The cement industry cannot achieve the target of reducing CO2 emission in 2030 and beyond, without adopting the technologies of carbon capture and utilisation.
To keep open a realistic chance of meeting the target of restricting the global average rise in temperature below 20C, as envisaged in the Paris COP21, intensive and innovative actions are required by the cement industry, which happens to be a significant contributor to the total anthropogenic emission of CO2. In this context, the technological options available to the Indian cement industry have been briefly dealt with in this article.

Although we do not spend much time thinking about cement, we, however, for certain, see that the traditional Portland cement based concrete provides the foundation for the built environment, in which we live, work, travel and relax. In order to meet these human needs, there is a perpetually increasing demand for cement and concrete on one hand, and growing environmental concerns, on the other, linked to increasing emission of greenhouse gases (GHG) and its consequent impacts on climate change.

The demands of sustainable development, however, place a responsibility on the construction sector to continually improve existing processes, products and practices and to innovate in order to reduce both energy used in service and embodied energy in products together with emission of GHG in manufacture.

In this context, it may be relevant to recall that under the banner of the UN Framework Convention for Climate Change (UNFCCC), 195 countries adopted the first-ever universal legally binding global climate deal in the 21st session of the Conference of Parties (popularly known as COP21), held in Paris in December 2015.

The agreement sets out a global action plan to put the world on track to avoid dangerous climate change. The key elements of this agreement are the following:

• The agreement is due to be effective from 2020, subject to 55 countries accounting for at least 55 per cent emissions ratifying the agreement.
• It envisages a long-term goal of keeping the increase in global average temperature to well below 20C above the pre-industrial levels.
• It further aims to limit the increase to 1.50C, since this would significantly reduce risks and impacts of climate change.
• In the event of immediate global peaking of emissions, particularly in developed economies, rapid reduction measures will be undertaken, using the best available technologies.
• The countries will come together every five years to set more rigorous targets as required by science.
• The countries will track progress towards the long-term goal through a robust transparency and accountability system.

It may be relevant to mention here that at the commencement of this conference, some 155 countries had announced their Intended Nationally Determined Contributions (INDC) towards reducing global GHG emissions. For example, India has committed to improving the CO2 emission intensity with respect to its GDP by 33-35 per cent by 2030 , taking 2005 as the base year. One of the important means of achieving the target has been indicated to be the option of increasing the use of renewable energy to the tune of 175 GW by 2022.

The World Business Council for Sustainable Development (WBCSD), the International Energy Agency (IEA) and the cement industry had already drawn up a roadmap for sustainable growth of the global cement industry in 2009 and of the Indian cement industry in particular in 2012. The Paris agreement and the INDCs will now provide the new guideline and impetus for recasting the roadmaps to the extent required. But since the existing roadmaps for the cement industry appear to comprehensively reflect the spirit of even the Paris COP21, this article is primary based on the existing roadmap in elucidating the future sustainability through adoption of green cement and low carbon technologies in the Indian cement industry.

CO2 emissions
The direct CO2 emission from the clinker manufacturing process is estimated at about 865 kg/t and the corresponding figure for cement would be proportional to the quantity of clinker used in the cement.
The key levers to reduce the emission level adopted by the industry in general include:

• Increased rates of addition of mineral admixtures at the time of finish grinding of cement so as to reduce the clinker/cement ratio
• Enhanced use of alternative fuels
• High levels of energy efficiency
• Waste heat utilisation for generating electricity
• Progressive adoption of renewable energy to substitute fossil-fuel based electricity.

With all the above measures being progressively taken by the industry, the average CO2 emission in India came down to 630 kg/t of cement produced in 2010. In the low carbon technology roadmap the Indian cement industry, however, is envisaged to grow significantly from 2010 and 2050, both under low demand and high demand scenarios as shown in Fig.1.

However, if one were to project the business-as-usual scenario up to 2050, CO2 emissions from the Indian cement industry under two scenarios of demand growth would reach a level of 488 to 835 million tonnes (mn t), which represent an increase of 255 to 510 per cent compared to the 2010 emissions. It is estimated that such levels of CO2 emissions would correspond to a 60C rise in the average global temperature.

In order to restrict the global temperature rise to 20C and correspondingly achieve a target of CO2 emission of 275 mn t under low demand scenario or 350 kg/t of cement produced in 2050, the technology levers mentioned above may not be adequate and a paradigm shift in our strategy for developing and adopting new carbon dioxide emission technologies will be essential (see Fig. 2).

From Fig. 2, it is also evident that CCS, (Carbon Capture and Storage), will turn out to be a key technological option in achieving the emission target, the alternative for which is still not available. It should also be borne in mind that CCS technology is still in a trial mode and not at all applied to cement plants. Based on the available knowledge it is estimated that 50 per cent of the investment in the cement industry for the projected growth will be consumed by CCS technology. As a result, this option may not be sustainable for the cement industry. Hence, the search for newer technological options for low carbon pathways is essential.

Table 1: Summary of new clinker compositions as available in the public domain.

Innovative research thrusts

The traditional levers like energy efficiency, biogenic fuels and clinker substitution have been in practice for long and yet some incremental developments are being pursued, which, however, are not dealt with here for the sake of brevity. For a broad understanding of the innovative research areas for low carbon pathways one may emphasise the following trends:

• Reformulation of clinker composition
• Innovative processes for reducing carbon dioxide emission in the traditional manufacturing process
• Carbon capture and transformation
• Carbon negative new cements and concretes.

A run-through of the above potential options is furnished in the following section.

Reformulation of Clinker Composition
Some of the major multinational cement manufacturers have been working on new clinker formulations, the approaches being two-fold ? one, using less limestone in the raw mix; and two, choosing a chemistry with lower heat of reaction. If commercially feasible and adaptable, this technology can reduce the CO2 intensity per tonne of clinker by 20 – 30 per cent. A summary of new clinker formulations is provided in Table 1 (2). From the above table, the reduction in the lime content is evident. Some of the compositions are likely to contain even raw materials with lime requiring no calcination. Some developers might be using hydroxides in place of carbonates in order to reduce the extent of calcination energy requirements.

Innovative Processes for Reduction of CO2 Emission
Use of alternative fuels is a lever in practice for CO2 reduction in clinker burning. Although it started in the mid-1980s, the thermal substitution rate is still very low in our country and many other countries as well. Enhancing the utilisation of alternative fuels requires innovative adaptation of the combustion process. Modern multichannel burners and the thermograph systems allow control of the flame shape to optimise the burning behaviour of the fuels and the burning conditions of the clinker.

Gasification of fuel as well as oxygen enrichment of primary or secondary air are proving to be promising for advanced alternative fuel combustion. Further, while on the subject, it may be relevant to mention about the attention that the technologies such as "Hotdisc" of FLS, "Step combustor"of Polysius and "RDF combustor"

of KHD are receiving for complete and efficient combustion of alternative fuels. It may also be borne in mind that for achieving higher thermal substitution rate by alternative fuels, it is essential to have high-quality or even tailormade alternative fuels with guaranteed moisture content, particle size and heating value in order to avoid possible process repercussions and clinker quality problems. Thus, considering the fuel quality parameters and the processing requirements together, innovation has to continue in making the use of alternative fuel a really effective lever in reducing CO2 emission.

Carbon Capture and Use (CCU)
It is common knowledge that separation of CO2 from the flue gas is referred to as the carbon capture process. The process can be carried out broadly in the following three ways:

• Pre-combustion technology
• Oxy-combustion technology
• Post-combustion technology

The pre-combustion technology based on fuel gasification and separation is obviously not beneficial for the cement industry as the limestone component falls outside the purview of this technology; oxy-combustion is interesting but requires special facilities and therefore can only be considered for new installations. This leaves the post-combustion technology as the most likely solution for the existing cement plants as well as other industries. Consequently, substantial amount of research is being carried out in this area and the research studies are focused on:

• Application of sorbents
• Use of membranes
• Adopting cryogenic techniques.

Once the CO2 is separated, it can be utilised in many ways, either by itself or by transforming into other usable forms. Leaving aside in this article the transformation of CO2 into fuel and chemicals, which is a subject by itself, the recycling of CO2 for producing green cements is narrated briefly in the following section.

Carbon Negative Novel Cements and Concretes
Use of CO2 in producing new binders can be illustrated with the help of the following technologies:

• Calera process for calcium carbonate cement
• TecEco cements based on reactive magnesia
• Calix processes and products
• Novacem cement and concrete
• Solidia cement and concrete
• CO2- SUICOM concrete.

These are not the only technologies that have the potential of recycling CO2, but the present discourse for brevity will be limited to these ones as they are displaying higher promise of success. There are cements which are in more advanced stages of development having lower carbon footprint than the Portland cement derivatives, viz., Calcium Sulpho-Aluminate Belite cement, Calcium Aluminate based formulations and Alkali-Activated Geopolymer cements. But these cements are not made by the process of end-of-the-pipe recycling of CO2 from the emitted flue gases and hence not dealt with here.

Calera process for calcium carbonate cement
The Calera process for CO2 reduction involves the capture of CO2 gas from industrial emitting sources and converting the gas into a novel calcium carbonate cement system that can be used to make a variety of valuable products. The process of removal of carbon dioxide from the emitting sources does not require any concentration step and converts the gas into solid calcium carbonate powder, thereby permanently sequestering the CO2, in accordance with the following chemical reaction:
CO2 + Ca(OH)2- CaCO3 + H2O
Or
CO2 + 2NaOH + CaCl2 – CaCO3
+ 2 NaCl + H2O
It appears that there is a proprietary technology in the Calera process to produce the vaterite polymorph of calcium carbonate, which in the absence of water is stable. When water and other proprietary additives are added to vaterite, the latter transforms via dissolution and re-precipitation process into aragonite. This polymorphic transformation yields high strength to the product. The Calera product is produced as a fine free-flowing powder. It can function as supplementary cementitious material in traditional concrete. It can be used as an independent binder. Calera Corporation is developing wallboard and cement board products, substituting gypsum, calcium silicate or Portland cement. The process has been scaled up to a capacity of 2TPD of calcium carbonate, using raw flue gas without any concentration of CO2 gas. A simplified schematic depiction of the Calera process is given in Fig.3 (3).

Figure3. The simplified Calera process.
THE PROCESS

TecEco cements based on reactive magnesia
The process of manufacture of TecEco cements consists of the following unit operations:

• Carbonation of raw feed material
• Calcination with CO2 recycling
• Grinding if and as required
• Agglomeration or blending.

In the carbonation step, the waste magnesium cations as found in process water and bitterns are carbonated to produce large quantities of nesquihonite (MgCO3GCo3H2O), which is then calcined at 750oC or preferably at lower temperatures in a specially designed Tec-Kiln to produce reactive magnesia. The carbon dioxide generated in this step is fed back into the process of carbonation.

It may be pertinent to mention here that the Tec-Kiln is a specially designed system for pyroprocessing and simultaneous grinding of Nesquihonite. Because of the low temperature of calcination, the proposed kiln may make use of non-fossil and renewable sources of energy such as solar or wind. Since grinding is carried out in the same system and since the grinding process converts most of the energy into heat, this may also be a supplementary source of thermal energy. On the whole, the Tec-Kiln is expected to be 25-30 per cent more efficient than other conventional systems. It operates in a closed circuit without releasing carbon dioxide and other volatiles to the atmosphere.

After obtaining the required fineness of the low-temperature calcined magnesia, it is used as a binder to agglomerate large amounts of nesquihonite to produce synthetic carbonate aggregate or to be blended with other materials to form TecEco cements. Three products are differentiated: Tec-cements, Eco-cements and Enviro-cements. Tec-cements are essentially produced from hydraulic Portland varieties of cement, in which a small quantity (5-10 per cent) of reactive magnesia is incorporated, and are meant to be used for making high-strength concrete. Eco-cements and Enviro-cements contain large proportions of reactive magnesia, differing in their gas-permeability properties by design.

Eco-cements, it appears, are well-suited for masonry products like bricks, blocks, pavers, mortars, etc., which set by absorbing CO2. Eco-cement products have high thermal mass, low embodied energy, insulating properties (depending on aggregates used) and are favoured for energy-saving buildings.

Enviro-cement based concretes are ideal for immobilising and using toxic and hazardous wastes such as fly ash, bottom ash, iron slags, red mud, etc. (see Fig. 4) (4).

Figure 4: The comprehensive TecEco process.

Calix processes and products
Calix Limited of Australia has been engaged in innovating processes and systems for reducing the intensity of carbon dioxide emission in the fields of building and construction, agriculture, energy, water and other industries. At the core of Calix?s business are the following new technologies and systems:

• Catalytic Flash Calcination (CFC)
• Direct separation for lime and cement
• Endex reactor for fossil fuels
• Vortex pyrolysis

The company operates a demonstration production and testing facility at Bacchus Marsh ? about 40 km northwest of Melbourne in Victoria, Australia. The new cement formulation patented by this company is composed of 30-80 per cent of the magnesian component having MgO and at least one form of magnesium carbonate and 20-70 per cent of another silico-aluminous component. Such formulations can be used to produce building materials such as cements, mortars, grouts, etc., with low carbon footprint than those based on Portland cement (5).

Novacem cement and concrete
Since researches have been continuing to develop binding materials that should absorb more CO2 than they produce, it is relevant to mention here that a spin-off company of Imperial College, London, named Novacem Limited, developed a carbon negative cement making process from magnesium silicates in relatively larger scales (6), more or less on the basis of the fundamental work of Calix Limited. The magnesium carbonate obtained from the silicate phase is decarbonated at 7000C and the carbon dioxide released during this part of the process is returned back to the process for carbonation of magnesium silicate. Novacem has already operated an experimental batch pilot plant, which was planned to be upgraded to continuous operation mode. Planning was also done to set up a semi-commercial plant of 25000 t capacity. However, due to lack of funds, Novacem went into liquidation in October 2012 and the company?s technology and Intellectual Property have been sold by the Liquidator to Calix Limited.

According to the inventors, the following benefits are foreseen, if this technology can be commercialised :

• Use of non-carbonate feedstock with no carbon dioxide emission.
• Low calcination temperature opening up the possibility of using biomass fuel by keeping the carbon dioxide emission within 0-150 kg per tonne of cement.
• The carbonate phase in the cement composition promotes absorption of carbon dioxide (net sink 300-500 kg CO2/t carbonate)
• Total typical emissions of -50 to +100 kg CO2 per tonne of cement
• The total energy requirement in producing the Novacem product is 60-90 per cent of what is spent in making OPC.

Figure 5. CO2-cured Solidia
concrete phases.
precast concrete manufacturers

Figure 5: Microstructure of Co2 – cured Solidia
Cement (The green area is calcite (CaCO3).
The red area is amorphous silica (SiO2),
and the yellow area is unreacted wolliastonite
(CaO.SiO2)

Solidia cement and concrete
Solidia cement, a trade-marked product developed by Solidia Technologies USA, is a low-lime non-hydraulic binder, the setting and hardening characteristics of which are derived from a reaction between carbon dioxide and the calcium silicates such as wollastonite and pseudowollastonite (CaO.SiO2), rankinite ( 3CaO.2SiO2) and an amorphous meliolitic phase (Ca-Al-Si-O). During the carbonation process, calcite (CaCO3) and silica gel (SiO2) form and impart binding properties to the product.

Figure 5: Microstructure of Co2 – cured Solidia Cement (The green area is calcite (CaCO3). The red area is amorphous silica (SiO2), and the yellow area is unreacted wolliastonite(CaO.SiO2)
The process of making Solidia clinker is the same as that of Portland cement clinker but with significantly lower energy requirements. The Solidia concrete does not set, harden and cure, until it is exposed simultaneously to water and gaseous CO2. The process is mildly exothermic and the reactions occur in an aqueous environment as follows:
CaO.SiO2+ CO2 – CaCO3 + SiO2
3CaO.2SiO2 + 3CO2 – 3CaCO3+ SiO2
The above reactions demand a CO2-rich atmosphere but it can be conducted at ambient gas pressures and at fairly low temperature, viz., 20 to 500C (see Fig.5). In totality, the CO2 footprint associated with the manufacturing and use of Solidia cement can be reduced by 70 per cent. Interestingly, this level of achievement can be accomplished from the present and familiar plant, equipment, process, raw materials and supply chains (7).

Figure 6. A schematic representation of the CO2-SUICOM process.

CO2-SUICOM concrete
CO2-SUICOM?was jointly developed by the Chugoku Electric Power Co., Inc., Denki Kagaku Kogyo Kabushiki Kaisha and Kajima Corporation of Japan for a new ecological concrete which can achieve CO2 emissions levels below zero by capturing CO2 emitted from thermal power stations. The technology makes use of a special additive in the form of n?P-C2S and coal ash as well as a special carbon dioxide curing chamber. The nomenclature of the concrete has been derived from ?CO2 Storage Under Infrastructure by Concrete Materials?. A concrete block of certain given dimension, made of patented compositions, reportedly absorbs 14 kg CO2 per year, which is equivalent to the quantity absorbed by a cedar tree in a year. A schematic diagram of the process is shown in Fig.6 (8). The process involves the manufacture of the special additive with low level of carbon dioxide emission and having the property of reacting with CO2 in the concrete to achieve its hardening properties. The exhaust gases from the plant are drawn into the carbonation chamber containing the precast concrete elements for a curing period of two weeks.

After the concrete elements attain a strength level of 17-19 MPa as tested by the Japanese standard testing procedure, they are used as paving blocks, boundary wall masonry, etc.

In conclusion
The cement industry has made large strides in reducing its carbon footprint with the help of traditional levers but more is required. All projections indicate that the target of CO2 emission in 2030 and beyond cannot be achieved without adopting the technologies of carbon capture and utilisation. A large amount of work is being carried out globally in developing green cements by recycling CO2 captured from the flue gases. Given the newness of the green cements, it is difficult to judge which, if indeed any, of these novel methods of cement production will be commercially successful. All have environmental advantages over Portland cement but their production costs are likely to be higher particularly due to the limitations of process scale-up; data on material integrity of these new products needs to be firmly established; and more research is yet to be undertaken to understand the unforeseen environmental issues of the new production processes. Notwithstanding these uncertainties, there are no alternatives before the cement industry than to seriously and intently track the developments of the green cement sector, without which it may not be possible to secure real sustainability for human society at large. (By Anjan K Chatterjee, Materials & Process Consultant, Kolkata)

References
1.WBCSD-CSI and IEA, Technology roadmap GCo low-carbon technology for the Indian cement industry, 2012.
2.John Kline and Charles Kline, Cement and CO2: WhatGC?s happening ?, ZKG. 9, 2014.
3.Calera Corporation. The Science, 2013.
4. TecEco Pty Ltd, TecEco Cements, 2013.
5.Calix Ltd, http://www.calix.com.au/calix_overview.html
6.N. Vlasopoulos, Novacem carbon negative cement, SCI technical update, 25 November 2010. http://novacem.com/wp-content/uploads/2010/12/20101125-Technical-update.pdf
7.N. DeCristofaro and Sada Sahu, Exploring the chemical properties and performance results of sustainable Solidia cement and Solidia concrete, The Masterbuilder, February & March 2015.
8.Y. Yoshioka, D. Obata, N.Nanjo et al., New ecological concrete that reduces CO2 emissions below zero level, Energy Procedia, 37, 2013.

ABOUT THE AUTHOR
Dr Anjan K Chatterjee is an internationally familiar personality in the field of Cement, Concrete and Materials Science. He is an Ex. Director of ACC Ltd., had a stint with National Council of Building Materials and also with IIT Kharagpur. Presently he is associated with Pidilite Industry, besides being an advisor to several industrial and academic organisations within and outside the country. Academically he is a Materials Scientist with a Doctoral degree from the Moscow State University, Russia and carried out extensive research work in Baikov Institute of Metallurgy in Moscow as well as Building Research Establishment in UK. He is also a Fellow of the Indian National Academy of Engineering, Indian Concrete Institute and Indian Institute of Ceramics.