The Adani Group has announced the opening of its Rs 31,000 crore open offer to buy a further 26 per cent of the publicly traded Indian subsidiaries of Swiss company Holcim, ACC Ltd. and Ambuja Cements, from the public shareholders. The Adani Group stated in May of this year that it had reached an agreement to pay US$ 10.5 billion for a majority interest in Holcim Ltd.’s Indian assets. Last week, the open offer received approval from the market regulator Sebi.
Exploring New Secondary Cementitious Materials
Dr S B Hegde, Visiting Professor, Pennsylvania State University, United States of America, discusses innovations in supplementary cementitious materials in the face of the challenges faced by cement manufacturers to become more sustainable.
Due to rapidly expanding urbanisation, environmental sustainability in the construction industry is facing serious challenges. To put it into perspective, concrete preparation requires a significant quantity of nat ural reserves worldwide and necessitates the development of alternative materials and sources. The manufacturing of concrete needs around 27 billion tonnes of raw material inventory, representing 4 tonnes of concrete per person per year!
By 2050, concrete production will be four times higher than in 1990. Aggregates and cement represent around 60 per cent to 80 per cent and 10 per cent to 15 per cent of the total weight of concrete, respectively.
Along with processing a substantial quantity of aggregates and around 3.5 billion tonnes of cement per year, concrete generates approximately 5 per cent to 7 per cent of the global total carbon dioxide emissions.
By 2025, around 4 billion tonnes of carbon dioxide (approximately) are estimated to be released to the atmosphere during cement production. The possible solution for more sustainable production could be to explore and develop SOPs for utilising the locally available waste materials or recyclable materials. The abundance of these materials and their different chemistries and physics compel the development of a common strategy for their application in concrete production.
Numerous industrial solid by-products containing calcareous siliceous, and aluminium materials (fly ash, ultrafine fly ash, silica fume, etc.), along with some natural pozzolanic materials (volcanic tuffs, diatomaceous earth, sugarcane bagasse ash, palm oil fuel ash, rice husk ash, mine tailings, etc.) can be used as SCM.
Sewage sludge ash (SSA) is an urban waste that may be used as fertiliser, as well as a cement substitute. SSA was not only considered as SCM in blended cements but also in a large scale of building materials like pave-stones, tiles, bricks, light aggregates production.
Marble dust, too, could be explored as one of the SCM. Marble is a finely crystallised metamorphic rock originating from the low-intensity metamorphism of calcareous and dolomitic rocks. Calcium carbonate (CaCO3) can form up to 99 per cent of the total amount of this carbonated rock. Additional phases may also include SiO2, MgO, Fe2O3, Al2O3 and Na2O and, in minor ratio, MnO, K2O, P2O5, F, Cu, S, Pb and Zn.
Construction and demolition debris (CDD) constitute one of the massive flows of solid waste generated from municipal and commercial activities of the modern era. Usually, CDD are in the shape of brick bats, mortars, aggregates, concrete, glass, ceramic tiles, metals and even plastics. The review of these new SCM for life cycle is very much imperative and will mention whether it will be environmentally feasible to apply the SCM for the life cycle of concrete.
Supplementary Cementitious Materials
Supplementary Cementitious Materials (SCM) play a significant role in performance of concrete specially to impart additional durability potential. They encompass a wide spectrum of aluminum-siliceous materials, including natural or processed pozzolans and industrial by-products like ground granulated blast furnace slag (GGBS), fly ash (FA), ultra-fine fly ash (UFFA) and silica fume (SF). Though there is higher fluctuation both in properties and chemistry across the various types of SCM, they share in common capacity to react chemically in concrete and form cementitious binders replacing those obtained by OPC hydration. The key feature of SCM is their pozzolanicity, i.e., their capability to react with calcium hydroxide (portlandite, CH) aqueous solutions to form calcium silicate hydrate (C–S–H).
In the right proportion, SCM can improve the fresh and hardened properties of concrete, especially the long-term durability.
Rice Husk Ash (RHA): An agricultural by-product that is suitable for cement replacement in rice-growing regions is Rice Husk Ash. Various research investigations have demonstrated that the principal chemical composition of rice husk ash consists of biomass-driven silicon dioxide (SiO2) as a result that the nature of silica in rice husk ash is sensitive to processing conditions. The ash obtained through open-field burning or uncontrolled combustion in furnaces generally includes a high percentage of crystalline silica minerals, like tridymite or cristobalite, with inferior reactivity. The highest amount of amorphous silica is obtained when RHA is burnt at temperatures ranging from 500°C to 700°C. The superior reactivity of RHA is due to its large amount of amorphous silica, which has high surface area due to the porous architecture of the host material. RHA can be used as a substitute in Portland cement (acceptable up to 15 per cent), thanks to its pozzolanic activity. Fine RHA can increase the compressive strength of cement paste and can lead to preparation of mortars with low porosity.
As a cement substitute, the usage of RHA in concrete production has advantages and disadvantages. Improved compressive strength of concrete is one of the essential advantages of using RHA as a substitute. Recent studies have highlighted important benefits of replacing cement with RHA in small percentages. In the context of durability, the use of RHA as a substitute in concrete production can lead to notable improved water permeability resistance, Cl penetration and sulphate deterioration.
Sugar cane bagasse: Sugarcane bagasse ash (SBA) is a by-product of producing juice from sugar cane by crushing the stalks of the plants. The addition of SBA in concrete production can decrease the hydration temperature up to 33 per cent, when 30 per cent of OPC is substituted by SBA. Also, water permeability considerably decreases when compared to control concrete samples. With the aim of superior compressive strength, OPC was substituted in the range from 15 per cent to 30 per cent. SBA incorporation has improved concrete durability.
Other wastes: Wastes of different sources have been investigated for their possibility in re-use, to reduce their environmental impact, in landfill volume and decomposition by-products. Sewage sludge ash (SSA) is an urban waste that may be used as fertiliser, as well as a cement substitute. SSA was not only considered as SCM in blended cements but also in a large scale of building materials like pave-stones, tiles, bricks, light aggregates production. The impact of SSA in mortar was a decrease in the compressive strength, when SSA was used as a partial cement substitute. Therefore, use of SSA as an SCM was shown to be limited, in the construction industry. The cement community does not include SSA in the group of pozzolanic materials.
Palm oil fuel ash (POFA): Palm oil is an important cash-crop in tropical countries, especially in Malaysia and Indonesia. For every 100 t of fresh fruit bunches handled, there will be about 20t of nut shells, 7t of fibres and 25t of empty bunches released from the mills. POFA can be used in concrete either as aggregates, SCM or as filler material. Comparable to RHA and SBA, the amorphous SiO2 (around 76 per cent) content of POFA offers relatively high pozzolanic activity, when used as binder in concrete production. Even though a few performance parameters of concrete (especially setting time and strength) are negatively influenced by POFA, several studies claimed that palm oil fuel ash may be appropriate in different applications.
Mining wastes: The quantity of mine wastes has increased hugely due to increasing demand for metal and mineral resources. Mining wastes are produced during mineral extraction by the mining industry and is at present one of the largest waste available worldwide.
At present, they are being used mainly as backfilling both in open cast mines and underground areas. They pose potential long-term risks for environmental pollution. However, use of tailings is not only relevant to environmental conservation, but can also benefit the mining industry. These solid wastes contain compounds with potential pozzolanic properties and can decrease the amount of cement used to produce concrete, reducing simultaneously the ecological impact of the cement and mining industries. An additional benefit of mine tailings is that they are already finely ground. Most of the other SCM require mechanical grinding, as a pre-treatment for use, to improve their reactivity.
Marble powder: Marble is a finely crystallised metamorphic rock of calcareous and dolomitic rocks. Calcium carbonate (CaCO3) can form up to 99 per cent of the total amount of this carbonated rock. Additional phases may also include SiO2, MgO, Fe2O3, Al2O3 and Na2O and, in minor ratio, MnO, K2O, P2O5, F, Cu, S, Pb and Zn.
Through the shaping, sawing and polishing operations, around 20 per cent to 25 per cent of processed marble is converted into powder or lumps. As a result, dumps of marble dust have become an important environmental issue worldwide. Marble powder (MP) has successfully been demonstrated as a viable SCM in self-compacting concrete (SCC). The research proved that marble powder used as a mineral substitute of cement can enhance some properties of fresh concrete and/or hardened concrete.
In the cement-related literature, there are just a few research studies related to the application of marble powder in concrete or mortar production. Thus, more detailed studies are needed in order to define the properties of concrete or mortars with marble powder. The use of marble powder in ternary cementitious blends demands further caution to remove or reduce its adverse effects on the fresh properties of self-compacting concrete and/or mortar.
Construction and demolition debris (CDD): CDD constitute huge solid waste generated from municipal and commercial activities of modern urban styles. Usually, CDD are in the shape of brick bats, mortars, aggregates, concrete, glass, ceramic tiles, metals and even plastics. They must be mechanically sorted according to size and quality level. They are then crushed down to desired size.
There is a need to study the ‘life cycle’ of construction materials to develop a global understanding of sustainable building construction and the feasible use of CDD as SCM for OPC replacement materials.
The materials like low grade/marginal grade limestone, red mud, bio wastes including vegetative wastes calcined under controlled conditions are some examples of potential SCM in future.
Concrete is one of the most widely used materials after water worldwide by volume. Portland cement production is highly energy intensive, and emits significant amounts of CO2 through the calcination process, which contributes substantial adverse impact on global warming. Efforts are needed to produce more ecologically friendly concrete with improved performance and durability.
The conventional SCM are not enough considering the quantity of concrete requirement for infra development world wide and to mitigate global warming issue; there is a pressing need to explore the new SCM, its characterisation, performance evaluation, standardisation and adoption.
However, it is clear that more research is needed to assess the feasibility of long-term performance and to develop a more ecologically sound production SOPs, in addition to quality assessment of these materials.
It is envisaged that introducing new cementitious materials in cement and concrete manufacturing is a time consuming process. Not only from the viewpoints of plants but from standards or codes issuing bodies like Bureau of Indian Standards (BIS) in India, ASTM, EN Standard organisations plus local nodal agencies of the particular countries. Many researches have been done in Universities, and other R&D institutions but issuing relevant codes (specifications) by these organisations for commercial usage is utmost important.
About the author:
DrS B Hegde is a Winner of Global Visionary Award for notable contribution to Cement and currently Visiting Professor, Pennsylvania State University, United States of America. Dr Hegde has more than 30 years of experience in the cement industry both in India and abroad.
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By 2050, concrete production will be four times higher than in 1990. Aggregates and cement represent around 60 per cent to 80 per cent and 10 per cent to 15 per cent of the total weight of concrete, respectively.
The main task in cement production is improving sustainability
Prakhar Shrivastava, Head – Corporate Quality, JK Cement Limited, discusses the smart use of supplementary cementitious materials to improve cement production and make cement manufacturing more integral to a circular economy.
What are supplementary cementitious materials? Tell us more about their nature
Supplementary Cementitious Materials (SCM) are materials that are obtained from other industrial waste as by-product and none have their own/individually hardened properties but contribute by grinding with clinker or blending with Ordinary Portland Cement (OPC) through hydraulic and/or pozzolanic activity. These waste products are used as supplementary cementitious materials so that the maximum utilisation of wastes is possible. SCM play a significant role in increasing the workability of the product and enhance the serviceability or durability, thus, decreasing the permeability, aiding in pumpability and finishability.
Typical SCM are flyash, slag, silica fume, natural ashes, rice husk ash, burnt shale, metakaolinite, calcined clay and natural pozzolana i.e., volcanic glass, etc. Among them, flyash and slag are widely used by cement industries for production of PPC and PSC.
Flyash or pulverised fuel ash is formed during combustion of coal from coal-fired electric and steam generating plants and obtained by electrostatic or mechanical precipitation of dust like particles from the flue gases. Earlier, it was recognised as an industrial waste but now has become an important industrial by-product.
Steel slag, a by-product of steel industries, formerly referred to as ground, granulated blast-furnace slag, is a glassy, granular material formed when molten, iron blast-furnace slag is rapidly chilled – typically by water sprays or immersion in water – and subsequently ground to cement fineness.
Tell us about the supplementary cementitious materials and their composition used by your organisation?
Supplementary cementitious materials are soluble siliceous, alumina-siliceous or calcium alumina-siliceous powders used as partial replacements of clinker in cements or as partial replacements of portland cement in concrete mixtures.
At JK Cement, we manufacture Portland Pozzolana Cement (PPC) from all our plants with addition of flyash up to 35 per cent and PPC in premium category with 20 per cent flyash to promote usage of only blended cement to fulfil customer requirements by achieving equivalent strength properties of OPC (Ordinary Portland Cement). At our south India plant in Muddapur, we also manufacture Portland Slag Cement (PSC) with the addition of slag at approximately 65 per cent, meeting all the internal product quality norms.
In our plants, flyash is sourced from different thermal power plants in accordance to the quality, cost and suitability criteria of the plants. Similarly, slag is sourced from steel plants located in Karnataka and Goa. The typical chemical composition and quality requirements as per Indian standards of flyash and slag are mentioned in the table:
Does the use of supplementary cementitious materials impact the process of cement manufacturing?
Impact of SCM can be categorised in two aspects i.e., challenges and benefits. Below are the few challenges faced during the process of cement manufacturing.
- Major SCM are available across the country, such as, dry flyash and pond ash; however, less availability of dry flyash directly connected with thermal power plants (TPP) operation.
- Though there is abundance of pond ash, the major concern in its usage is the high moisture content and coarser size, which creates constraint of jamming, leading to lower production, higher power consumption, blended cement quality and slower production.
- Additional feeding systems are required.
- Challenges of further grinding of abrasive/harder to grind materials such as coarser pond ash, GGBS, copper slag.
- It may increase the cost of the product especially where some SCM are more expensive than cement. i.e., the availability of SCM.
- SCM used for the clinkerisation process required high grade limestone to maintain the desired quality of clinker which affects the mine life.
What are the key advantages of using supplementary cementitious materials in the cement manufacturing process?
The key advantages of using supplementary cementitious materials are:
- Increased clinker substitution; reduces CO2 emission per ton of cement production.
- Reduces use of fossil fuel per ton of cement production.
- Increases the life of limestone mines.
- Reduces consumption of thermal and electrical energy.
- Reduces water consumption.
- Reduces generation of garbage materials at the location, which in turn leads to clean India.
How does the use of supplementary materials increase the profitability of the cement manufacturing for your organisation?
SCM play a vital role in increasing the profitability of the cement manufacturing; with the addition of SCM during cement production, it enhances the overall cement capacity. All our plants are using SCM which are available nearby to plant location. We are investing a lot at locations where SCM are available at a lower cost value and hence reducing the overall cost of cement as compared to clinker cost. Also, these SCM help in reducing the power consumption per ton of cement due to increase in cement volume. Another benefit is the increased cement volume that results in intangible benefit by increasing limestone mine life and conserving natural resources of compendious materials.
Tell us about the quality standards and checks implemented for the final product made using supplementary materials.
The Indian standards have been framed to define the quality of SCM by BIS. Each SCM has a specific Indian standard with specific quality norms like for pulverised fuel ash (IS 3812 Part-1), slag (IS 12089), calcined clay pozzolana (IS: 1344-1981 (Part-II) etc. According to IS specification; internal quality standards have been specified to monitor the SCM quality and these quality specifications are specified in the purchase order for vendor reference. A structured and systematic approach is made to check the SCM quality by the quality control department and all test results are recorded in SIT formats.
In order to make different grade products following checks have been implemented
- Has established a distinct location/yard/silo for proper storage of SCM and to avoid contamination.
- Different hoppers are assigned for each type of material storage and to introduce during the manufacturing process.
- For controlled and calculated addition; weigh feeders are installed.
- For each process or step, quality norms have defined and organised the monitoring and testing in stipulated frequency as per IS requirement.
- Prior to dispatch and release of product in market or to customer the prescribed quality testing performed for quality reassurance.
Tell us about the role of technology in deciding the proportions of supplementary cementitious materials.
Today, the main task in cement production is improving sustainability by reducing emissions. This is achieved by promoting the use of green fuels that lower the conventional fuel consumption and by utilising the alternative raw materials i.e. SCM while producing reliable products at a competitive cost for the construction industry. Less clinker and more SCM is the challenge for the cement industry. The control and optimisation of clinker and cement reactivity is one important key to reach these targets. A problem today is that clinker and cement reactivity are not quantified at cement plants, except by slow and indirect methods like compressive strength testing.
XRF and XRD studies are valuable to understand the composition. However, quantitative XRD does not directly assess the reactivity of SCM. Recently isothermal heat flow calorimetry techniques have been suggested as a new analytical tool for process control and deciding the proportion of SCM in cement.
Recently, the beneficiation or processing of flyash has become hugely important. Flyash Beneficiation Technology or process converts waste from coal-fired power stations (pulverised fuel ash or flyash) by separating the constituent minerals to generate a range of sustainable, environment-friendly products with unique physical and chemical characteristics.
What are the major challenges you face while using supplementary materials for cement manufacturing?
The major concern is availability in terms of quality and quantity; the second factor is cost because the overall cost depends on the distance between the generation unit to the cement manufacturing plant which eventually impacts the cost of cement.
Constantly the SCM demand is increasing and the availability of good quality SCM is very limited and on high cost, the high moisture content of slag and pond ash creates operational challenges. The quality of SCM, largely influenced by the existence of high quartz, heavy metals, alkalis and the fineness that determine the quality of cement. Indian flyash is more crystalline compared to what is generated in other countries and the ratio of formers (SiO2,+Al2O3+Fe2O3) to network modifiers (Na2O+K2O+CaO+MgO) in the Indian flyash is very high and imbalanced.
Depending on the source of coal that varies from mine to mine impacts the composition of flyash like bituminous coals, sub-bituminous and lignite coal determine the flyash colour, fineness and other radicals. Among all SCM, flyash is mostly used in cement plants and as thermal power plants (TPP) are the source of flyash, the present availability of coal and its high cost is a major concern for TPP operations that is affecting the flyash generation. The availability and sources of slag in India are limited, which are affecting its usage in blended cement. Except for flyash and slag, other SCM availability is very less and not too economical.
How does the use of cement made of supplementary materials impact its carbon footprint?
We have committed to achieving our SBTi goal by cutting our GHG emissions according to climate science and as a Global Member of GCCA, by pledging for UNFCCC’s ‘Race to Zero Campaign’ to achieve Net Zero Carbon by 2050.
Clinker manufacturing is responsible for 80 per cent of the carbon emissions and supplementary cementitious materials reduce the clinker content in cement to a great extent without compromising the quality of the product. JK Cement’s green vision is to deliver a sustainable product to meet the stakeholder’s demands while taking several measures that can reduce CO2 emissions in the clinker manufacturing process. This can be achieved by using different types of alternative fuels, RDF/MSW, biomass fuels etc. and various industrial waste such as raw mix components like red mud, GCP dust, iron sludge, zinc slag etc.
Supplementary cementitious materials such as flyash, slag, waste gypsum and industrial waste are the crucial components of JK Cement’s business strategies for conservation of the mineral resources which enables us to produce sustainable construction materials in terms of low embodied carbon at a competitive cost. This has transformed our operations by setting up a benchmark for achieving the best sustainable business practices in the industries and producing Green Certified Cement.
Tell us about the impact of cement made with supplementary materials on the construction and allied industries.
As the construction sector is incessantly challenged by the growing societal demands for safer and cost-effective infrastructures, more and more environment-friendly products and processes must be developed and adopted into our industrial practice. Although supplementary cementitious materials are one of the most used construction materials worldwide, there are still some major concerns about their sustainability and durability.
Firstly, the production of concrete is releasing large volumes of carbon dioxide into the atmosphere, one of the greenhouse gases attributable to
climate change. Secondly, even though cementitious materials are very versatile and robust they may suffer from various deteriorative processes, leading to shortened service life, and consequently, intrusive or expensive costs for maintenance and repair.
To meet the expectations of consumers, demanding more durable, less labour and service intensive materials at a competitive price, numerous new composite materials and technologies have been developed over the last couple of decades including blended cements with Supplementary Cementitious Materials (SCM).
Some of the positive impacts are summarised as follows:
- The use of supplementary cementitious materials in construction not only improves the mechanical property of cement matrix but also reduces its impacts on the environment.
- Blended cement helps to reduce the damage to the concrete from alkali-silica reaction and provides higher resistance to chloride ingress thus reducing the risk of reinforcement corrosion.
- Mitigating sulphate phase formation, which takes place when sulphates found in seawater and some soils react with tricalcium aluminate in concrete.
- Some of the allied industries have started making limestone bricks, AAC blocks, hollow blocks, flyash bricks which are not only considered as green products but also reduce the cost of construction works.
How do you foresee the future of the global cement industry in terms of using alternative materials for cement manufacturing and running the race of decarbonisation?
The production of Ordinary Portland Cement (OPC) is continuously declining, with a simultaneous increase in the production of blended cement like PPC, PSC, and Composite Cement based on flyash and granulated blast furnace slag. SCM are increasingly used to minimise cement-related CO2 emissions and increase plant efficiency from an economic and environmental perspective.
At present, blended cements have a greater share (73 per cent) in comparison to ordinary portland cement (27 per cent). Other cement formulations such as Portland Limestone Cement (PLC) and Limestone Calcined Clay Cement (LC3) are also at different stages of development in India.
In recent years, globally and in India several research has been conducted for the development of environment-friendly and less CO2 emission cement i.e., Calcium Sulfo-Aluminate Cement, Reactive Belite Cement, Alkali Activated Cement etc., that is found to be more energy-saving, less carbon intensive and optimises waste-utilisation. Further studies were carried out on carbon capture storage and usage, zero emission mining, oxyfuel combustion in kiln etc. If these solutions become economically viable, they may contribute to a considerable reduction in CO2 output from the cement industry.
Environmental concerns and depleting natural resources, and the impact of cement production on the two are imminent issues that cement companies need to address on priority. Supplementary cementitious materials procured from industrial wastes is one way of looking at this colossal problem. ICR examines the changes made in company protocol with regards to sourcing of alternative materials and their overall impact.
Before we dive into the subject of supplementary cementitious materials, let us look at some of the key facts about cement production. India is the second largest producer of cement in the world. Limestone is at the core of its production as it is the prime raw material used for production. The process of making cement involves extraction of this limestone from its quarries, crushing and processing it at the cement plant under extreme temperatures for calcination to form what is called a clinker (a mixture of raw materials like limestone, silica, iron ore, fly ash etc.). This clinker is then cooled down and is ground to a fine powder and mixed with gypsum or other additives to make the final product – cement. The reason we are elucidating the cement production process is to look at how supplementary cementitious materials or SCM can be incorporated into it to make the process not only more cost effective but also environmentally responsible.
Limestone is a sedimentary rock composed typically of calcium carbonate (calcite) or the double carbonate of calcium and magnesium (dolomite). It is commonly composed of tiny fossils, shell fragments and other fossilised debris. This sediment is usually available in grey colour, but it may also be white, yellow or brown. It is a soft rock and is easily scratched. It will effervesce readily in any common acid. This naturally occurring deposit is depleting from the environment due to its extensive use in cement manufacturing process. Its extraction is the cause of dust pollution as well as some erosion in the nearby areas.
The process of calcination while manufacturing cement is a major contributor to carbon emission in the environment. This gives rise to the need of using alternative raw materials to the cement making process. The industry is advancing in its production swiftly to meet the needs of development happening across the nation.
According to the India Brand Equity Foundation (IBEF), the cement demand in India is estimated to touch 419.92 MT by FY 2027. As India has a high quantity and quality of limestone deposits through-out the country, the cement industry promises huge potential for growth. India has a total of 210 large cement plants out of which 77 are in the states of Andhra Pradesh, Rajasthan, and Tamil Nadu. Nearly 33 per cent of India’s cement production capacity is based in South India, 22 per cent in North India, 13 per cent in Central and West India, and the remaining 19 per cent is based in East India. As per Crisil Ratings, the Indian cement industry is likely to add approximately 80 million tonnes (MT) capacity by FY24, the highest since the last 10 years, driven by increasing spending on housing and infrastructure activities.
The Indian cement production overall stood at 263.12 million tonnes in 2021, and it is expected to reach 404.11 million tonnes by 2029 with a CAGR of 5.51 per cent during the forecast period, suggests a report published by Maximize Market Research in September 2022.
The production capacity and demand of cement in the country is increasing and is expected to grow at a steady rate in the years to come. The country is moving towards urbanisation and is building projects for the development of the nation. However, it is also imperative that the industry holds accountability of the environment and emission from this production activity and creates sustainable solutions to meet the demands as well as safeguard the planet as well.
India has pledged to achieve Net Zero by 2070 at the Glasgow Climate summits.
Environmental concerns and depleting natural resources are edging the cement industry to look at alternative materials for their manufacturing process.
Composition and Impact of SCM
Cement manufacturers know that to reduce CO2 emissions in the process of cement making, it is essential to change its composition. The raw mix of approximately 90 per cent limestone should be substituted with other materials with similar properties.
These materials, known as supplementary cementitious materials contribute to the properties of hardened concrete through hydraulic or pozzolanic activity. Typical examples are fly ashes, slag cement (ground, granulated blast-furnace slag), silica fumes etc. These can be used individually with portland or blended cement or in different combinations. SCM are often added to concrete to make concrete mixtures more economical, reduce permeability, increase strength, or influence other concrete properties. SCM may be added during cement manufacturing for a more consistent blended cement.
Some of the commonly used supplementary cementitious materials are:
Fly Ash: This material contains a substantial amount of silicone dioxide and calcium oxide. It is a fine, light, glassy residue, most widely used SCM in concrete and is a byproduct of coal combustion in electric power generating plants. Fly ash can compensate for fine materials that may be lacking in sand quantities and can be very beneficial
in improving the flowability and finishability of concrete mixtures.
Ground Granulated Blast-furnace Slag (GGBS): It is a by-product of the iron and steel industry. In the blast furnace, slag floats to the top of the iron and is removed. GGBS is produced through quenching the molten slag in water and then grinding it into a fine powder. Chemically it is like, but less reactive than, Portland cement.
Silica Fume: It is a by-product from the manufacture of silicon. It is an extremely fine powder (as fine as smoke) and therefore it is used in concrete production in either a densified or slurry form.
Slag: It is a by-product of the production of iron and steel in blast furnaces. The benefits of the partial substitution of slag for cement are improved durability, reduction of life-cycle costs, lower maintenance costs, and greater concrete sustainability. The molten slag is cooled in water and then ground into a fine powder.
Limestone Fines: These can be added in a proportion of 6 to 10 per cent as a constituent to produce cement. The advantages of using these fines are reduced energy consumption and reduced CO2 emissions.
Gypsum: A useful binding material, commonly known as the Plaster of Paris (POP), it requires a temperature of about 150oC to convert itself into a binding material. Retarded plaster of Paris can be used on its own or mixed with up to three parts of clean, sharp sand. Hydrated lime can be added to increase its strength and water resistance.
Cement Kiln Dust: Kilns are the location where clinkerisation takes place. It leaves behind dust that contains raw feed, partially calcined feed and clinker dust, free lime, alkali sulphate salts, and other volatile compounds. After the alkalis are removed, the cement kiln dust can be blended with clinker to produce acceptable cement.
Pozzolanas: These materials are not necessarily cementitious. However, they can combine chemically with lime in the presence of water to form a strong cementing material. They can include – volcanic ash, power station fly ash, burnt clays, ash from burnt plant materials or siliceous earth materials.
SCM used in conjunction with Portland cement contribute beneficially to the properties of concrete through hydraulic or pozzolanic activity or both. Hydraulic materials (e.g., slag cement), like Portland cement itself, will set and harden when mixed with water. Pozzolanic materials require a source of calcium hydroxide (CH) to set, which is supplied by Portland cement during the hydration process. The right dosage of strategically chosen SCM can improve both the fresh and hardened properties of a concrete mixture.
Prakhar Shrivastava, Head – Corporate Quality, JK Cement Limited, says, “We manufacture Portland Pozzolana Cement (PPC) from all our plants with addition of flyash up to 35 per cent and PPC in premium category with 20 per cent flyash to promote usage of only blended cement to fulfil customer requirements by achieving equivalent strength properties of Ordinary Portland Cement (OPC). At our south India plant in Muddapur, we also manufacture Portland Slag Cement (PSC) with the addition of slag at approximately 65 per cent, meeting all the internal product quality norms.”
“The production of Ordinary Portland Cement (OPC) is continuously declining, with a simultaneous increase in the production of blended cement like PPC, PSC, and Composite Cement based on flyash and granulated blast furnace slag. SCMs are increasingly used to minimise cement-related CO2 emissions and increase plant efficiency from an economic and environmental perspective,” he adds.
Achieving Sustainability through Substitution
Cement is the most used man-made material globally. The rising demand for infrastructure and development of the nation is showing a clear indication of increased production of cement, thus raising concerns about natural resources, environment, and emission of carbon. One of the widely adopted solutions for ensuring sustainability in cement manufacturing is reducing the clinker-to-cement ratio by adding supplementary cementitious materials.
In his authored article, Dr S B Hegde, Visiting Professor, Pennsylvania State University, United States of America, states, “Concrete is one of the most widely used materials after water worldwide by volume. Portland cement production is highly energy intensive, and emits significant amounts of CO2 through the calcination process, which contributes substantial adverse impact on global warming. Efforts are needed to produce more ecologically friendly concrete with improved performance and durability.”
“The conventional SCM are not enough considering the quantity of concrete requirement for infra development worldwide and to mitigate global warming issue; there is a pressing need to explore the new SCM, its characterisation, performance evaluation, standardisation and adoption,” he adds.
The CO2 emissions from cement production are the third largest source of difficult-to-eliminate emissions, after load-following electricity and iron and steel. Beyond greenhouse gas (GHG) emissions, the production of concrete and mortar causes over approximately three per cent of global energy demand, over five per cent of global anthropogenic particulate matter (PM10) emissions, and approximately two per cent of global water withdrawals. These environmental impacts may be reduced through various technical (energy, emissions, and material efficiency) measures, of which cementitious materials (CM) substitution (including complete and partial substitution) is one of the most promising.
The manufacturing process of cement can become sustainable by measuring the impact of supplementary materials that can be added to the raw meal of cement. Various materials, naturally occurring or man-made or wastes should be studied and consequently should be included in the cement production process to create blended cements that not only meet the rising demands of the world in terms of quality and strength, but at the same time meet environmental concerns. Research, innovation and technology is key to making a difference in the segment of cement manufacturing by studying more materials that can be used as supplementary materials in cement and concrete, by crafting new compositions and blends of cement and crafting equipment that support the same.
One of the most important ways of reducing carbon emission in cement manufacturing is the use of alternative raw materials from various other industries. This gives way to a circular economy, utilising waste from other industries and bettering the environment with reduced emission of harmful gases, especially carbon dioxide. It also helps the avoidance of landfills or ocean pollution, as waste of industries is utilised in manufacturing cement. Overall, new compositions of cement are the future. The nation’s economy can greatly benefit from a growing cement industry and business sector, however, it should pay keen attention on finding pathways to safeguard the environment its people reside in.