Every year, manufacturers world over produce around 5 billion tonnes (bn t) of Portland cement, the gray powder that mixes with water to form concrete. For every tonne of cement produced, the process emits approximately a tonne of carbon dioxide, all of which accounts for roughly 7 per cent of the world´s CO2 emissions. The demand for cement is rising, and the developing world has started using more and more of Portland cement.
As per the estimates by the Indian Bureau of Mines, the limestone deposits that are available to produce cement have a limited life and are not going to last for more than 35-40 years. Being a developing country, our per capita consumption of cement is increasing every year, and therefore it is more pertinent for us to look for alternatives. However in the last 15 years, we have not seen much progress on this front. The other facet of the story is the technological alternative for the product. Do we really have an alternate product which will replace the existing cement? This is a million dollar question yet to be answered. The ICR team decided to have a close look at the alternatives which are on the scene.
While looking for substitute products, the category of developments can be divided into different sections: cements developed without the use of limestone; cements still using limestone, but with less of carbon footprint; making use of gaseous CO2 for production of concrete and producing calcium carbonates which is a raw material for cement from sea water or algae.
Cements without limestone! Geo Polymers
The most striking development on this front has been that of geo polymers. The term geo polymer was coined by Davidovits in 1978, to represent a broad range of materials characterised by chains or networks of inorganic molecules. In the case of geo polymers based on alumino silicate, suitable source materials must be rich in amorphous forms of Si and Al, including those processed from natural mineral and clay deposits (e.g., kaolinite clays) or industrial byproducts (e.g., low calcium oxide fly ash or ground granulated blast furnace slag) or combinations thereof. The source material is mixed with an activating solution that provides the alkalinity (sodium hydroxide or potassium hydroxide are often used) needed to liberate the Si and Al, and possibly with an additional source of silica (sodium silicate is most commonly used). The temperature during curing is very important, and depending upon the source materials and activating solution, heat often must be applied to facilitate polymerisation, although some systems have been developed that are designed to be cured at room temperature.
Till date, there are no widespread applications of geo polymer concrete, although the technology is rapidly advancing in Europe and Australia. One North American geo polymer application is a blended Portland geo polymer cement known as Pyrament (patented in 1984), variations of which continue to be successfully used for rapid pavement repair. Other Portland geo polymer cement systems may soon emerge. In addition to Pyrament, the US Military is using geo polymer pavement coatings designed to resist the heat generated by vertical takeoff and landing aircraft. In the short term, there is potential for geo polymer applications for bridges, such as precast structural elements and structural retrofits using geo polymer-fibre composites. Geo polymer technology is most advanced in precast applications due to the relative ease in handling materials and the need for a controlled high-temperature curing environment required for many current geo polymer carbon dioxide binder that has similar properties to Portland cement. In addition, current research is focusing on the development of user-friendly geo polymers that do not require the use of highly caustic activating solutions.
Cement based on Mgnesium Oxide
Novacem, a UK-based Institute that has developed a new class of cement, offers performance and cost at par with ordinary Portland cement, but with a carbon negative footprint. It is uniquely positioned to meet the challenge of reducing cement industry´s carbon emissions.
Novacem cement is based on magnesium oxide (MgO) and hydrated magnesium carbonates. The production process uses accelerated carbonation of magnesium silicates under elevated levels of temperature and pressure (i.e. 180oC/150oC bar). The carbonates produced are heated at low temperatures (700oC) to produce MgO, with the CO2 generated being recycled back in the process. The use of magnesium silicates eliminates the CO2 emissions from raw materials processing. In addition, the low temperatures required allow use of fuels with low energy content or carbon intensity (i.e. biomass), thus further reducing carbon emissions. Additionally, production of the carbonates absorbs CO2; they are produced by carbonating part of the manufactured MgO using atmospheric/industrial CO2. Overall, the production process to make 1 tonne of Novacem cement absorbs up to 100 kg more CO2 than it emits, making it a carbon negative product.
This production process is based on 20 years of research on the mineral carbonation of magnesium silicates. These minerals are widely dispersed with accessible worldwide reserves estimated to significantly exceed 10,000 bn t.
One perplexing thing about this product is that there doesn´t appear to be any independent research on the properties of Novacem cement, and that would be important to examine, for example, the durability and water-resistance of Novacem´s product compared with Portland-based cement. So, some caution must be exercised with regards to accepting their claims. It is very unfortunate that after this invention Novacem cement company, announced a Creditors´ Voluntary Liquidation and does not exists any more.
One reason why engineers and contractors haven´t embraced alternatives, such as geo polymers and Novacem is that most building and construction codes are formula based rather than performance based. In other words, the codes tend to spell out in detail what mix of concrete can be used where, instead of establishing a set of characteristics. Unless these codes are modified - and there doesn´t appear to be any hope of wide-scale application.
LC3 technology, quite promising
In an effort to reduce the carbon footprint of cement by 40 per cent, three Indian Institutes of Technology are working on a new blend that will substitute up to half of the Portland cement normally used to make concrete. Researchers from IIT-Delhi, IIT-Bombay and IIT-Madras are collaborating in the project with the Swiss Federal Institute of Technology in Lausanne and the Central University of Las Villas in Cuba. The new blend, a combination of calcined clay and ground limestone, is expected to replace half of the Portland cement. Using LC3 (Limestone Calcined Clay Cement) will mean lower CO2 emission, lower cost because the substitution material is abundantly available, and lower capital because no change of equipment or additional training of construction workers is required. The researchers also expect it to be energy efficient as lesser energy will be required to manufacture and manage industrial waste as there is a possibility of using existing mining and other wastes. It will also extend the life of limestone reserves.
The first phase of the project is expected to be completed in three years. Researchers are closely working with Technology and Action for Rural Advancement (TARA), a social outfit that works to create sustainable livelihoods by providing development solutions, in association with some cement companies. The Swiss Agency for Development and Cooperation has sanctioned considerable funds for developing and testing of LC3. LC3 has the potential to reduce CO2 emission by 20-30 per cent compared to traditional cement; a major reduction considering that cement accounts for 5-8 per cent of today´s manmade emissions. LC3 is a low-carbon and low-cost cement that delivers similar performance properties compared to Portland cement. The blend can be easily manufactured in existing production lines, requiring only minor capital investments.
"The potential impact of the LC3 project is very encouraging It is estimated that using LC3 instead of regular cement can save up to 500 mn t of CO2 per year by 2050. "If we want to advance the sustainability of concrete - its cost, availability and environmental footprint - we have to act before demand increases," says one of the scientist associated with the project.
Industrial scale pilot projects were implemented in Cuba and India, and in both countries several structures were successfully built using the cement. Cuba will be the first country where LC3 will be produced at a commercial scale. It will enable meeting growing demand without large capital investments, whilst lowering impact on the environment. If applied globally, LC3 could help bring down future emissions considerably, researchers feel, having a major impact on global climate change.
Prof Gaurav Sant of the California Nano Systems Institute at UCLA, recently completed a research that could eventually lead to methods of cement production that give off no carbon dioxide. Sant, found that CO2 released during cement manufacture could be captured and reused. During cement manufacturing, there are two steps responsible for carbon emissions. One is calcination, when limestone, the raw material used to produce cement, is heated to about 750oC. That process separates limestone into an unstable solid - calcium oxide, or lime - and CO2 gas. When lime is combined with water, a process called slaking, it forms a more stable compound called calcium hydroxide. The major compound in Portland cement is tricalcium silicate, which hardens like stone when it is combined with water. Tricalcium silicate is produced by combining lime with siliceous component and heating the mixture to 1,500oC.
Cements with advanced amine technology
Cement flue gas contains about 18 per cent CO2 by volume. Initial studies have demonstrated that the carbon capture plant can be created which will be compact and cost-efficient to capture CO2 emitted in the manufacturing process of cement. Heat integration studies have concluded that about half of the CO2 emission from cement plants can be captured by utilisation of waste heat from production. Cost of heat supply is normally one of the highest operating cost factors for the capture plant. One such pilot project right now is running at Norcem plant in Norway.
Aker Solutions´ CO2 Capture Technology is in use by Norcem for commercial-scale. This is the world´s first commercial-scale carbon capture facility for use in cement production. The company is looking to capture around 400,000 tonnes of CO2 a year at Norcem´s cement plant in Brevik, Norway, using Aker Solutions´ technology. The extended feasibility study will contain an overall design for the facility, including its utility systems, CO2 liquefaction and ship offloading as well as integration with the cement plant.
The work for Norcem is part of a feasibility study that will be submitted to Gassnova and the Norwegian Ministry of Petroleum and Energy by the end of May 2016. The Brevik plant has been nominated to become a national carbon capture and storage demonstration project in Norway by 2020.
Cements with enhanced fly ash content
Fly ash, produced during burning of black coal or lignite in power stations and caught in fly-ash separators, has been used for production of concrete for the last 50 years. The reasons are mainly reduction of hydration heat, hence elimination of inner strain and consequent cracks in concrete structure. Fly ash also takes part in the process of hydration and combined with cement, creates compact structure; under certain circumstances it can show higher strength than cement matrix.
The technique of using fly ash as active addition for correction of end-use properties of concrete and consequent reduction of input calcareous material has been known for quite a long time. However, use of readily available fly ash, slag or similar materials are in vogue. However, the quantum of addition has almost reached its limit in the normal way and there are attempts being made by the scientific community to enhance these limits. Three approaches are commonly tried out successfully-one is chemical activation of fly ash and the others are thermal activation and mechanical activation of fly ash.
The chemical activation means chemical activating agent is added, usually alkali based compound. The second way is thermal activation where fly ash is heated before use. The last way is mechanical activation, which is based on the fact, that smaller grains are more active and ensure higher density of cement stone. Fly ash of smaller fractions shows more advantageous chemical and morphological properties. Fly ash is sorted by means of various filters. In this way, fly ash of high quality is obtained. Fly ash is sorted by means of various filters. In this way, fly ash of high quality is obtained, the rest is going as a waste. This situation is solved by means of the last way - treating of fly ash by grinding. In this way, fly ash of average properties, chemical and morphological is obtained; however, granulometry and specific surface are in the area of higher activity. Grains of fly ash can fill matrix better and have higher specific surface, which increases their reactivity.
Fly ash produced by conventional burning can be divided into black coal fly ash and lignite fly ash.
Most of fly ash is produced during burning lignite, the quality is no good. Quality of black coal fly ash is better not only from the point of view of chemical composition but also because of the shape and surface of grains, which are round and the size of cement grains.
VHSC Patented Process
Texas-based VHSC Cement LLC has a proprietary transformative process to make a high-quality cementing material using coal fly ash. VHSC´s product - PozzoSlag - is a replacement for Portland cement. This technology - though still at development stage - is slightly ahead of the rest. It activates fly ash through a mechanical and chemical process which costs 50 per cent less than cement, and has over 90 per cent lower emissions. The company´s main aim is to improve the performance of concrete while reducing material costs and the carbon footprint.
Though VHSC process looks quite simple, the end product produces different characteristics. Various approvals from regulatory institutions for the product have been obtained and a couple of jobs have been carried out as claimed by the company.
VHSC converts pozzolans into a cementitious material that has better performance than ordinary Portland cement. End product could replace over 60 per cent of Portland Cement without sacrificing performance. The investment is relatively lower than traditional cement plants, and plants can be smaller, cheaper and easier to build, in locations closer to local markets.
One of the cement companies in India experimented with use of the technology developed by VHSC with their own fly ash and found the results quite encouraging.
We at ICR have tried to capture few of the developments aimed at making cement with low carbon foot print. The subject is so vast that it is difficult to complete it in just one issue. Some technologies like the formation of Calcium Carbonate from Algae is also a novel route, the other one being precipitating Calcium Carbonate from sea water also is not covered. As and when the subject is again taken up we shall provide the readers more information.
The Concrete Pavement Technology Program (CPTP) is an integrated, national effort to improve the long-term performance and cost-effectiveness of concrete pavements. Solidia Technologies® makes it easy and profitable to use CO2 to create superior and sustainable building materials. Based in Piscataway, N.J. (USA)
VHSC´s Texas facility has demonstrated the technology´s commercial viability
WHAT IS THE MOST EFFICIENT WAY TO ACTIVATE THE REACTIVITY OF FLY ASHES?
A complex technology made simple
For over 50 years, scientists have been trying to cure concrete with CO2 knowing the resulting product would be stronger and more stable. Solidia Technologies is the first to make this commercially viable. This complex technology made simple, has been verified by leading independent industry laboratories.
Solidia´s technology sequesters CO2by injecting it into concrete during the manufacturing process, transforming CO2 into a usable element that gives large scale as well as small scale concrete producers a competitive edge. Solidia will modernise one of the world´s oldest industries through a collaborative partnership that innovates with greater sustainability in process and products.
By adopting Solidia´s process, cement and concrete producers can manage CO2 responsibly-either by emitting less CO2 during cement production or consuming it during concrete curing. In doing so, they can manufacture innovative materials that are not only stronger but also more energy and resource efficient, and profitable. Solidia Technologies´ utilisation of carbon dioxide in concrete curing was featured as a leading emerging technology in civil infrastructure during the American Concrete Institute (ACI) Fall 2015 Convention that took place in Denver. Non-hydraulic processes reduce carbon footprint of cement and concrete 70 per cent with 100 per cent reclamation.
Solidia Cement™¢, which is silicate-based and has a low-calcium content, gains strength through carbonation instead of hydration. Presenting findings of ´CO2-Cured Concrete based on Calcium Silicate Cement (CSC)´, Solidia Technologies Principal Scientist Sada Sahu, Ph.D., explained that, when compared to Ordinary Portland Cement (OPC), 30 per cent less CO2 is emitted during the production of Solidia Cement. Additionally, Solidia Cement-based concrete (CSC) can consume up to 300 kg of CO2 per tonnes of cement during the curing process.
Currently in commercialisation for large scale and small scale applications, Solidia´s additional R&D collaborators include LafargeHolcim, CDS Group, DOE´s National Energy Technology Laboratory, the EPA, Rutgers University, Purdue University, Ohio University and the University of South Florida.
ACI features Solidia´s CO2 curing processes as a leading emerging technology for civil infrastructure.
When the reduced CO2 emissions associated with Solidia Cement production are considered along with the ability of that cement to consume CO2 during concrete curing, the CO2 footprint associated with the manufacturing and use of cement can be reduced by up to 70 per cent, compared to OPC. As water is not consumed during the curing process, the mix water can be recovered and recycled.
Caijun ShiA and Yixin ShaoB
A CJS Technology Inc., Burlington, Ontario, Canada
B Department of Civil Engineering, McGill University, Montreal, Quebec, Canada