewed as an undesirable component in cement manufacture. This means that despite it being readily available, existing cement plants are built in locations away from this resource. The cost of
transporting the raw material would therefore be high in the first instance. Magnesium carbonate is also slightly toxic to the nervous system over prolonged exposure, which may give rise to health concerns, especially in environment and health-conscious economies such as North America and Europe.
The company is currently taking orders for Eco-Cement from contractors on an experimental basis.
Municipal solid waste incinerator ash cement: Municipal solid waste incinerator ash (MSWIA) can be used to make cement in two different ways. Firstly, it can be substituted for up to 50% of OPC clinker, a practice seen in Japan. Cements made with MSWIA are made by heating in a kiln, as with OPC, but the process can be performed at 1350°C instead of 1450-1550°C. This lowers the fuel requirements of the kiln by a small but noticeable extent and thus cuts the CO2 emissions of the process. With the exception of some specialist types of cement, these are very similar to OPC and hence can be used and handled in the same way as OPC.
Harmful components of MSWIA such as dioxins and chlorides are burnt off and decompose above 800°C. To ensure that dioxins do not reform, the clinker is rapidly cooled down to below 250°C, once it is formed. Harmful metals such as cadmium, zinc, lead and copper may be contained within the clinker dust at this point. By further processing, it is possible to remove these metals for refining and re-use. This represents not only a health benefit, but also reduces the need to mine these metals from virgin sources.
Novacem: Novacem is a company based at Imperial College, London, UK. It is developing magnesium chemistry as a replacement for calcium chemistry, but it aims to go one step further than TecEco by replacing the magnesium carbonate starting material with magnesium silicates. It is investigating the accelerated carbonation of the silicates (to give MgCO3) by using elevated temperatures (180°C) and pressures (150bar). This forms a complex magnesium carbonate, which then undergoes a similar calcining process to that of TecEco's material to give magnesium oxide (MgO).
French cement giant Lafarge has recently signed up to Novacem's 'Green Cement' Bond, giving a nominal UK£1m (Euro1.22m) to Novacem to help accelerate its research programme and assist with the construction of an experimental low-scale cement plant of 25,000t/yr next to an existing OPC plant in the UK. Lafarge and any other subscribers that may come onboard will be able to sell Novacem under licence from Novacem once the plant is operational.
Novacem's cement is composed of between 50% and 80% magnesium oxide and hydrated magnesium carbonates, which allow rapid strength development in applications where CO2 is not readily available, (ie: underwater). Novacem says that the cement is still in development, but also that it is already suitable for low-level, non load-bearing applications, such as producing masonry products.
RockTron: RockTron is a UK-based company that processes fly ash from coal-fired power stations for the recovery of various materials. The fly ash is firstly refined using froth-floatation to separate the alumino-silicates and magnetite from carbon. Secondly, the magnetite is removed using magnetic separators before polishing and grading of the alumino-silicates for addition to OPC.
By replacing a percentage of the clinker in a 1:1 ratio, less clinker is required per ton of cement. By removing up to 90% of the carbon, the material is able to have all of the benefits of fly-ash-containing cements; reduced permeability, improved long term strength and reduced risk of alkali silica reaction. RockTron estimates that 450,000t of CO2 are saved for every 500,000t of fly ash processed.
By cleaning up fly-ash into a number of useful mineral products, RockTron is reducing the amount of fly-ash waste sent to landfill and by substituting OPC clinker, it is reducing the need to mine for raw materials. Both of these factors clearly have additional environmental benefits.
RockTron has two plants in the UK, a full-scale production facility at Fiddlers Ferry, Merseyside, which can process 0.8Mt/yr of fly-ash and an R&D/demonstration plant at Gale Common, Yorkshire which can operate at 3.2t/hr. RockTron plans to expand operations at Gale Common by constructing two new plants, both with a fly-ash processing capacity of 0.8Mt/yr.
The company's current range of products includes two types of cement constituent for inclusion with OPC clinker and CenTron brand cenospheres, a range of light-weight hollow spheres suitable for inclusion in light-weight cements for specialty applications.
SlagStar: SlagStar is a non-OPC product, which has been developed over 18 years by Austrian firm Wopfinger Baustoffindustrie. Now patented in 50 countries, it is the result of the search for a cement that has a low heat of hydration. Essentially, it is a modern supersulphated cement, which consists of granulated blast furnace slag, alkaline sulphate agents, gypsum and other additives. The manufacture of SlagStar requires no calcination and so it emits 73% to 90% less CO2 per ton than comparable CEMIII products. From an environmental perspective this is clearly a major advantage.
SlagStar has greater compressive strength than OPC and also has higher acid and sulphate resistance that make it suitable for use in harsh soils or for corrosive industrial applications.
The heat of hydration of a typical concrete made using SlagStar is 9°C, 75% lower than that of conventional concrete products. This makes it suitable for applications where thermal expansion during setting needs to be avoided, such as in dam construction. If appropriately marketed to contractors in these areas, SlagStar may be able to displace some OPC usage.
Conclusion
It is clear that there are a large number of potential OPC replacements and substitutes being developed by start-up companies, universities and established cement manufacturers. This article only scratches the surface of the work being undertaken. Some of the processes have chemistry on their side or the backing of major producers and others are even in a position to make full production a reality.
Unfortunately, any low-CO2 successor to OPC will initially be an expensive alternative. It may be unfamiliar in terms of its behaviour and it will need to be developed in terms of its supply chain and undergo the process of being standardised if it is to become a commodity. This means that there will be a risk associated with attempting to market any of the perfectly chemically sound options that simply may not pay off. Major attractions of OPC are that it is predictable, 'safe' and well known. It is likely that at some point, increased environmental pressure will come to bear on OPC producers and they will have to look beyond efficiency measures and alternative fuels. Cement producers as we know them may have to become suppliers of less specific cementitious materials, rather than producers of OPC.
The inaugural Future Cement Conference is taking place in London, UK on 8 February 2011. Please see www.FutureCement.com to register your interest and for more information.