Cement is one of the most important building materials worldwide. It is used mainly for the production of concrete. Concrete is a mixture of inert mineral aggregates, e.g. sand, gravel, crushed stones, and cement. Cement consumption and production is closely related to construction activity and, therefore, to the general economic activity. Because of the importance of cement as a construction material, and because of the geographic abundance of the main raw materials, cement is produced in almost all countries. The widespread production is also due to the relatively low price and high density of cement that, in turn, limits ground transportation because of high transport costs. Cement production is a highly energy-intensive production process. Energy consumption by the cement industry is estimated at about 2% of the global primary energy consumption, or almost 5% of the total global industrial energy consumption.
Because of the dominant use of carbon-intensive fuels, such as coal in clinker making, the cement industry is a major source of CO2 emissions. Besides energy consumption, the clinker-making process also emits CO2 from the calcining process. Because of both emission sources, and because of the emissions from electricity production, the cement industry is a major source of carbon emissions and deserves attention in the assessment of carbon emission-reduction options.
Hence we need to restrict the use of such a material and try to shift to another more greener sustainable one. If we look in the history, vernacular materials have proved to be the best and environmental friendly. But with changing demands of faster life they have failed and need modification. Also new contemporary materials have come to replace them. The term sustainability once rare to find in dictionary has in the last few years begun to appear more regularly. So,
What are the possible ways to minimize the use of cement which helps for a sustainable future?
1.2 NEED IDENTIFICATION
Environmental issues are growing in importance in especially the developing countries like ours. However growth expectation in such economies are still high, particularly to accommodate unsolved social problems and consequently the introduction of environmental concerns in policymaking is becoming more complex. Also government intervention is inevitably faced with the challenges of balanced costs and benefits among contemporary future generations to justify policy actions in a way which maximizes social welfare. As a part of the society and also as a contributor to the environmental crisis, it is also our responsibility now to understand the situation and start reacting to it. The construction technology is being developed by the architects, artists, builders and the owners. Building with locally derived, unprocessed materials, materials as simple as the soil beneath our feet, is a natural response to this crisis. It significantly reduces the amount of energy and secondary resources needed for extracting, processing and transportation. Many developed countries have started to adopted methods to tackle the issue. If non-industrialised building materials can be respected in this most industrialized of countries, vernacular methods stand a chance of being valued in our country as well. But being a developing country, you need metro and mega cities and they cannot be build using vernacular materials. There we need new contemporary materials which help us lay foundations for a better future.
1.3 SCOPE OF STUDY
As the subject is very vast and spans basically to entire foundation of architecture and engineering, which is beyond the scope of this kind of research. I would restrict myself to the study of materials which independently can replace cement in low floor structure and mineral admixtures in concrete to reduce the proportion of cement. Also my study of new materials is from environmental point of view hence there are few materials which were commonly used for construction but now have come to a situation where they need to be conserved e.g. Wood. Hence I have not included such materials in my study.
1.4 LIMITATIONS
As it was not possible to collect primary data on some issues, reliance on lot of secondary data is there. The technical evaluation of the materials is not possible due to time limitations. As many of the construction techniques are not practised in India, the applications of such materials are taken from international platforms.
1.5 METHODOLOGY
In this dissertation I will review the role of the cement in global CO2 emissions. Then I will describe the cement production process and the main emission sources. This is followed by an overview on why/how to provide a new building context. Finally, provide a brief review of the new opportunities for finding low-cost, alternatives to concrete, concrete reduction or concrete admixing/re-use within the present real world context. This is divided into two categories, first, the direct replacements which can be used directly in place of cement and second, are the additive mixtures which are to reduce the proportion of cement and hence reducing the use of cement.
The method of the study can be followed broadly under the heads of:
Understanding need for the study
Literature Survey
References
Case studies
Discussions
CHAPTER: 2 CLIMATE CHANGE AND CEMENT INDUSTRY
2.1 THE CLIMATE CHANGE
In Devotions upon Emergent Occasions, the seventeenth century English metaphysical poet John Donne wrote, "No man is an Island, entire of itself." Through this statement, Donne asserted that we all share a common humanity. In today's increasingly complex and interrelated world, not only is a man an island but, similarly, no building stands alone. Every building exists within an environmental context upon which it not only acts but which also has an impact upon the building. Due to today's increased complexities and interrelatedness, no building can be constructed as a microcosm. The people in charge of building project must consider the impact it will have on the environment into which it will be placed, locally and globally. Donne's assertion that no man is an island is also an affirmation of sustainability. Sustainability is commonly interpreted to mean living in such a way as to meet the needs of the present without compromising the ability of future generations to meet the needs of the future.( Spiegel & Meadows, second edition)
The first thing to realise about the Earth's climate is that it is enormously variable, not only from place to place today, but also over a number of timescales. For example, the climate in the early history of the Earth was radically different to that of today, partly due to the different luminosity of the Sun, and partly due to large-scale variations in oxygen and carbon dioxide contents of the atmosphere. Within the last million years, there have been seven major glaciations, spaced at roughly 100,000 year intervals. We are currently in a relatively warm interglacial period, and calculations of the orbit of the Earth suggest that a variety of factors will combine to create another ice age in 5000-7000 years. Within the last thousand years there have been smaller but still significant variations. It has now been recognised that there has been a gradual warming of the atmosphere in the last 140 years (Figure1).
The Intergovernmental Panel on Climate Change, the most respected of all bodies investigating climate change, and one which represents a broad consensus among the world's scientists, has recently published its latest report on climate change.
Among its most significant findings are the following:
• "The global average surface temperature has increased over the 20th century by about 0.6°C"
• "Temperatures have risen during the past four decades in the lowest 8km of the atmosphere"
• "Snow cover and ice extent have decreased"
• "Average sea level has risen and ocean heat content has increased"
• "Changes have also occurred in other important aspects of climate"
• "Some important aspects of climate appear not to have changed".
Question we might usefully ask ourselves include, 'Why is it happening?' and 'Is it anything to do with us?'
2.2 CLIMATE CHANGE FACTORS
There are a variety of factors which influence climate on both long and short time scales. For example, volcanoes emit CO2, sulphur compounds and dust during eruptions. Sulphur and dust emitted during the eruptions tend to form aerosols which
block solar radiation from reaching the Earth's surface, and which therefore tend to produce a cooling effect. Carbon dioxide, however, is a so called greenhouse gas. It tends to block radiation from the earth's atmosphere, and therefore tends to produce a warming effect. Variations in the orbit of the earth, including its eccentricity, the precession of the Earth's angle of rotation and variations in its angle of tilt, all have a role to play in determining the amount of solar energy which falls on a particular spot on the earth's surface, as well as on the total amount of energy which is received by the Earth as a whole. In addition, variations in solar activity (such as that experienced during the regular 11-year sunspot cycle, as well as longer-term variations in total solar luminosity) will have an obvious effect on terrestrial climate systems. Changes in atmospheric composition can have a major effect on climate.
This is where humanity may have a role to play in climate change.
2.3 CEMENT INDUSTRY EMISSIONS
The global cement industry contributes around 6% of all man-made CO2 emissions and is consequently responsible for around 4% of man-made global warming. CO2
emissions trading is likely to be of huge importance to the industry in the future. The cement industry has four major emissions which have a cooling or heating effect on the Earth through an effect on radiative forcing - the change in net irradiance in Wm-2.These emissions are dust emissions, sulphur, NOx and CO2. The contribution to global warming by the cement industry of the first three is not significant. For example, the total amount of natural dust produced each year in around 3Bt, whereas all industrial sources of dust total 100Mt. The global cement industry contributes a small proportion of this total. Again, the sulphur emissions of the
cement industry are not significant on a global scale. Nearly 100% of anthropogenic emissions of sulphur are from oil and coal combustion, and from copper production. In any case, sulphur is currently thought to play a role in global cooling, through the formation of aerosols. The NOx emissions of the cement industry are again not significant on a global scale. Advanced technology, such as low-NOx burners, and specially-designed pyro processing lines, has a major part to play in the reduction of NOx emissions from the cement industry. However, in contrast to dust, sulphur and NOx emissions, CO2 emissions from the global cement industry are significant - and they are increasing. Global cement production is currently around 1.6Bt/y, and through the calcination of limestone to produce calcium oxide and carbon dioxide, approximately 0.97t of CO2 is produced for each tonne of clinker produced. Taking into account gypsum addition, and the use of substitutes such as limestone, and slag, on average around 900kg of clinker is used in each 1000kg of cement produced. In this simple and presently unavoidable way, the global cement industry produces around 1.4Bt of CO2 each year. Total global manmade CO2 production is around 23.1Bt now, so that the contribution of the global cement industry to worldwide man-made CO2 production is about 6%. What is also important to remember, however, is that the total global production of CO2 runs into billions of tonnes. And remember also that CO2 is not just active as a greenhouse gas for a day or a week, but for decades and possibly centuries after its creation.
2.4 Increasing Production
Cement production increases at about 3%/year at the moment. This rate is set to increase as developing nations rapidly become richer, and spend proportionately more on cement-intensive infrastructure. The lowest projections suggest that by 2050, global per capita income will be more than 50% higher than in 1990, while similarly India's per capita income will be more than 100% higher than in 1990, and Asia's per capita income will be more than 500% higher than in 1990. These two countries alone account for more than one third of the world's population. It is likely that the contribution of the cement industry to CO2 emissions is likely to keep pace with or overtake global population increases. The fraction of radiative forcing due to CO2 is projected to increase from about 50% in 2000 to about 75% in 2100. Since the cement industry produces around 6% of man-made CO2, and CO2 is responsible for 65% of radiative forcing, at the moment, the cement industry can be
seen to be responsible for about 4% of all man-made global warming. It is likely that CO2 emissions will become more important to society at large - and less acceptable.( McCaffrey, R., (2002). Climate change and the cement industry)
2.5 PROCESS DESCRIPTION OF CEMENT MAKING
Cement Properties
Cement is an inorganic, non-metallic substance with hydraulic binding properties. Mixed with water it forms a paste, which hardens owing to formation of hydrates. After hardening, the cement retains its strength. There are numerous types of cement because of the use of different sources for calcium and different additives
to regulate properties. The exact composition of cement determines its properties (e.g., sulphate resistance, alkali content, heat of hydration), whereas the fineness is an important parameter in the development of strength and rate of setting. Because of the importance of cement as a construction material, and because of the geographic abundance of the main raw materials, cement is produced in virtually all countries. The widespread production is also due to the relatively
low price and high density of cement, which in turn limits ground transportation
because of high transport costs. In 1996, global cement tradewas 106 Mt of cement,
7% of global cement production.
Process Description
Cement production is a highly energy-intensive process. Cement making consists of three major process steps (Figure 4): raw material preparation, clinker making in the kiln, and cement making. Raw material preparation and cement making are the main electricity-consuming processes, while the clinker kiln uses almost all the fuel in a typical cement plant. Clinker production is the most energy-intensive production step, responsible for about 70%-80% of the total energy consumed. Raw material preparation and finish grinding are electricity-intensive production steps. Energy consumption by the cement industry is estimated at 2% of the global primary energy consumption, or 5% of the total global industrial energy consumption. In the process described below, we focus on energy use because of its
importance as one of the potential sources of CO2 emissions.
2.6 Energy Use in Cement Making
The theoretical energy consumption for producing cement can be calculated based on the enthalpy of formation of 1 kg of Portland cement clinker, which is about
1.76 MJ. This calculation refers to reactants and products at 25±C and 0.101 MPa. In addition to the theoretical minimum heat requirements, energy is required to evaporate water and to compensate for the heat losses. Heat is lost from the plant by radiation or convection and, with clinker, emitted kiln dust and exit gases leaving the process. Hence, in practice, energy consumption is higher. The kiln is the major energy user in the cement-making process. Energy use in the kiln basically depends on the moisture content of the raw meal. Most electricity is consumed in the grinding of the raw materials and finished cement. Power consumption for a rotary kiln is comparatively small, and generally around 17 and 23 kWh/t of clinker. Additional
power is consumed for conveyor belts and packing of cement. (Worrell, E., Price, L., Martin, N., Hendriks, C., & Meida, L.O.(2001). Carbon Dioxide from the global cement industry)
INDIA'S CONTRIBUTION TO THE PREDICAMENT
India is a major stakeholder in global climate change discussions along with other developed nations. Its share of CO2 emissions from total energy consumption - at about 1.5 billion tonnes in 2009 - occupies a significant position in the world89. It is the third largest emitter following U.S. and China and contributed around 5% of global CO2 emissions from consumption of energy in 2009 (EIA). In comparison to the share by two leading nations (U.S. and China together contribute around 43% of the global emissions); India's share is insignificant. In spite of low cumulative historical emissions relative to most industrialised countries and low per capita emissions [1.7t of CO2 (Atteridge et al., 2009)] with regard to other major developing economies and even compared to the world average of 5.8t of CO2 per capita per year, the emissions are increasing. CO2 emissions from consumption of fossil fuels contribute towards more than half of total GHG emissions in India, as in several other nations. Maintaining a high rate of economic growth is a key to poverty reduction in India and there is indeed a great demand for energy to support this growth. So India is faced with the challenge of adapting a "low carbon" growth strategy.
CHAPTER: 3 SUSTAINABILITY OF ENVIRONMENT
3.1 THE ENVIRONMENT IMPERATIVE
Since the World War II, lightwood framing has become the predominant building method in the United States because it was expedient and cheap. Stick frame, as known in the United States, has only very recently been parts of the world. At a time when forests thought that the world are being clear-cut at unprecedented rates, it is tragic that wood framing should now take the fancy of builders in wood-poor countries, where masonry and other indigenous building system have predominated within a more or less balanced ecology for centuries, if not millennia.
The ecosystem on our planet, forest is the most discernibly threatened by human exploitation. We have lost nearly half (46 percent) -3 billion hectares of forest that originally blanketed the earth, and deforestation continues to expand. Most of this forest cover was cleared during the twentieth century for timber or to convert land. The World Resources Institute has reported that only 22 percent remains of the world's irreplaceable "frontier forest"-areas of "large, ecologically intact, and relatively undisturbed natural forest." In Europe and United States, the percentage drops down to 3. (Frontier Forest, World Resource Institute,1997, Washington DC.
Ancient forests support roughly half the world's biodiversity; they also renew our air, stabilize our climate, and maintain our watersheds and soils. Wood frame residential construction in the United States is a leading cause of global deforestation. 45 percent of all the wood harvested in the world in 1995 (3.33 billion Cubic meters) was used for industrial round wood-wood that is used to make lumber, paper, plywood, and similar products. Nearly one-quarter of that round wood is consumed in the United States; and 40 percent or more of this is used for construction. Ultimately, about 10 percent of the world's industrial round wood is used by the U.S. construction industry, and most of that for residential buildings. (J Abramovitz and A Matton, Reorienting the Forest Product Economy, 1999). Despite the critical need to stop this voracious forest consumption, little if any incentive for significantly changing building practices was given.
Organizations have published recommendation for reducing wood demand, which include more efficient framing techniques and engineered wood products. Specifying lumber from sustainably managed forests is gaining more awareness as an important solution. But this is just one story, there are many such practices going with other materials worldwide. If we sum them all then the situation gets even worst.
3.2 PRIORITIZATION OF ENVIRONMENTAL IMPACTS
Environmental impacts are not all equal. Some arc more critical than others and should be given more weight. Important prioritizing factors include:
Sphere of influence: Some impacts have a more widespread area of influence than others (global warming versus streams siltation).
Duration: Some impacts last only a few months or years while others continue forever (nuclear waste dumps versus patchwork clear-cutting).
Magnitude of risk to human or ecosystem health: Some impacts have severe consequences for human or ecosystem health while others have little or no effect (toxic waste dumps versus well-managed municipal landfills). Sometimes the same behavior will have different effects (the reaction to VOC emissions by healthy people versus those with environmental illnesses).
Reversibility: Some impacts are irreversible while others are technologically possible to repair (destruction of a genetically diverse ecosystem versus reclamation of a former strip mine site).
It is important to identify which relationships between environmental problems and a particular design tradition (or construction behavior) are causal relationships; that is, where the environmentally destructive behavior is driven by construction industry demand and where the discontinuation of the practice will improve the environmental situation. If there is a direct causal relationship, then the practice should be avoided. However if the driving force comes from some other segment of society and the construction industry is only making efficient use of leftover wastes, then a potentially bad environmental practice becomes environmentally beneficial. An example illustrating this point would be the factors underlying destruction of world forests. In those regions where agricultural conversion is the primary motivating factor behind forest destruction (such as in some tropical forests), it would be better to use the timber than to burn it.
Resource intensity is also one factor to be kept in mind. Some building products consume less raw materials than others in fulfilling the same use. Therefore some preference should be given to the product that uses raw material more efficiently. This consideration is more important for products whose raw materials are in limited supply than for abundantly available materials.
3.3 ALTRUISM AND PROFESSIONAL RESPONSIBILITIES
The costs that may be tracked on a typical assets summary may appear significant to the bottom line of a particular project or product, but they pale in comparison to the environmental costs. It may be hard to economically justify basic it's -the right thing to do- logic, but it will be impossible to continue without it, economically and otherwise. We have only this one planet. It has the same amount of resources water, air, minerals that it has had since the beginning of time, yet demand for them is continually increasing. Most goods are derived from the Earth's natural resources, to be used briefly and then buried in a landfill: By the middle of the next century, the same limited amount of resources is expected to support nearly 12 billion people. (Spiegel, R., Meadows, D. (2006). Green building materials: A guide to product selection and specification). We need to be extremely careful of the resources we use and how much of them we use. We must ask and answer these questions:
What are we using?
How well are we using it?
Furthermore, our limited resources are not spread out evenly. There are centres of biodiversity. We rely on biodiversity, the different characteristics of different species, for medical, agricultural, and industrial advances. When we remove all existing vegetation during the construction process, even if we landscape with native vegetation afterward, we destroy a portion of the biodiversity of the area forever. We also contribute to the destruction of the Earth's biodiversity when we rely on a single species. Reliance on a single species or a limited number of species promotes monoculture, the antithesis of biodiversity. It was, in part, monoculture that devastated Ireland in the Great Potato Famine of the mid-1800s. Some techno-enthusiasts have argued that a little DDT would have put an end to the potato famine in a hurry. In the short term, that may have been true. But the next generation of Irish would have been much worse off.
By specifying green products, products that are nontoxic, have recycled contents, and are themselves easily recyclable we can make it safer and easier to cycle materials responsibly and eliminate waste. Waste costs money. It pollutes the planet and consolidates the Earth's resources in singularly useless pits around the world. Most landfill pits are hygienically isolated and rigorously compressed such that the contents are not exposed to oxygen or water and, consequently, do not readily decompose. Assuming a site that promotes decomposition, however, decomposition time for plastics one million years; for paper, one month; for glass, over one million years; for apples, three to four weeks; and for aluminium, 200 to 500 years. (Spiegel, R., Meadows, D. (2006). Green building materials: A guide to product selection and specification
CHAPTER: 5 ADMIXTURES IN CONCRETE
5.1 MINERAL ADMIXTURES AND CLASSIFICATIONS
Mineral Admixtures
Mineral admixtures are finely divided siliceous materials which are added to concrete in relatively large amounts, generally in the range 20 to 100 percent by weight of Portland cement. Although pozzolans in the raw state or after thermal activation are being used in some parts of world, for economic reasons many industrial by products are fast becoming the primary source of mineral admixtures in concrete.
Power generation units using coal as fuel, and metallurgical furnaces producing cast iron, silicon metal and ferrosilicon alloys, are the major source of by-products, which are being produced at the rate of millions of tons every years in many industrial countries. Dumping away these by-products represents a waste of the material and causes serious environmental pollution problems. With proper quality control, large amounts of many industrial by products can be incorporated into concrete, either in the form of blended portland cement or as mineral admixtures. When the pozzolanic and/or cementitious properties of a material are such that it can be used as a partial replacement for portland cement in concrete, this results in significant energy and cost savings.
Classification
Some mineral admixtures are pozzolanic; some are cementitious, whereas others are both cementitious and pozzolanic. A classification of mineral admixtures according to their pozzolanic and/or cementitious characteristics is shown in table 04 below. By using mineral admixtures in concrete it is possible to have a favorable influence on many properties through either purely physical effect associated with the presence of very fine particles or physic-chemical effects associated with pozzolanic and cementitious reactions, which result in pore-size reduction and grain-size reduction phenomena. Among the properties that are favorably affected are the rheological behaviors of fresh concrete mixtures, and the strength and durability of hardened concrete. Resistance to chemical attacks and thermal cracking are the two aspects of concrete durability that can be improved significantly by the incorporation of mineral admixtures. It is necessary to have a proper understanding of the mechanisms by which mineral admixtures improve properties of concrete because above mentioned potential benefits are not always realized.
5.2 FLY ASH IN CONCRETE
Introduction
Fly ash, also known as pulverized-fuel ash, is the ash precipitated from the exhaust gases of coal fired power station and it is the most common artificial pozzolana. They are generally finer than cement and consist mainly of glassy-spherical particles formed during cooling. Use of fly ash in concrete started in the United States in the early 1930's.In addition to economic and ecological benefits, the use of fly ash in concrete improves its workability, reduces segregation, bleeding, heat evolution and permeability, inhibits alkali-aggregate reaction, and enhances sulphate resistance. It is recognized that it's morphological, physical and chemical characteristics are such that fly ash can very effectively be used as a partial replacement of portland cement in concretes required to meet stringent specifications. Even though the use of fly ash in concrete has increased in the last 20 years, less than 20% of the fly ash collected was used in the cement and concrete industries.( Patel, R., SBST, CEPT,2001. Combination of fly ash & silica fumes in concrete as a cement replacement material). Fly ash can be used in concrete for all types of applications including pumpable concrete, high strength concrete, precast masonry units, precast structural concrete, concrete pipes, roller compacted concrete for dams and road pavements.
Classification and specification
Two major classes of fly ash are specified on the basis of their chemical composition resulting from the type of the coal burned; these are designated as class F and class C (ASTM C 618). The chemical composition of fly ash depends on the original composition of coal ash. Class F is fly ash normally produced from burning bituminous coal, which is known Ordinary Fly Ash, while Class C fly ash is produced from burning of sub-bituminous and lignite coal and also named as high calcium fly ash. Class C fly ash usually has cementitious properties in addition to pozzalanic properties due to free lime, whereas Class F is rarely cementitious when mixed with water.
ADVANTAGES OF FLY ASH IN CONCRETE
Fly ash is a pozzolan. Concrete containing Fly Ash pozzolan becomes denser, stronger and generally more durable long term as compared to straight Portland cement concrete mixtures.
Fly ash improves concrete workability and lowers water demand.
Sulfate and Alkali Aggregate Resistance, Class F and a few Class C Fly ashes impart significant sulfate resistance and alkali aggregate reaction (ASR) resistance to the concrete mixture.
Fly Ash has a lower heat of hydration; Portland cement produces considerable heat upon hydration. In mass concrete placements the excess internal heat may contribute to cracking. The use of Fly Ash may greatly reduce this heat buildup and reduce external cracking.
Fly ash generally reduces the permeability and adsorption of concrete. By reducing the permeability of chloride ion egress, corrosion of embedded steel is greatly decreased. Also, chemical resistance is improved by the reduction of permeability and adsorption.
Fly Ash is economical. The cost of Fly Ash is generally less than Portland cement depending on transportation. Significant quantities may be substituted for Portland cement in concrete mixtures and yet increase the long term strength and durability. Thus, the use of Fly Ash may impart considerable benefits to the concrete mixture over a plain concrete for less cost. (http://www.ashgroveresources.com/showcase4.html)
Because of physical, chemical and mineralogical properties of fly ash, it can be effectively and economically utilized in cement manufacture or concrete and other related products in the following ways:
As raw material in the manufacture of portland cement
For the manufacture of blended cements
For the manufacture of sintered lightweight aggregates
As finely divided mineral admixture
As partial replacement of fine aggregate (sand)
As partial replacement of cement
By weight
By volume
For the manufacture of aerated cement concrete blocks
For production of flexible concrete when used along with water soluble polymers.
In addition to its use in cement concrete, it may be used as asphalt filler, road bed material, and structural fill for embankments, grout material for coal mines, drainage improvement material and as a soil stabilizer for foundations and pavement. In architecture fly ash is used in the manufacture of tiles, bricks, ceramic insulating material mineral wool, etc.
5.3 SILICON FUMES IN CONCRETE
INTRODUCTION
Silica fume is a mineral composed of ultrafine, amorphous glassy spheres of silicon dioxide, produced as by-product resulting from the reduction of high purity quartz with coal or coke and wood chips in an electric arc furnace during the production of silicon metal or ferrosilicon alloys. The quality of the raw material and the operation of the furnaces determine the purity of the silica fume. It has also been referred to as silica dust, condensed silica fume and microsilica. The initial interest in the use of silica fume was mainly caused by the strict enforcement of air-pollution control measures ¡n various countries to stop release of the material into the atmosphere.
Silica fume is available commercially in many countries in several countries in several forms. In India, the productions of silicon and ferrosilicon alloys are very less and hence silica fume is not easily available, more over it is costlier. In India, silica fume is available in manufactured form in bulk or in bags and handed over and transported like Portland cement.
The ultrafine spheres fill the gaps between the relatively coarser cement grains, which can be otherwise occupied by water, refining the voids in the fresh concrete. They are easily dispersed in the space between and around each cement grain. This densifies the whole concrete structure resulting in increased strength and significant reductions in permeability.
EFFECT OF SILICA FUMES
Water Demand, Workability: The water demand of concrete containing silica fume increases with increasing amounts of silica fume, without water reducing admixture. This increase is due to the high surface area of the silica fume. For achieving maximum improvement in strength and permeability, silica fume concrete should be made with the water reducing admixture (WRA) or high range water reducing admixture (RWRA). Addition of silica fume could reduce water demand because the silica fume particles were occupying space otherwise occupied by water between the cement grains.
Setting Time: It is found that the addition of silica fumes to concrete in the absence of a WRA or RWRA causes a delay in setting time, compared to a non-silica fume concrete of equal strength.
Abrasion and Erosion of Concrete: Silica fume Concretes exhibit greater abrasion resistance than similar concretes made without silica fume, especially when inferior aggregates are employed. The use of silica fume reduces the porosity and improves the wear resistance of the cement past.
APPLICATION OF SILICA FUME IN CONSTRUCTION INDUSTRY
Silica fume as a cement replacement material in concrete, for achieving durability of structure has been used in many countries since 1980. As far as India is concerned, very limited application of silica fume has been reported, this is mainly due to higher cost of about 10 times than ordinary portland cement. Different application of silica fume are as under:
Silica fume concrete has been used in industrial floors, parking structures, bridge decks, bridge deck overlays and pavement to provide better abrasion resistance and chemical resistance.
Silica fume has been used for repair of hydraulic structures subjected to abrasion-erosion damage and for rehabilitation of parking structure subjected to chemical attack.
Silica fume has been used in blended cement to prevent excessive expansion caused by the alkali-silica reaction in Iceland.
Because of its high pozzolanic activity, silica fume can replace portland cement while maintaining essentially the same level of concrete performance and for the purpose of reducing the amount of heat generated in concrete.
Silica fume has been used in grout to anchor post-tensioned tie-back cables, since silica fume grout exhibited longer pot life and resistance to corrosion.
Use of silica fume can improve the early age strength of concrete containing fly ash and blast furnace slag cement.
Precast and cast-in-place high-strength silica fume concrete has been used for building and parking structure in columns and in post tensioned beams and for Bank-vaults.
5.4 UTILIZATION OF AGRICULTURAL WASTES
Due to increasing population the housing problem has become very severe and building materials are not available in sufficient quantity. They are also costly. India being an agricultural country, enough agro waste material is available. Hence, there is utmost necessity to produce cheap building materials from agro waste materials and synthetic resin or cement.
A variety of agrowastes such as saw dust, sugarcane bagasse, coir fibres, rice husk, wheat stalk are available. Various types of building boards and panels can be produced from the agro- industrial wastes. Unpulped straw such as rice and wheat straw can be converted into bilnderless boards and rigid building panels by applying heat and pressure techniques. Methods for the preparation of building boards and blocks from rice husk have worked in many countries. Glued hot mat boards have been prepared from reeds & straw. They are being used for certain walling and roofs for cattle. (C.B.R.I Roorkee(1990). Building materials & components: Tech specs for developing countries)
Coconut fibre building panels have been produced using cement binder. Mineral bonded panel products also have been made from wood shavings and portland cement. Similarly there are heavy investigations aiming agro-industrial waste to produce a variety of building products by employing cold hot pressing techniques or use of a range of organic and inorganic binders.
Feasibility of producing normal particle board and building components from husk has been studied by CBRI. A normal particle board requires 8 - 1O% adhesive by weight for a workable strength & finish which amounts to nearly half of the cost of the finished board. Even the raw materials for making the adhesive are quite cheap. Argo waste like coconut husk and its by-product, groundnut husk, rice husk etc. have no inhibiting action on normal Portland cement and these have been used to produce such boards. A corrugated roofing sheet using some agricultural waste has also been developed. (C.B.R.I Roorkee(1990). Building materials & components: Tech specs for developing countries)
Tiles of sizes 25X25X1 cm have been produced in strong iron mould and pressed which weighted 1.1kg and flexural strength of 7.5 to 3.5 kg/cm2. To increase flexural strength, wire mesh and bamboo mesh was also incorporated. These tiles can be bonded with epoxy resin and fixed on to wooden frame to form a partition wall. Such partition is cheap, decorative, handy and light in weight. (C.B.R.I Roorkee(1990). Building materials & components: Tech specs for developing countries)
5.5 GEO-POLYMERS IN CONCRETE
Geopolymers are a type of inorganic polymer that can be formed at room temperature by using industrial waste or by-products as source materials to form a solid binder that looks like and performs a similar function to ordinary portland cement (OPC). Geopolymer binder can be used in applications to fully or partially replace OPC with environmental and technical benefits, including an 80 - 90% reduction in CO2 emissions and improved resistance to fire and aggressive chemicals.
Geopolymer cement is made from aluminium and silicon, instead of calcium and silicon. The sources of aluminium in nature are not present as carbonates and therefore, when made active for use as cement, do not release vast quantities of CO2. The most readily available raw materials containing aluminium and silicon are fly ash and slag.
The main process difference between OPC and geopolymer cement is that OPC relies on a high-energy manufacturing process that imparts high potential energy to the material via calcination. This means the activated material will react readily with a low energy material such as water. On the other hand, geopolymer cement uses very low energy materials, like fly ashes, slags and other industrial wastes and a small amount of high chemical energy materials to bring about reaction only at the surfaces of particles to act as glue.
This approach allows the use of measured amounts of chemicals to tailor the product to specification, rather than using an amount of very high-energy material required for OPC, regardless of whether the material is used to build strength. This approach results in a very large energy saving in the production of geopolymer cement.
The properties of geopolymer cement, when used to make concrete, have been repeatedly and independently shown to be equivalent to other cements in terms of the structural qualities of the resulting concrete. Indeed, the fire resistance of geopolymer has been tested to be well in excess of double that of traditional concrete. This is a highly significant technical benefit of geopolymers and will drive wide scale adoption in high-rise construction in the near term.
ADVANTAGES AND APPLICATIONS OF GEOPOLYMER
Compared with portland cement, geopolymers possess the following characteristics:
• Abundant raw materials resources: any pozzolanic compound that is readily dissolved in alkaline solution will suffice as a source of the production of geopolymer.
• Energy saving and environment protection: geopolymers don not require large energy consumption. Thermal processing of natural alumino-silicates at relative low temperature (600° to 800°) provides suitable geopolymeric raw materials, resulting in 3/5 less energy assumption than portland cement.
• Simple preparation technique
• Good volume stability: Geopolymers have 4/5 lower shrinkage than Portland cement.
• Reasonable strength gain in a short time: geopolymer can obtain 70% of the final compressive strength in the first 4 hours of setting.
• Ultra-excellent durability: geopolymer concrete or mortar can withdraw thousands of years weathering attack without too much function loss.
• High fire resistance and low thermal conductivity: geopolymer can withdraw
1000° to 1200° without losing functions.(Development of SUSTAINABLE CEMENTITIOUS MATERIALS, DMSE, Southeast University,Nanjing)
CHAPTER: 6 CONCLUSION
6.1 AN OUNCE OF PREVENTION
Buildings impact the Earth directly through their use of resources. They work directly on the quantity and quality of the Earth's resources-the amount they use and the degree to which they contaminate what they use. Buildings impact the Earth indirectly through their performance and through their effect on the performance of adjacent structures. Buildings impact the Earth indirectly through design decisions that help drive the market. If you select a green product, you make a philosophical and an economic statement.
Architects, have an opportunity and an obligation to confront these issues. Architects can have a huge impact not only on the design of the building, which can affect the people who use it, but also on the design process, which can affect the market, regulatory requirements, and accepted practices.
Often, however, the question is not so much whether a greener, more efficient solution exists but rather how to identify and implement such a solution. The expectations of the design and construction industry tend to limit design choices to current industry standards, which are not necessarily the most efficient. They also tend to focus attention on problem solving during the construction phase rather than problem identification during the design phase, further limiting the range of possible solutions. Standard design and construction strategies often require a pound of cure. Green strategies offer an ounce of prevention.
6.2 CONCLUSION
Given the present scenario we don't have an equal alternative for cement, in near future. We will have to modify the process where cement is harming the environment. We will have to rethink on few things and improve technologies so that at least we can reduce the effect it is causing
Demand of the construction industry is increasing at an exponential rate. If the steps are not taken from today, the future is going to be dark and its coming very soon. As to respond to the demand in cities, the vernacular or traditional materials cannot be used as they restrict at their own properties. It is not possible to build the cities from any of these materials. But these are most suitable to the rural demands where today also we have two storey structures.
But anyhow we need to reduce the use of cement. The admixtures should be used in cities by which at least the use of cement will go down until a strong alternative for cement is discovered…
