In this era which we live in, there are more than 7 billion (1) people on Earth and its resources are limited and quickly depleting. As a response to this high demand and burden on the Earth’s precious goods, there has been a ‘green’ movement. Governments and companies are implementing laws and constantly looking for ways to be more efficient and conserve whatever little we have at our disposal. In the light of all this innovative research and as university students studying environmental engineering, concrete evolution has sparked our interest. Concrete is very much a large part of the environment, being one of the most widely used materials in construction, concrete is virtually everywhere. Its high durability and versatility has made it superior to all other building materials however there are some downsides to concrete that has made it a threat to the environment, mainly the greenhouse gas emissions that come with making cement. The cement manufacturing industry is under increasing pressure to reduce these emissions due to the fact that it releases a lot of gases, namely carbon dioxide and nitrogen oxide. The real struggle is to find ways to produce a concrete that is environmentally safe, without losing the integrity of the concrete’s durability and reliability.
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In this paper, the making of concrete and its advantages and disadvantages will be discussed, alongside some different alternatives that have been implemented at present time to deal with energy efficiency and environmental security. Economic and social effects are also looked at and discussed. The main alternatives in focus are; the use of chemical admixtures, recycled concrete materials and fuel alternatives for the kiln.
Kiln and Efficiency
The most energy consuming part of the cement making process is the burning of the mixture of the constituent parts of cement within the kiln. A large amount of emissions is released by the fossil fuels used to heat the kiln up and the chemical reactions that take place within the kiln itself. A kiln is a thermally insulated chamber or oven, in which a controlled temperature is maintained and kilns used for making cement get to temperatures of about 1500 degrees Celsius (2). In order to get to these temperatures, large quantities of coal are burned to generate the energy needed for the kiln. Coal is the primary fuel burned in cement kilns, however, the use of alternative fuels in cement kilns is now common and increasing.
This high energy consumption however leads to high carbon emissions, about 7% of the world’s total carbon emissions. Cement production is an energy-intensive process consuming thermal energy of the order of 3.3 GJ/tonne of clinker produced. Electrical energy consumption is about 90 – 120 kWh/tonne of cement.(3)
These are the reasons why more efficient fuel alternatives are being investigated to firstly help improve the quality of air we breathe and secondly protect the earth from adverse conditions that come with too much carbon dioxide in the atmosphere.
There are two types of kilns being put to use, one utilizing a wet process and the other dry. The wet process is the older method and involves a slurry mixture of water and the cement ingredients being transferred to the kiln. The wet process however uses a lot of energy and therefore the modern dry process is more commonly used. It uses the dry ingredients blended together and then transferred to the kiln, the only disadvantage it that a lot of dust is released. Both diagrams below will illustrate the the cement making process and more importantly the two different kinds of kilns.
Material Science
According to the Merriam-Webster dictionary, material science is the scientific study of the properties and applications of materials of construction or manufacture (as ceramics, metals, polymers, and composites).(5) The concise encyclopedia further explains how material science goes into how the properties of different materials depend on their composition such as atomic mass and electron configuration.(5) It also points out the importance of material science to engineers of all disciplines as they need to know as much as possible about different materials in order to come up with designs and fix problems in their respective fields.
With a sound knowledge of materials and their properties, they can be manipulated in any way necessary to be an asset to us. In our case, understanding the chemical reactions that take place in concrete will help to understand why the methods chosen have been picked in the first place to help rectify any problems. From the manufacturing of the cement in the kiln to the demolition of concrete structures, knowing and understanding the reason behind different aspects surrounding the whole concrete process is very beneficial in finding alternatives to make it more environmentally safe and efficient.
Alternative Fuels
Fossil fuels, such as coal and natural gas, have been used as energy sources in the cement manufacturing industry for decades. In more recent years, these traditional fuel sources have become increasingly substituted with alternative fuels typically of waste sources such as municipal solid waste, scrap tires, waste wood, agricultural biomass, meat and bone meal, and petroleum coke. The list of candidate materials is continuously expanding and regulatory pressures, economic factors and the fact that we are running out of landfill space are all reasons why these alternative fuels are continuously sought for and studied.
Alternative fuels used today in cement manufacturing and the different potential alternative fuels differ considerably from the traditional fuels, and the cement manufacturing industry is faced with several challenges in making the switch from traditional to alternative fuels. Some of these challenges include, inadequate heat distribution, blockages in the preheater cyclones, unstable precalciner function, higher SO2, NOx, and CO emissions, congestion in the kiln riser ducts and dusty kilns (6). Furthermore, due to the fact that the cement industry is strictly regulated by national and international legislation for environmental related issues, health and safety of practices, and the quality of cement produced, special approval is required for the use alternative fuel since they all different and can potentially introduce harmful environmental effects or affect the quality of the cement. The type of combustion implemented, which is determined by the type of fuel used can have a direct affect on the composition and characteristics of the output product, and the function of the kiln as different manufacturing plants may differ in their design.
A common practice in cement manufacture is the addition of the ash produced by the fuels, which are comprised of compounds containing silica and alumina, into the clinker (6). The composition of the fuel ash created by different fuels can determine in which proportions an alternative fuel can substitute a conventional fuel, for example some can yield silica rich fuel ash which can later reduce the amount of ground sand needed as a raw material to make cement. Moreover, the inclusion of constituents that can have a deleterious effect on concrete performance must be controlled, since this can happen even at very small concentrations. An example of this would be alkalis such as potassium oxide and sodium oxide, which can in the presence of moisture can cause reactions in concrete called ACR and ASR which can cause cracking in the structure. These alkalis can also react with SO3 to form alkali sulphates, which can affect the reactivity of the cement with aggregates, resulting in hardening problems (6). Therefore, the inclusion of alkalis from the kiln system should be minimized.
In certain kilns that have preheaters, the use of alternative fuels can lead to the volatilization of certain molecules they introduce, which can lead to their subsequent recirculation in increasingly higher loads. Their recirculating can lead to their condensation in cooler areas, binding to circulating dust particles and can potentially cause blockages, thereby affecting the heat-exchange system. Some of these molecules are sodium oxide, potassium oxide, alkali sulphates, and chlorine, which not only are responsible for deposits, preheater blockages, and kiln rings but can also affect the quality of the cement produced if they are retained to some proportions(6).
Petroleum coke or petcoke is a solid residue from the crude oil refineries. It is considered a low volatile fuel with a typical volatile content of 5-15 %. The fact that its volatile content is low means that it has a low reactivity / burning rate and therefore is not possible to burn 100% petcoke in kiln or precalciner without using other high volatile fuels along with it (6). Consequently, this alternative fuel requires finer grinding and is pushing new kiln designs into the market to allow for their complete burning. Another negative is the fact that it has a high sulphur and vanadium content. This can result in increasing the sulphur circulation in the kiln and precalciners and as mentioned before causing build-ups and blockages, and increasing sulphur dioxide emissions. Sulphur contamination of the cement can cause cracking and high vanadium content can cause reduce the strength of concrete (6). A 0.2 percent addition is reported to lead to a 10 percent reduction in 28-day strength of cement. However, due to low ash content of petcoke such high contents of vanadium in cement are unlikely (6). This is an attractive fuel as it has a high calorific content and relatively less expensive than coal and other fossil fuel conventionally used.
Sewage Sludge is generated from wastewater treatment from industrial, residential, commercial, and institutional sources. Sewage sludge is usually disposed of by throwing it in the sea, its use as fertilizer, its incineration, or it is dumped in a landfill. Due to stricter environmental specifications associated with its disposal, the possible health and environmental risks in using it as a fertilizer and the increasing cost for its disposal in landfills, its use as an alternative fuel in cement manufacture is becoming more attractive. The organic components of the sludge are entirely destroyed when it is burned as fuel and the inorganic components and heavy metals are combined and included in the final product. The sulphur content of sludge is not greater than coal so it does not pose a major concern in comparison, and although it has higher nitrogen content the nitrogen oxide emissions are lower than when fossil fuels are burned (7). However, there are higher contents of volatile content, ash, and low fixed carbon compared to coals. Sludge usually requires pretreatment before it can be used as a fuel and has to be burned in controlled conditions as with most alternative fuels. Using sludge is also attractive economically, as it resulted in an increase in return when used instead of fossil fuels, in spite of its lower energy content than coal and the fact that it needs to be stored in special silos in order to avoid contamination (7). Its storage and handling and reduction of water content are the most difficult part of its use as a fuel, however it is definitely a far better option than use of non-renewable resources since it is widely available and a nuisance to dispose of. The use of sludge as a fuel source cannot have much of a social impact other than perhaps peoples perception of it, some may regard it as a better option than incinerating sludge which forms poisonous by-products, while others may be bothered by its use in anything else.
Scrap tires have become utilized as an alternative source of fuel for various parts of the developed world instead of fossil fuels in many industries including cement manufacture. When tires are burned the rubber is completely destroyed and the inorganic component and heavy metals are included in the cement product. Different cases present different conclusions about the emission of SO2 and NOx, which may suggest that it depends on the kiln system and the burning process implemented. However, two Portland Cement Association (PCA) reports (2008, 2009) found that nitrogen oxide, sulphur oxide, and particulate emissions were reduced when scrap tires substituted a portion of the conventional fuels (7). Heavy metal, dioxins and furan emissions showed different results in different studies however, again Portland Cement Association studies collected data from 31 cement plants that used tire as fuel and found a significant reduction in the emissions of dioxins and furans (7) . Some problems with tire derived fuel is incomplete combustion and zinc oxide present at concentrations that may be detrimental to the quality of the cement. Overall, the use of tires as fuel is an environmentally, and economically sound option compared to other end-of life alternatives of tires and the use of 100% fossil fuel. Tires have a higher energy content than coal and allow for savings in the purchase of coal.
Another source of fuel being used is agricultural biomass, which includes all forms of biomass not included in the categories of meat and bone meal, or sewage sludge. Some common sources are rice and coffee bean husks, palm kernels, algae, and cottonseed oils. The use of agricultural biomass has been proven to be an effective way to reduce greenhouse gases and the dependency for fossil fuel (7). Furthermore, its been determined to have low SO2 emissions, low dioxin and furan emissions, and very low heavy metal emissions. Biomass in the form of waste from industrial or agricultural processes is less expensive than fossil fuels, and therefore its use would reduce operational costs. However, equipment specific to the processing of biomass may be needed and may incur additional costs. Also, supply seems to be a major concern, a continuous supply may be difficult to achieve. Socially it can be beneficial to some agricultural communities, allowing them to make an additional income from selling their agricultural by-products (7).
Finally the last alternative fuel to be discussed is the meat and bone meal (MBM), a by-product of the rendering and food industries. Their co-incineration with fossil fuels in cement kiln systems has become a common way for their elimination. MBM has a lower fixed carbon and high ash content and high levels of phosphate, sodium, potassium, magnesium and chlorine (7). Due to the fact that chlorides can volatilize and condense at high temperatures in the kiln and can react with alkalis and sulphates to form compounds with low melting points, which can lead to their recirculation and condensation. As mentioned earlier this can can have harmful effects on the production process and cement produced . Consequently the MBM used as substituted fuel and the compounds introduced into the cement needs to be controlled and monitored. The sulphur content, on the other hand, is a little lower than can be found in coal, and the high calcium content in MBM can help retain most of the SO2 released from its combustion (7). The use of MBM in cement production reduces CO2 emissions, SO2 emissions, and introduces a safe and environmentally friendly way to disposing of them. And as with the previous alternative fuels mentioned, it reduces the demand for landfills and their associated environmental and health risks.
In summary the cement kiln provides numerous advantages over other end-of life alternatives for much of these wastes. The high temperatures, oxygen rich environment, and adequate residence time provided by the kiln system allows for the complete destruction of the organic material. Also, aside from being able to process a wide range of waste materials, since the ash is incorporated into the final product there is no additional waste to manage from the use of these wastes. However, these alternative fuels are derived from selected waste streams and usually require some level of pretreatment, such as the shredding of tires, drying of sewage sludge and reducing its pathogen levels, etc. This is a extra investment of time and money that the cement manufacturers will need to take on as the pretreatment of these wastes is an integral part of their recovery and in most cases is taken care of externally by waste treatment experts or outside suppliers. Despite, these extra costs for the preprocessing of these wastes, the cement manufacturers are expected to make a larger return on this investment in comparison to the purchase of fossil fuels. Also, the use of these wastes as fuels would create a market for these them in neighboring communities, which will help reduce the number of operating landfills and put to use the calorific value in these wastes rather than have them wasted. The burning of carbon neutral wastes which include agricultural biomass, municipal waste, animal waste and paper waste are considered as GHG sinks since they would otherwise decay in landfills and form methane which is a more harmful GHG than CO2 (8). Other wastes that are derived from fossil fuels such as tires, are not carbon neutral, however burning them in cement kilns rather than incinerating them, which also induces GHG emissions, can result in significant CO2 reductions. Although the kiln emerging technologies and their capacities to process these fuels was not discussed, since it is too broad of a subject to cover and is not the main purpose of this paper, it is understood that some alterations to tradition kiln systems is required to adapt to the different combustion of these fuels. An example of these changes is features such as a multi-channel burner design and thermograph systems which allow for the control of the flame and optimize burning of different fuels (10). There are also different mathematical models, which look at which combination of alternative fuels in which proportions can produce optimal burning conditions (11). There has been much progress over the years in the substitution of fossil fuels in cement kilns, especially in the EU where substitution rates are much higher than in North America, however, there is much more work that needs to be done in the evolution of the cement industry towards greener and more sustainable practices.
Advanced Chemical Admixtures
Our society relies greatly on building materials, concrete being one of the oldest, and most important of those materials. Concrete is a combination of 60% to 75% aggregates and 25% to 40% paste. The paste is comprised of 7% to 15% cement, 4% to 8% of air content, and 14% to 21% water (15). Although paste only contribute less than 40% of concrete, the components greatly affects the overall quality. An example is with the reduction of water to cement ratio, and in turn increases the compressive and flexural strength, increases resistance to weathering reduces shrinkage and cracking, and lowers permeability. To achieve these characteristics in concrete, engineers came up with admixtures, an important ingredient’s used when the goals are to reduce the cost of concrete, maintaining the quality of concrete during the different stages of its production, basically to achieve desirable properties of concrete. Admixtures are classified under the following; Air-entraining mixtures, water reducing admixtures, plasticizers, accelerating admixtures, retarding admixtures, corrosion inhibitors, etc.
Superplasticiser/High range water reduction is made up of synthetic polymers, which are admixtures that increase slump flow, essentially used for low to normal slump and water-cement ratio. The use of superplascticiser not only brings the water-cement ratio down, but it drastically increases the workability, as well as increasing the strength at an early stage up to 200% within 16 hrs (13) . A great example of a new and innovative plasticizing admixture is Glenium SKY.
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The third generation high range water reducer or superplasticizer also known as Glenium was introduced in the 1990’s. Glenium is a polycarboxylic ether polymer, that attracts entrringite molecules, through a static electric charge. This entrringite provides a protective barrier around the surface of cement particle, which prevents hydration and crystallization. In September 2003 Glenium SKY (Synthesis of Key performance and Yield) was introduced. This new superplasticizer was developed for ready mixed concrete, concrete that contains high performance quality from the production to the usage. Unlike the other types of Glenium; Glenium 21 and Glenium 27 , Figure 1 Glenium SKY, new chemical monomers controls the rate in which the entrringite molecules cover the cement molecules. Therefore the cement molecules are not completely covered, which allows the crystallization to take place at a slower rate, and accelerated strength at an early stage without compromising the consistency.
Glenium SKY was tested against Glenium 27, it is noticeable from the table below that Glenium needing less water and lower cement-water ratio, but still was a able to produce a more consistent slump and greater strength according to the test results in table 1. Within 90 min, the slump dropped only by 2 mm. The usage of Glenium SKY benefits socially, economically, and environmentally.
With the increase in workability, decrease in water-cement ratio, and high early strength, allows the most optimum concrete design which in turns have a positive effect on the economical aspect
The definition of sustainability means to “meet the needs of the present generation without compromising the ability of the future generations to meet their need” (15). Sustainability is achieved when these 3 factors are properly balanced, economy, social, and environmental.
It is evident that the addition of the superplasticiser used in concrete mix designs will help to achieve environmental sustainability. The concrete industry’s main goal is to produce a superior material with a positive impact to the environment. This super-plasticiser or high range water reducer will decrease the water-cemnet ratio, meaning less cement needed as well as water. Water usage can be reduced from 10% to over 30%. A 10% reduction is equivalent to 600 000 tonnes of water saved annually (14). Not only are the usage of water is reduced but also other environmental factors.
Analyzing the effects of the super-plasticiser shows a generally positive effect on the environment. Figures 3 and 4, shows 2 concrete structures impact on the environment with the addition of super-plasticiser in relation with the structures without the admixture. For the flat Slab concrete, adding the super-plasticiser admixture decreases these negative impacts by a great margin. It is evident that the total energy requirement is reduced by 8%, the toxic impact on human health is reduced by 10 %, the acidifying pollutants is reduced by 8%, and the CO2 is reduced to almost 20%. However, using this admixture, there is an increase in chemical waste, the values have been normalized to 100% for the control, which means the increase of the non-hazardous chemical wastes will only increase by 1% (13). Overall for this cement mix the superplasticier admixture is beneficial because it decreases major negative impacts such as abiotic depletion and eootox sediment by 3% and another 3 categories by over 10%. The effect of the admixtures vary depending on the different type of mixes and their purpose. The concrete mix for a precast wall unit was also analyzed using these admixtures. Based on that analysis we can conclude that this admixture has the desired effect by decreasing the energy by 10% by volume of concrete. As with the previous concrete mix the superplasticiser has a greater impact on a number of impacts. While there is an increase in chemical waste by 10% by volume of control concrete the decrease in energy is of 20%. This outweighs the negative of this admixture. Overall the super-plasticiser admixture has a great effect on a number of impacts but also a negative impact on chemical waste.
Figure 3 and 4 compare the strength with and without the admixture versus the energy and climate change. It is evident that the super-plasticiser are effective in reducing the CO2 emissions and the energy consumption. A cement mix with this superplasticiser admixture will have a positive overall effect and has very beneficial impact on the environment.
The use of this super-plasticiser admixture in concrete mixes allows for social sustainability. As previously stated before, with the addition of this admixture, concrete mix can be made to have a higher strength. An increase in the strength of the concrete will result in a more durable material, and a longer life expectancy. With concrete having higher strength and a longer life, maintaining these structures will be reduced especially within major highways and roadways within a city. Traffic congestions greatly impacts our lives, career, and safety. By reducing the amount of traffic will allow a better quality of life. In central Ontario alone, there are 11 zones where major structures are being repaired and maintained. On the Queen Elizabeth Way (QEW) near Hamilton, a $7.3 million dollar contract has been approved for the structural rehabilitation (14). According to the traffic reports, the QEW Burlington Skyway Bridge, the Millen and Fifty Road structures and glover road will be under construction, which means a delay up to 30 minutes (14). The 30 minute construction delay with the addition of bottlenecked areas, and overcrowded vehicles can increase the delay to be even longer than 30 minutes. Let’s say 30 minutes delay for construction, and because of a large amount of the population commute to the city of Toronto for work, another 10 minutes due to vehicles overcrowding the roads. We have a total of 40 minutes delay, and a total of 80 minutes delay a day, which comes up to 6400 minutes or 106.6 hours in traffic until this specific project is completed in 4 months. These delays will cause people to be late for work, school, or other commitments, which can lead to stress, and road safety. By producing an optimal concrete design using a super-plasticiser such as Glenium SKY, we can increase the strength and life expectancy of the structure, which will reduce traffic caused by maintenance and repair, and create a more socially sustainable environment.
Reaching economic sustainability is just as important as social and environmental sustainability. With the increase in workability, decrease in water-cement ratio, and high early strength, allows the most optimum concrete design which in turns will help to achieve economical sustainability. The increase in workability allows proper installation into areas of low clearance, underwater placements, and areas where consolidating methods cannot be used. Which means savings on equipment, transportation, and time. When mix designing, with the addition of Glenium SKY admixture, the water-cement ratio is decreased, which results in a smaller amount of water and cement needed. Economically, this is a positive result, not only does it reduce the cost of the amount of cement needed it also reduces energy cost. According to the Ministry of Transportation, Ontario is increasing driver and vehicle fees in order to maintain bridges and roads. The price of maintenance has increased. Consequently this increases taxes and other fees. Using this admixture the amount of maintenance required can be reduced, with increase in tax will not be necessary.
Recycled Concrete
Concrete as one of the mostly used building materials when produced and transported creates a lot of CO2 and when disposed generates a huge amount of waste; therefore it causes a lot of concerns for environmental activists. In order to address these environmental issues, it is necessary to recycle the concrete when demolishing buildings built using concrete. Also reuse of this construction waste is important in terms of life Cycle Assessment that is the standard method of evaluating environmental impacts associated with different stage of products’ life, which includes recycling (22). There are 3 basic concepts to promote the proper reuse of the construction waste, (A) assurance of safety and quality, (B) decrease of environmental impact, and (C) increase of cost effectiveness of construction. In this paper we focus on some benefits of proper recycling of concrete for the environment.
First, we are going to address some of the main environmental problems with concrete. Concrete production emits huge amount of CO2, which is the main issue of this industry that leads into global warming. Up to 8% of all the CO2 produced in the world comes from concrete production. Using recycled concrete can dramatically reduce the amount of emitted CO2 and fight against global warming. Nitrous oxide emission and other articulated air emissions on one hand, and on the other hand the traffic congestion caused by delivery of the ready concrete wastes a lot of energy and cause air pollution. Water pollution and adverse effects of concrete on health are among the other problems that make concrete recycling more essential. (19)
In the past, the resulting concrete from demolishing the buildings was released in the environment which had enormous negative impacts. Conventionally recycling concrete has been considered as a difficult task, however recycling technology has been improved and now it has become a feasible technology. Recycling concrete has become a simple process that involves breaking the concrete pavements, removing them from the sites to the recycling machines that can be also installed near the construction sites and finally crushing the concrete into pieces that can vary in quality and size. (18)
Furthermore, recycling technology has reached the stage that can prepare the recycled concrete to produce superior recycled aggregate for structural concrete. Recycled concrete has become one of the best construction materials as it is stronger than new concrete. There are only few restrictions on the type of concrete that can be used as recycled concrete aggregates (RCA) (20).
Recycling now has become more common method of developing the waste produced by demolishing or renovating the structures made of concrete rather than transferring them by truck and leaving them in landfill. Environmental awareness and also the desire of contractors to keep construction costs as low as possible, has made concrete recycling an attractive proposal in any construction project involving concrete.
Unlike most of the materials, such as, glass, bottles or metals that can be reused to produce the same material, once concrete has been made from cement, it cannot be decomposed to its initial component of sand, cement and water. However, Crushed concrete can be combined with virgin aggregate in producing concrete. (17)
It is important to develop standardized guidelines to create new materials. These standards are needed for quality control of Recycled Concrete Aggregate (RCA), and the correct use of this recycled materials to produce new concrete.
Recycling can reduce the amount of waste concrete that must be landfilled so it saves landfill space by keeping the waste concrete out of landfill. In addition, it reduces the need of virgin aggregates which help to cuts the negative environmental issues of extraction process. Recycled concrete can be used as gravel and it reduces the need for gravel mining. Another positive impact of recycling is the reduction of transportation requirements to transfer the new material to the construction sites, which in turn can reduce air and water pollution significantly and also decrease the greenhouse emission.
One of the most important environmental advantages of concrete recycling rather than leaving the concrete in the landfill and buying the new material is to save up to 1,360 gallons water by recycling one ton of concrete. Using developed recycling system, to recycle the concrete waste produced from demolishing structures or roadways, can reduce t
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