CECR: The cement industry is known to be a high emitter of CO2. How can it be reduced? What are the cement manufacturers doing to achieve low emission of CO2?
A.K Chatterjee: The carbon dioxide emission continues to remain a major concern for the Portland cement manufacturing process. The concern emanates from the release of about 535 kg CO2 per tonne of clinker from calcination of limestone and about 330 kg CO2 per tonne of clinker from combustion of fuel, resulting in direct emission of 835 kg CO2 per tonne of clinker. The corresponding figure for cement would vary, depending on the quantity of clinker used in making a tonne of cement and the grinding technology adopted. In this context, the industry at large has adopted the following key levers to reduce the CO2 emission level:
A. Clinker Making Stage
- Use of alternative non-carbonate calcium-rich raw materials.
- Enhancing use of alternative fuels in place of conventional coal. - Making unit operations in the total manufacturing process more energy efficient.
- Generating electricity with waste heat.
- Increasing the use of renewable energy.
B. Cement Making Stage
- Use of supplementary cementing materials in substitution of clinker in binary blended cements
- Manufacture of Portland ternary composite cements with even lower clinker factor than the binary blended cements.
With all the above measures taken and initiated by the industry, the present CO2 emission intensity is tentatively estimated at 0.7 t CO2/ t cement, which was about 0.72 in 2010 and 1.12 in 1996. Although this shows a significant reduction in CO2 emission during the last two decades, the above levers may not be adequate to meet the future targets committed and required for the climate change demand. It is projected that the 2020 target for CO2 emission should be 0.58t/t and it should go down further to 0.50t/t by 2050. These targets do not appear achievable with the traditional levers like energy efficiency, use of biogenic fuels and clinker substitution. Search for newer technological options for the low carbon pathways is essential for the cement industry. Globally, the following innovative research areas are receiving increasing attention:
- Reformulation of clinker composition
- Low carbon emitting manufacturing processes
- Carbon capture and use to produce carbonated cements and concrete
- Carbon capture and transformation into fuel.
The above areas are futuristic and have barriers of development from concept proving to becoming viable processes. While some countries have directed their resources quite substantially towards developing low carbon pathways, the Indian industry is yet to take up the challenge of low carbon process and product research in a mission mode. It is important to note that the Indian cement industry is not only second in terms of production volume in the world; it is also the most efficient in energy consumption as compared to the world average. Further, improvements in the energy and environmental parameters are expected through more extensive use of alternative fuels and raw materials, and manufacture of Portland
Dr. Anjan K Chatterjeecements with clinker factor of 0.50 or even less but still it is not enough, and it is time for the industry to chalk out a blue print of more robust and disruptive innovation and cooperative research on a national plane.
Former Wholetime Director,
ACC Limited, Mumbai &
Chairman, Conmat Technologies Pvt Ltd, Kolkata
CECR: How feasible is the production of low alkali Portland cement in India, as our code of practice specifies this type of cement for combating alkali aggregate reaction in concrete, especially in concrete dams? What about the alkali by-pass systems? Are such systems being used by the cement manufacturers?
A.K.C: It is true that in India the occurrence of reactive aggregates has been noticed more widely than known previously. In fact, some regions, which were taken for granted as sources of non-reactive aggregates, are now seen to have pockets of reactive aggregates. Aggregates can be reactive in most cases due to the presence of certain forms of silica which result in alkali silica reaction (ASR). The types of silica that can take part in ASR include strained varieties of quartz, tridymite, cristoballite and amorphous forms like opals, flints, cherts, etc. In certain other situations the aggregates can have a reactive form of dolomite that causes ‘aggregate carbonates reaction (ACR)’.
In ASR, hydroxide ions in the pore solution react with reactive silica present in the aggregates, resulting in internal stresses that can cause expansion and cracking. Failure may occur within days or only after years. The necessary conditions for ASR in Portland cement concrete are a sufficiently high content of alkali oxides in the cement, a reactive constituent in the aggregate and a supply of water. ASR is unlikely to occur in concrete made with Portland cements, if the content of equivalent Na2O (Na2O + 0.66 K2O) in the concrete is below 4 kg/m3 and a practical limit of 3 kg/m3 has generally been proposed to allow for day-to-day variations in cement composition. The alternative criterion based on cement composition (Na2O < 0.6%) does not allow for varying cement content in concrete. It should also be borne in mind that alkali cations may also be supplied from external sources including mineral admixtures or aggregates. It may also be relevant to mention that in great majority of cases, with use of composite cements with supplementary cementing materials like fly as or granulated slag; the ASR related expansion is sufficiently reduced to prevent failure of concrete. Hence, the entire ASR related concrete failure issue must be seen from various perspectives including the sources of input of alkalis other than cement. However, the provision in our codes and standards of testing the aggregate for its reactivity in large and critical projects where concrete is likely to be exposed to humid atmosphere or wetting action, is certainly an important safeguard but the recommendation of using low alkali cement is only one option for preventing concrete failure. Unlike ASR, in ACR the destructive expansion occurs in concretes made with some aggregates containing dolomite, which reacts with (OH) - ions in the so-called de dolomitization reaction, resulting in the formation of CaCO3 and Mg(OH)2 or brucite. The expansion is mainly caused by the growth of brucite crystals around the surfaces of the dolomite grains. In this type of expansion and failure of concrete, the alkali content is not directly responsible.
The manufacture of low alkali cement is obviously dependent on the alkali contents in the raw materials used in making clinker. In preheated kilns with suspension preheated cyclones and electrostatic separators, volatiles present in the kilns practically have no escape route and the circulating volatiles inside the kiln may often lead to entrapment of higher amounts of alkali/sulphur in the clinker. This is partially overcome by diverting a portion of the kiln exit gases with a bypass duct. A modern bypass system consists of an air quench chamber, a shut-off valve, a water-quench chamber and a separate dust collector. The installation of a bypass system leads to additional energy and material losses. As a thump rule one may indicate that with a 30% bypass system, the fuel consumption increases by about 15% and the material losses by about 7.5%. Technically speaking, in a preheated kiln provided with a pre - calcination stage it is possible to produce low-alkali clinker from high-alkali raw materials with more than 60% bypass, but its feasibility is doubtful in most circumstances due to additional energy consumption, problems of disposal of bypass dust and limited and sporadic market of low alkali cement. The Indian cement industry is fully familiar with the bypass technology and such facilities have been installed in a few plants for tackling chlorine and sulphur problems, not so much for alkalis. It must be understood that any bypass system is not selective for a single volatile component the exit gases carry all the different volatiles present in the system. Therefore, the degree of bypass required for different components is different. In a practical situation the bypass is decided upon for the most affecting components.
The above discourse is not intended to give an impression that the low alkali cement cannot or is not made in India. It is made in selected plants where there is availability of low alkali raw materials and where the economics are favourable. Quite a few dams in India have been constructed with indigenous low alkali cement. If required, it will be produced under appropriate conditions. What might be helpful in future planning is, to prepare an authentic demand projection for low alkali cement in the country.
CECR: The new BIS standard for composite cement has been issued. What are the prospects of this cement for its utilization in concrete construction?
A.K.C: It is certainly a very welcome development in India that we have embarked on ternary composite cement specification with the adoption of IS 16415:2015 with the following composition:
- Clinker/OPC (IS 16353): 35-65%
- Fly ash (IS 3812 Pt I): 15-35%
- Granulated slag (IS 12089): 20-50%.
It is important for two reasons – one, the clinker factor can be reduced to a level that perhaps cannot be achieved in only fly ash-based PPC; two, high particle packing density may be attained in concrete with durability characteristics that are not normally encountered. There is also a third advantage that the limitation of expanding the PSC production due to limited availability of granulated slag may be overcome with this ternary blend. Recognizing the likely benefits, the Indian cement industry is engaged in commercializing the product. The product is expected in the market any time now.
Notwithstanding these developments, it is important to note that the composite cement specification in its present form is limited in scope and options. The blending components are restricted to only fly ash and slag, the latter being available in specific regions and the quantity available is almost entirely tied up with different PSC producers. Hence, the exploitation of the opportunity to produce new composite cement is tilted in favour of properly located plants. Expanding the use of additional blending components like calcined clay and limestone powder must be considered in the revision of the standard, if wider adoption of multiblend composite cement technology must be achieved in the country.
Further, the present standard specification is still prescriptive and not performance oriented. There is a pressing need in the country to move towards ‘performance oriented’ specification of cement to make use of large variety of supplementary cementing materials. This only will pave the way for resource conservation, environmental protection and making concrete more durable. In addition to these aspects, one must recognize that the concept of multi-blend composite cement is based on the principle of high particle packing density, which is yet to sip into the production and application professionals. The success of new composite cements would depend on realizing the scientific and technological concepts behind the products.
CECR: Concrete code specifies PPC and PSC for aggressive environmental conditions, e.g., the underground construction. In the concrete piles and pile caps of bridges they should be used instead of OPC. But the construction companies and Metro authorities have preference for OPC. What are your views?
A.K.C: In simple terms, the concrete piles fall in two basic categories: precast and cast-in-situ. Precast piles can be further divided into two general classes: normally reinforced piles and pre-stressed piles. Castin- situ piles are subdivided into piles with casing and piles without casing. It is possible to have several variations of these basic types including variation of cross-sectional area and longitudinal shape. Depending on the foundation conditions and the type of concrete pile selected, the load carrying ability of the pile can be developed either in skin friction or point loading or a combination of the two. Concrete cast-in-situ piles and more particularly pre-stressed concrete piles can sustain high bending stresses and are frequently used in viaducts and trestle types of structures with the pile extending above ground or channel bottom level. From this very summary of pile types it is obvious that piling is a specialized and project-specific engineering activity of fairly complex nature and it may not be desirable to extend the conventional concept of choosing a particular type of cement, based on broad environmental considerations.
Durability of piles is generally ensured by the composition and density of concrete, use of sound and hard aggregates, and designing proper concrete cover over the reinforcing steel. Often the concrete mix is rich and proper care is taken in mixing, placing, consolidating and curing to achieve hard dense concrete. Piles can be protected against some of the agents of deterioration by use of coatings and jackets applied to vulnerable areas. Plain or reinforced concrete piles embedded in earth are generally considered not subject to deterioration. The water table, if free from deleterious substances, does not affect durability. In extremely infrequent situations there are possibilities of permeation and damage by ground water saturated with either acids or alkalis or salts. Dense rich concrete with sulphate-resisting cement is a means of minimizing the effect of a deleterious environment. It may also be kept in mind that for precast and pre-stressed types of piles, use of blended cements is not feasible.
Notwithstanding what has been stated above in the context of pile foundation, the benefits of using blended cements or supplementary cementing materials in concrete for placement in adverse environmental conditions cannot be ignored. The example of the construction of Confederation Bridge across the Northumberland Strait between New Brunswick and Prince Edward Island in Canada way back in mid-nineties may prove the point. To withstand a very aggressive marine environment there including freezing/thawing a concrete was designed with 480 kg/m3 total cementitious content, of which 10% was a class F fly ash and 6.75% was silica fume. The water-cementitious materials ratio was less than 0.30. The average 90-days strength of the concrete was 80 MPa against the structural requirement of 60 MPa with an air content of 6-8%. The above prescription of concrete served quite well in the aggressive marine environment. This example will perhaps bring forth the fact that the design of concrete is at the core of its durability in aggressive environments including those of the pile foundation. Use of blended cements is not ruled out but its application must be accompanied with material knowledge and good practices.
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