Introduction Of Low Carbon Concrete And Environmental Impact Construction Essay

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Concrete is a very versatile and stable building material. For this reason, it is the second most widely used resource in the world after water. It is the most widely used building material in the world because of its beauty, strength and durability, among other benefits. Concrete is used in nearly every type of construction, including homes, buildings, roads, bridges, airports and subways, just to name a few. And in an era of increased attention on the environmental impact of construction, concrete performs well when compared to other building materials. As with any building product, production of concrete and its ingredients does require energy that in turn results in the generation of carbon dioxide, or CO2. The amount of CO2 produced during manufacturing and the net impact of using concrete as a building material is relatively small.

With the rapid climate changes in the recent times, it is important to focus on the biggest concern of our environment and make it to be a better place. As stated by Ecocem, "if an individual in their normal life reduced their emissions by 50% they might save 8 tons of CO2 annually," but "if professionals in the construction industry reduce the emissions of their projects during construction by 50% they can save up to 4,000,000 tons of CO2 annually" (Overview - National Climate Change Strategy, 2011). (ECOCEM, 2010).

The following features of concrete construction help minimize its carbon footprint (Erin Ashley, 2008):

Concrete is resource efficient and the ingredients require little processing.

Most of the materials for concrete are acquired and manufactured locally which minimize transportation energy.

Concrete building systems combine insulation with high thermal mass and low air infiltration to make homes and buildings more energy efficient.

Concrete has a long service life for buildings and transportation infrastructure, thereby increasing the period between reconstruction, repair and maintenance and the associated environmental impact.

Concrete, when used as pavement or exterior cladding, helps minimize the urban heat island effect thus reducing the energy required to heat and cool our homes and buildings.

Concrete incorporates recycled industrial by-products such as fly ash, slag and silica fume which helps reduce embodied energy, carbon footprint and quantity of land filled materials.

Concrete absorbs CO2 throughout its lifetime through a process called carbonation, helping reduce its carbon footprint.

Concrete production accounts for approximately 5% of worldwide greenhouse gas emissions. The majority of these emissions derive from the cement binder. Although developments in cement manufacture have led to a significant reduction in CO2 emissions, further substantial reductions will require radical change. Low carbon concrete is an improvement on normal concrete.

1.1 Background and Context

Due to the high emission of carbon dioxide into atmosphere, Cement is considered as one of the most environmentally hazardous materials in the world. According to the Sustainable Development Commission, 4% of Co2 is caused by aviation. Depending on how conservatively you do the sums, cement-based building materials, including concrete and asphalt, account for between 5% and 10% of all carbon dioxide emissions (Adam, 2007). Making the 2bn tons of cement used globally every year pumps out 5% of world's CO2 emission- more than the entire aviation industry. And the long-term trends are upwards: a recent report by the French bank Credit Agricole (Dodson, 2006) (jha, 2008) estimated that, by 2020, demand for cement will increase by 50% (Erin Ashley, 2008) compared to today. Finding an alternative product to cement would, therefore, make excellent environmental sense, especially if Britain is to meet the government's ambitious target of a 60% reduction in carbon dioxide emissions by 2050. Previous attempts to make cement greener have included adding more aggregate to a concrete mixture, thereby using less cement. But this still does not tackle the problem of the carbon emissions from making the cement in the first place.

1.2 Scope and Objectives

As discussed earlier this work aims to investigate about low carbon concrete and findings of a study of options for 'low carbon concrete'. The prompt for the study was a newspaper article discussing low carbon alternatives to concrete, including use of a bituminous binder from processing heavy fuel oils, which was claimed to have negative carbon emissions. At the outset, it was decided that the study should not focus on a single proprietary material but should review other alternatives to Portland cement binder. Concrete production accounts for approximately 5% of worldwide greenhouse gas emissions. Most of the embodied CO2 emissions in concrete derive from the Portland cement (Figure 1) and the worldwide production of Portland cement accounts for approximately 3% of annual CO2 emissions. Each tonne of Portland cement produced generates approximately 1 tonne of CO2; 42% derived from the fuel required in processing and burning the constituents and grinding the cement and 58% being a by-product of de-carbonation of the limestone within the kiln. A number of factors have acted to mitigate the potential carbon emissions associated with Portland cement use in concrete:

Changes in the production process have increased cement strength development and allowed the content to be reduced by over 25% for the same concrete strength.

Blending of Portland cement with secondary cementitious materials (such as slag and fly ash) resulting in an estimated annual UK reduction in greenhouse gas emissions equivalent to 1.2million tonnes CO2.

Use of water-reducing admixtures resulting in over a 10% reduction in cement content for the same concrete strength.

Improvements in kiln efficiency and use of alternative waste fuels (such as scrap tyres, waste solvents and biofuels) have reduced fuel consumption by 50% since the 1970's.

Despite significant work in the cement and concrete industry to enhance and emphasise the sustainability of concrete, including its whole life benefits of thermal mass and longevity, further significant reductions in embodied CO2 in concrete will require fuller application of these existing technologies across the industry, as well as more radical change. This is reflected in the results of the desk study.

The Main Objective of this dissertation to study the various low carbon concrete constituents which is being experimented worldwide and finding the constituents that emits very low carbon along with considering few conditions like, to use during flood alleviation and the availability of them in UK. Also, providing the amount of CO2 emission by the Conventional concrete containing these constituents.

The study comprises three stages:

A desk study to review candidate materials and strategies being developed in different countries.

Screening the options to produce a maximum short-list of three candidate materials.

A detailed appraisal of the shortlisted materials for engineering suitability and effectiveness in reducing CO2 emissions.

1.3 Achievements

An extensive desk study identified seven groups of technologies with potential to reduce the carbon footprint: secondary cementitious materials modified Portland or non-Portland cements, low cement concrete, ultra high strength concrete changes in Portland cement (PC) manufacture, alternative binders and carbon capture. A short list was produced of three candidates with potential for high CO2 reduction and suitability for construction: (i) Agent-C (a bituminous binder from the processing of heavy fuel oils); (ii) high slag binder comprising a blend of 20% Portland cement and 80% ground granulated blast furnace slag; and (iii) geo-polymer (an alumino-silicate material formed by the reaction of alumina and silicate rich materials with an alkali activator). These three candidates were studied in detail for engineering characteristics, availability, environmental impact and construction issues. The

CO2 emissions associated with manufacture and transport of the materials were calculated and recommendations made for their future development.

The test would be selected by the judgment choice methodology where the research test on a certain criterion supported. As discussed, earlier that the building industry becomes the more elaborately in the light of the new alternatives, environmental problems and the enclosed national economy.

Compare the different seven types of concrete with their co2 emissions during the various processing.

The observations made in this process would be graphically represented and statistically analysed for reaching an outcome.

The detailed appraisal of these three low carbon concrete constituents providing their engineering characteristics and the amount of carbon emission by the conventional concrete containing them. Also interviewing the concrete experts with questionnaire of these three constituents and analyse the result based on them.

1.4 Overview of Dissertation

Concrete production accounts for approximately 5% of worldwide greenhouse gas emissions. The majority of these emissions derive from the cement binder. Although developments in cement manufacture have led to a significant reduction in CO2 emissions, further substantial reductions will require radical change. This dissertation summarises the findings of a study of options for 'low carbon concrete'. The study was based on the paper in 2007 by UK Government's Environment Agency, through its Carbon Reduction Fund, with the aim of identifying low CO2 alternatives to concrete for use on flood alleviation schemes.

An extensive desk study identified seven groups of technologies with potential to reduce the carbon footprint: secondary cementitious materials modified Portland or non -Portland cements, low cement concrete, ultra high strength concrete changes in Portland cement (PC) manufacture alternative binders and carbon capture. A short list was produced of three candidates with potential for high CO2 reduction and suitability for construction: (i) Agent-C (a bituminous binder from the processing of heavy fuel oils); (ii) high slag binder comprising a blend of 20% Portland cement and 80% ground granulated blast furnace slag; and (iii) geopolymer (an alumino-silicate material formed by the reaction of alumina and silicate rich materials with an alkali activator). These three candidates were studied in detail for engineering characteristics, availability, environmental impact and construction issues. The CO2 emissions associated with manufacture and transport of the materials were calculated and recommendations made for their future development.

Chapter 2. Literature Review

2.1 Introduction of Low-Carbon concrete and environmental impact

Concrete is one of the building materials, which are being used widely in the world. This is because of its strength, durability and also many other benefits. It is been included in almost every type of construction, which includes housing, commercial buildings, bridges and roads. In attention on the environmental impact of construction, it performs and gives more benefits compared to other building materials. During any building product, the production of concrete and its ingredients does require energy, which in turn generates carbon dioxide. Even though the amount of CO2 produced during concrete manufacturing and the net impact of using it as building material is comparatively very small, the amount of CO2 emission can be decreased to minimum by usage of Low-Carbon concrete.

2.2 Feasible Low carbon concrete constituent types:

An initial desk study of construction industry databases and web sites indicated that the strategies being adopted or developed in twelve countries around the world could be divided into seven groups. The effect upon CO2 emissions of changes in kiln technology, such as oxygen enrichment and carbon sequestration, are not known. Introduction of such technology across the industry will be a slow process and given the need for immediate measures, it has not been considered further.

2.2.1 Secondary Cementitious Materials:

Secondary Cementitious Materials are one of the most sustainable construction materials which help to reduce the carbon footprint to a great amount. This includes Fly ash, ground-granulated blast-furnace slag (GGBS).Because of the following reasons they plays major role in development of Low Carbon environment.

When they incorporate into concrete, it recovers an industrial byproduct.

Industrial byproducts can be avoided completely.

Emission of green gas and usage of natural raw materials can be decreased due to the reduced content of Portland cement in concrete.

Durability of concrete can be improved by increasing the structure service.

The compressive strength of High Volume Fly Ash (HVFA) mixtures were 8, 55, and 80 MPa at 1, 28, and 182 days, respectively. Extensive laboratory tests (V.M, 1994) concluded that the Young's modulus of elasticity, creep, drying shrinkage, and freezing and thawing characteristics of HVFA concrete is comparable to normal Portland cement concrete. The HVFA concrete also has high resistance to water permeation and chloride-ion penetration.

Another by-product that is useful for cement substitution is ground-granulated blast-furnace (GGBF) slag. According to (Mehta, 1999) the world production of this slag is approximately 100 million tons per year; only approximately 25 million tons of slags are processed into the granulated form that has the cementitious properties. Because GGBF slag is derived as a by-product from the blast furnaces manufacturing iron, its use has environmental benefits. The use of GGBF slag in concrete significantly reduces the risk of damages caused by alkali-silica reaction, provides higher resistance to chloride ingress, reduces the risk of reinforcement corrosion, and provides high resistance to attacks by sulphate and other chemicals. The use of GGBF slag in concrete has increased in recent years and this trend is expected to continue.

Secondary Cementitious products comprise the glue that holds concrete together. These materials include traditional Portland cement and other cementitious materials, such as fly ash, ground granulated blast furnace slag (ggbs), limestone fines and silica fume. These materials are either combined at the cement works (to produce composite cement) or at the concrete mixer when the concrete is being produced (the cementitious product is called a combination in this case).

Fly ash and ggbs are the most commonly used of these materials in the UK. These secondary materials are useful by-products of other industrial processes, which would potentially otherwise be sent to landfill. Ggbs is a useful by-product recovered from the blast-furnaces used in the production of iron. It can be used un-ground as a coarse aggregate or as a supplementary cementitious material (where it can replace up to 70% of cement in a concrete mix). Fly ash is a useful by-product of coal-fired power stations and is environmentally beneficial. If it were not used in composite cements or as an addition at the concrete mixer then the material would be wasted and sent to landfill. Using ggbs or fly ash in concrete, either as a mixer addition or through a factory made cement, significantly reduces the overall greenhouse gas emissions associated with the production of concrete

According to (Concrete Credentials: Sustainability, 2010)Direct annual CO2 emissions have reduced by nearly 40 per cent since 1990 in absolute terms. The cement industry met the UK's 2010 Climate Change Agreement target four years in advance and is continuing its commitment to improvement. This compares favourably with the UK construction industry, which overall recorded an increase in CO2 of more than 30 per cent over the same period.

Silica fume is a 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 condensed silica fume contains between 85 and 98 percent silicon dioxide and consists of extremely fine spherical glassy particles (the average particle size is less than 0.1μm) (Tony C.Liu, 2008).

Because of its extreme fineness and high silicon dioxide content, condensed silica fume is a very efficient pozzolanic material. The worldwide production of silica fume is estimated to be about 2 million tons [Ref. 5]. Because of limited availability and the current high price relative to portland cement and other pozzolans or slag, silica fume is being used primarily as a property enhancing material [Ref. 10]. In this role, silica fume has been used to provide concrete with very high compressive strength or with very high level of durability or both. It has been used to produce concretes with reduced permeability for applications such as parking structures and bridge decks and for repair of abrasion damaged hydraulic structures.

(Tony C.Liu, 2008)One of the major barriers against the use of large quantities of fly ash and other supplementary cementing materials in concrete is the current prescriptive-type of specifications and codes. The prescriptive-type of specifications generally place limits on the maximum percentage of the cement that can be replaced by the supplementary cementing materials. For example, ACI 318 Building Code limits the maximum percentage of fly ash or other pozzolans to not exceed 25% of the total cementitious materials by mass for concrete exposed to deucing chemicals. High performance concrete mixtures being produced with HVFA concrete prove that prescriptive specifications hinder the widespread use of fly ash and other supplementary cementing materials.

Replacing the prescriptive-type of specifications and codes with performance-based specifications and codes will accelerate the rate of utilization of fly ash and other supplementary cementing materials and can provide economic and environmental benefits.

2.2.2 Non-Portland cements binders:

Non-Portland cements binders are the constituents which act as binders without involving Portland cement. Geopolymer is one of the best examples of this type of cement. Apart from geopolymer there are few other materials, which are made of calcium sulphate based and magnesite based cements. Also the materials that are made of calcium sulfoaluminate are also included in this category. Geopolymer is made of aluminosilicate materials with potential use in a number of areas, essentially as a replacement for Portland cement and for advanced high-tech composites, ceramic applications or as a form of cast stone. Geoploymers are example of the broader class of alkali-activated binders, which also includes alkali-activated metallurgical slags and other materials (Cajin shi, 2006)

The use of fly ash-based Geopolymer Concrete contributes to the potential for reduced global warming. A recent life cycle assessment of geopolymer concretes indicates that the global warming potential (GWP) of geopolymer concretes is between 26 and 45% lower compared to ordinary Portland cement concrete (Stengel, 2009). However, when other ecological impact factors are considered, geopolymer concrete does not rate as favourably as Portland cement concrete. This is largely ascribed to the sodium silicate and sodium hydroxide production (Stengel, 2009). The impact of each depends upon the processing employed. The use of alkaline solutions form waste streams of other processes, such as aluminium processing, may provide potential reduction in the environmental impact of geopolymer concrete.

According to (Mcleod, 2005) Geopolymeric cements are the only products reviewed that are clearly capable of achieving low carbon footprints.

2.2.3 Low cement concrete:

These are the concrete mixtures with low cement content used for blinding off and typically of low compressive strength. They are also known as Lean Mix Concrete.

2.2.4 Ultra high strength concrete:

The term Ultra High Performance Concrete (UHPC) has been used to describe a fibre-reinforced, super plasticized, silica fume-cement mixture with very low water-cement ratio (w/c) characterized by the presence of very fine quartz sand (0.15-0.40 mm) instead of ordinary aggregate (A.R.Lubbers, 2003). The absence of coarse aggregate was considered by the inventors to be a key-aspect for the microstructure and the performance of the UHPC in order to reduce heterogeneity between the cement matrix and the aggregate. However, due to the use of very fine sand instead of ordinary aggregate, the cement density of UHPC is as high as (900-1000 kg/m3) (S. Collepardi, 1996). One of the primary benefits of this class of concrete is that it can exhibit significant tensile strength and toughness. Much of such properties enhancement is imparted to the concrete by the addition of short, discontinuous fibres during the mixing procedure (P.E.Hartman, 2000)

2.2.5 Changes in Portland cement Manufacture:

Changes in Portland cement Manufacture can be obtained by either adding few materials in the manufacturing stage else replacing the Ordinary Portland cement. It is reduced CO2 cement. They are energy efficient production and follows environmental management system and it sustains fuel.

Oxygen enrichment of kiln atmosphere to enhance burning Belite cements, Alinite and Fluoralinite cement and Portland limestone cement are few examples of cements which are manufactured by making changes in Ordinary Portland cement.

2.2.6 Alternative Binder Type:

To reduce the production of greenhouse gas (CO2), people should attempt to use less cement in concrete. To achieve this, usage of cement in concrete has to be minimized to the least and at the same time; the strength and durability of the concrete should not be collapsed. There are bituminous-based binders which acts helps to reduce the CO2 emission from the concrete to the maximum.

Asphalt is one of the bitumen that is mainly used for the road construction to replace the cement concrete. Asphalt concrete, commonly known as asphalt, is used in the construction of highways and roads. It is produced in a variety of mixtures, including hot mix, warm mix, cold mix, cutback, mastic, and natural, each with distinct material and energy inputs. A highway or road is built in several layers, including pavement, base, and sub-base. (Levis, 2008)

2.2.7 Carbon Capture:

Limecrete is the method of carbon capture that helps to reduce the CO2 emission from concrete. Ty- Mawr Ltd designed this innovative flooring system in 1998 as an alternative to cement-based concrete for use in old and historic properties. Hempcrete is a combination of chopped hemp shiv and binder comprising of natural hydraulic lime and a small amount of cement. It is firm and self-insulating. Hempcrete is suitable for uses such as timber frame infill, insulation and, with the addition of aggregate, floor slabs. Hemp is a renewable biomaterial and lime is an abundant quarried material. Hempcrete is carbon negative and the obvious choice for buildings aiming to achieve a low carbon footprint and the highest sustainable building code levels.

According to (Ty- Mawr Ltd, n.d.) Hempcrete is carbon negative. A 300mm hemcrete wall absorbs in its construction 40kg per m2 CO2. A typical brick and block wall emits 100kg giving a net benefit of 140kg. We use British hemp. Hempcrete is also recyclable at the eventual end of the life of the building.

2.3 CO2 Emissions relative to Portland cement Concrete:

CO2 Emissions from various low carbon concrete constituents can be compared relatively with the Ordinary Portland cement concrete CO2 Emission respective to the percentage.

Based on the various experiments which is carried out to find the CO2 emission from the different type of concrete which is explained before are compared to the CO2 emission from the Portland cement concrete.

According to (Wimpenny, 2009), they are classified into five different categories as mentioned in the table below:

Category

CO2 emissions relative to Portland cement concrete

(%)

Very high

>85

High

66-85

Medium

33-66

Low

0-33

Very Low

<0

Long- term effects such as carbon capture by carbonation or extended service life were not considered in the screening process. A very approximate assessment of CO2 emission was undertaken based on the potential reduction in the cement content or concrete volume or the embodied CO2 of the binder relative to Portland cement. The CO2 emissions for the options were categorised as shown above.

CO2 emissions less than 0% indicate that the material 'captures' CO2 by incorporating a component that could release CO2 into the atmosphere if used or disposed of in an alternative way.

Carbon dioxide emission in UK through concrete manufacturing:

The average CO2 composition changes from year-to-year ('a rolling concrete mix') and reflects changes in the concrete mix used by the market. According to (Sustainable Concrete Forum, 2010) the rolling concrete mix, there has been an increase of 7% for 2010, compared to 2009, which still represents a reduction of 12.3% under the 1990 baseline. The increase from 2009 is the result of a higher average cement content of the rolling concrete mix. Using the same calculation, but with a standardised (fixed) concrete mix, the CO2 emissions have not changed significantly between 2009 and 2010. Compared to the 1990 baseline the 2010 data shows a 16.3% reduction. The CO2 emissions - Production (standardised mix) indicator measures improvements in the production of concrete and based on this indicator we are on track to meet our 2012 target.

(Sustainable Concrete Forum, 2010)Approximately half of the carbon emissions in the UK's Carbon Budget are covered by the European Union Emission Trading Scheme (EU ETS). In the case of concrete, the majority of emissions are from cement, which is covered by the EU ETS and is actively managed in order to meet the EU carbon reduction targets. The continuing investment in new technology and use of biomass fuels has enabled further reductions in the CO2 emissions from the production of cement. In most of the other sectors slight increases in the CO2 emissions per tonne have been observed. (Sustainable Concrete Forum, 2010) Consistent with the energy efficiency indicator, whilst the industry has continued to invest in energy and carbon efficiency measures, the fixed energy requirement of the manufacturing assets being allocated over a reduced volume of production during the market downturn has largely off-set the gains made. (Sustainable Concrete Forum, 2010) The inclusion of the UK average reinforcement content to the concrete has increased the CO2 emissions by around 9%. The emissions associated with the production and fabrication of steel reinforcement indicates a slight reduction in 2010 compared to 2009. All of the steel produced by BAR members is produced using Electric Arc Furnace which uses scrap material as its main raw material. As a consequence steel reinforcement bar has approximately a third of the embodied energy and a fifth of the carbon impact of primary steel production.

Geo Polymer Concrete and its Carbon footprint:

Geopolymer is a term used to describe a family of alumino-silicate materials formed by the reaction of alumina and silicate rich materials with a metal alkali activator. A key parameter in determining the characteristics and use of the geopolymer is the silica:alumina ratio, as indicated in Table 5. For civil engineering a silica:alumina ratio of approximately 2 is required.

The alumino-silicate component of geopolymer can be derived from materials such as metakaolin, slag or fly ash. The accelerator would typically be a potassium or sodium hydroxide or silicate. The calcium and carbon content of the binder have to be controlled to avoid variable properties. Fly ash should have a loss on ignition less than 7% and derive from a sub-bituminous coal with calcium content less than 10%.

The Commonwealth Scientific and Industrial Research Organization (CSIRO) has investigated the use of the material in construction, including marine structures, fluid containment and conduits, although it admits that the transition from a well-established, material such as cement to a novel one is difficult. Trials undertaken by Curtin University of Technology, Perth, Australia on reinforced beams and columns have proved promising (11) and concrete is being produced commercially using geopolymer binder (12). However, no published examples have been obtained of the material being used in reinforced structural elements in the field.

By virtue of their stable alumino-silicate structure, it would be expected that geopolymers would have good durability and they have been considered for stabilisation and encapsulation of the nuclear waste. Fire resistance should be superior to conventional concrete because of the negligible water in the mix and the sulfate and acid resistance should be enhanced because of the lack of calcium bearing material in the binder. The alkalinity of the constituents should provide a sufficiently high pH to passivate steel reinforcement to corrosion.

The short setting time of these materials and the need with some combinations for heat treatment is a major drawback, which is likely to restrict its use in the first instance to precast items, specialist fast-setting repair materials and sprayed concrete.

There are no standards and codes covering the use of geopolymer in construction and this would most likely be facilitated by testing against a performance specification.

With the current cement production growth ranges from 5% to 16% and suggest an average growth rate in favour of the 5% increase scenario. In 25years from now, world cement CO2 emission could equal the 3,500 million tonnes total CO2 production of Europe(EC); industry+energy+transportation, or 18% of present world CO2 emission (Davidovits, 1994). This addresses the need for a drastic change in the cementitious systems involved in the utilization of concrete, through the manufacture of new type of cement which doesn't involve the calcination of limestone which release CO2.

To produce concrete, instead of cement paste, geopolymer is used as the binder. Then this geopolymer paste helps to bind the fine and coarse aggregates along with other unreacted materials together to form the geopolymer concrete. Using the usual concrete technology methods, the geopolymer concrete can be manufactured. As in the Portland cement concrete, the aggregates occupy the largest volume, that is, approximately 75 to 80% by mass, in geopolymer concrete (Djwantoro Hardjito, 2004). The silicon and the aluminium in the fly ash are activated by a combination of sodium hydroxide and sodium silicate solutions to form the geopolymer paste that binds the aggregates and other unreacted materials (Djwantoro Hardjito, 2004).

Geopolymer is a type of amorphous alumino-silicate cementitious material. Geopolymer cement is also acid-resistant, because unlike the Portland cement, geopolymer cements do not rely on lime and are not dissolved by acidic solutions. Geopolymer can be synthesized by polycondensation reaction of geopolymeric precursor, and alkali polysilicates. Comparing to Portland cement, the production of geopolymers consume less energy and almost no CO2 emission. (Djwantoro Hardjito, 2004) Geopolymers are not only energy efficient and environment friendly, but also have a relative higher strength, excellent volume stability, better durability, good fire resistance, and easy manufacture process. Thus geopolymer will become one of the perspective sustainable cementitious materials in 21st century.

Geopolymer concrete is seen as a potential alternative to standard concrete, and an opportunity to convert a variety of waste streams into useful by-products. One key driver in geopolymer development is the desire to reduce greenhouse gas emissions from the production of concrete products.

Comparison of Low carbon constituents and Ordinary Portland cement Concrete:

The carbon impacts of Ordinary Portland Cement (OPC) and geopolymers in an Australian context, with an identification of some key challenges for geopolymer development. The results of the examination show that there is wide variation in the calculated financial and environmental "cost" of geopolymers, which can be beneficial or detrimental depending on the source location, the energy source and the mode of transport. (Djwantoro Hardjito, 2004) Some case study geopolymer concrete mixes based on typical Australian feedstocks indicate potential for a 44-64% reduction in greenhouse gas emissions while the financial costs are 7% lower to 39% higher compared with OPC. Geopolymer, with properties such as abundant raw resource, little CO2 emission, less energy consumption, low production cost, high early strength, fast setting, these properties make geopolymer find great applications in many fields of industry such as civil engineering, automotive and aerospace industries, non-ferrous foundries and metallurgy, plastics industries, waste management, art and decoration, and retrofit of buildings. Global warming and energy saves (Djwantoro Hardjito, 2004). It is well known that a great amount of CO2 is emitted during the production of Portland cement, which is one of the main reasons for the global warming (Djwantoro Hardjito, 2004). Studies have shown that one ton of carbon dioxide gas is released into the atmosphere for every ton of Portland cement which is made anywhere in the world (Djwantoro Hardjito, 2004). In contrast, geopolymer cement is manufactured in a different way than that of Portland cement. (Djwantoro Hardjito, 2004)It does not require extreme high temperature treatment of limestone. Only low temperature processing of naturally occurring or directly man-made alumino-silicates (kaoline or fly ash) provides suitable geopolymeric raw materials. These lead to the significant reduce in the energy consumption and the CO2 emission. It is reported by (Davidovits, 1994) that about less 3/5 energy was required and 80%-90% less CO2 is generated for the production of geopolymer than that of Portland cement. Thus it is of great significance in environmental protection for the development and application of geopolymer cement.

Ground Granulated Blast furnace Slag (GGBS) in Concrete:

Another by-product that is useful for cement substitution is ground-granulated blast-furnace (GGBF) slag. Although the world production of this slag is approximately 100 million tons per year, only approximately 25 million tons of slags are processed into the granulated form that has the cementitious properties (Mehta, 1999). Because GGBF slag is derived as a by-product from the blast-furnaces manufacturing iron, its use has environmental benefits. The use of GGBF slag in concrete significantly reduces the risk of damages caused by alkali-silica reaction, provides higher resistance to chloride ingress, reduces the risk of reinforcement corrosion, and provides high resistance to attacks by sulphate and other chemicals. The use of GGBF slag in concrete has increased in recent years and this trend is expected to continue.

Laboratory work by Lang and Geiseler (Lang, 1996) on a German blast furnace slag cement (405 m2/kg specific surface area) containing 77.8 per cent slag showed that excellent mechanical and durability characteristics were achieved in super-plasticized concrete mixtures with 455 kg/m3 cement content and 0.28 w/cm. The compressive strengths at ages 1, 2, 7, and 28 days were 13, 37, 58, and 91 MPa, respectively. The concrete also showed good resistance to carbonation, penetration of organic liquids, freezing and thawing cycles (without air entrainment), and salt scaling.

(Tony C.Liu, 2008)Approximately 5 million tons of GGBF slags were used in concrete mixtures annually in Taiwan and also Up to 55% of the Portland cement (ASTM Type V) had been replaced by GGBF slag in concrete mixtures where high sulphate resistance is required and in the moderate sulphate resistance applications, 45% of Portland cement (ASTM Type II) can be replaced by GGBF slag with excellent performance. Concrete containing 45-50% of GGBF slag was commonly used for concrete slurry wall constructions in Taiwan (Tony C.Liu, 2008).

(Hanson Heidelberg Cement Group, 2009)The use of cement replacements with lower environmental burdens offers opportunities for significant reductions in energy use and carbon dioxide emissions. The most effective alternative to Portland cement is ground granulated blast furnace slag (GGBS), which typically replaces 50 per cent of the Portland cement in a concrete mix. Greater proportions of up to 70 or even 80 per cent can be used with advantage in suitable situations (Hanson Heidelberg Cement Group, 2009).

The uses of GGBS in concrete are to reduce emission of carbon dioxide, conserve natural resources and also it saves energy.

As a result of its low environmental impacts, using GGBS can reduce significantly many of the environmental burdens associated with concrete. The following table shows the environmental benefits of replacing 50 per cent of the Portland cement with GGBS in a typical concrete mix.

Environmental issue

Effect of using GGBS

Emission of Carbon dioxide

40% reduction

Acidification

35% reduction

Winter smog

35% reduction

Eutrophication

30% reduction

Primary energy requirement

30% reduction

Source: Higgins D, Parrott L & Sear L, Effects of GGBS and PFA upon the environmental impacts of concrete, CIA/DETR Partners in Technology Project, 2000.

(Hanson Heidelberg Cement Group, 2009)To allow comparison of products, the Building Research Establishment has produced a scoring system that allocates Ecopoints to materials - the more Ecopoints, the larger the impact on the environment. Under this method GGBS scores 0.47 Ecopoints, while Portland cement scores 4.6 Ecopoints. (Hanson Heidelberg Cement Group, 2009) This classes GGBS as having only one tenth of the environmental impact of Portland cement. (Hanson Heidelberg Cement Group, 2009) BRE has also released Green Guide to Specification ratings for use with the government's Code for Sustainable Homes. This guide includes examples of 50 per cent GGBS concrete, which consistently achieves improved sustainability ratings relative both to normal concrete and concrete made with fly ash (Hanson Heidelberg Cement Group, 2009).

Asphalt Concrete

Carbon footprints of Low carbon concrete comparison with conventional concrete:

According to (Wimpenny, 2009)CO2 emissions were calculated for conventional concrete and the candidate materials allowing for production of the primary materials, secondary processing and transport (Table 7). These calculations include the energy for heating Agent-C, heated mix water for high slag and steam-curing the geopolymer. The EA carbon converter was used, which prescribes the CO2 emissions arising from different fuel types, eg 2.63 kg of CO2 for 1 litre of diesel. The conventional concrete was assumed to have a 350kg/m3 binder content comprising 40% slag. The notional use was at a construction site in St Ives, Cornwall, UK.

Table 7. Summary of CO2 calculation

Concrete Type

kg CO2 per cubic metre of concrete

Primary processing and Constituents

Secondary processing

Transport

Total

Conventional

202.4

29.9

16.2

248

Asphalt Concrete(Best)

-501.3

58.3

21.2

-422

Asphalt Concrete(Realistic)

-18.3

58.3

21.2

61

GGBS

99.2

29.9

14.8

144

Geopolymer

23.2

34.4

20.9

78

Source: Report on Low Carbon Concrete-Options for the Next Generation of Infrastructure by Don Wimpenny in 2009

The assumptions made in the calculation are extremely important. For example, it can be observed that the CO2 reduction of the Agent-C differs significantly between the 'best case', where the VFCR is replaced by a zero carbon fuel, and the 'realistic case', where it is replaced by a light fuel oil (Wimpenny, 2009).

(Wimpenny, 2009)The potential CO2 reduction associated with the three materials has been calculated based on the availability of the binder constituents in the UK and for the potential for use in place of conventional concrete based on the notional site and three other infrastructure schemes in the same locality (Table 8). The average amount of concrete used at the three schemes was approximately 970m3 representing 237 tonnes of CO2 when using conventional concrete.

It can be observed that high slag has the greatest availability and potential for CO2 reduction (40%). The limited opportunity to use Agent-C to replace concrete in infrastructure schemes restricts its impact to a 2% reduction in CO2. This value would be expected to be higher for schemes requiring armour blocks. (Wimpenny, 2009)Geopolymer is unlikely to be widely available in the next 10 years, until there is confidence in the material and sources of supply are established. Its potential use in the table is conservatively restricted to precast items and acid-resistant sewer linings, giving a modest 7% CO2 reduction..

Table 8. Potential CO2 reduction for candidate materials

Estimated CO2 reduction per annum

Material

CO2 reduction (thousand tonnes) in UK Based on availability of constituents

CO2reduction (tonnes) for typical

infrastructure scheme

Now

5years

10years

Asphalt

0

187

Future availability may limit use

4

GGBS

315

315

96

Geopolymer

0

0

850

17

Source: Report on Low Carbon Concrete-Options for the Next Generation of Infrastructure by Don Wimpenny in 2009

Chapter 3: Case Studies

3.1 Usage of ground granulated blast-furnace slag (GGBS) in White River Place, St Austell, UK:

3.1.1 Background:

This £75 million scheme, which involves the renovation of a brownfield site into a stimulating town centre for St Austell, features a mix of uses over seven floors including retail, catering, offices, cinema and a 550 space car park. 

The White River Place development was part funded by the South West Regional Development Agency (SWRDA) and the contractual requirement was to achieve BREEAM Very Good. But SWRDA and the joint venture client White River Developments had aspirations to achieve BREEAM Excellent, which were shared by the contractor Sir Robert McAlpine.

3.1.2 Overview of Environmental Features:

Rather than implementing costly elements that pulled the score up, the team and client applied themselves to attaining all available credits by:

Maximising the materials credits with effective procurement and awareness of issues,

Use of local labour and materials,

Highly efficient use of water and rainwater harvesting,

Implementing effective ecological input on a brownfield site,

Parkmark compliance for the car parking,

Implementing environmental policies and management.

3.1.3 The BREEAM Assessment:

From the outset the BREEAM Assessors coordinated the information required and assisted the team in providing it. By taking this route the project scored very highly in Land Use and Ecology, Materials and Waste, Transport and Water. Full marks were gained in the Management section by using exemplar construction processes, commissioning and environmental procedures. 

The BREEAM Assessment was an integral part of project meetings and continued through the project, allowing the principles of the guidance to be reviewed and implemented at very early stages so as not to impact on the cost or programme.

3.1.4 Building Services:

The mechanical and electrical strategy was designed from the outset with economy and sustainability in mind, but not at the expense of functionality or comfort.

Using passive technologies in a busy town centre was always going to be a challenge. They were employed where possible, but supplemented by low carbon products with high efficiencies, such as heat pumps.

The large expanse of roofing meant that rainwater needed to be attenuated to control flow into the drains, but was an opportunity to provide a large rainwater harvesting system, giving 'free' water for use, not only in landlord areas, but also for irrigation, wash down and to individual tenant premises.

Carbon monoxide sensors are linked to pulse fans in the car park which run at low speed when required to provide fresh air. These fans are also linked to the smoke control system and when the fire alarms are activated can be used to create safe fire brigade access.  

3.1.5 CO2 footprint:

The main contractor has ISO 14001 EMS systems and set very high standards for environmental management on site, with a dedicated environmental manager on site throughout the contract.

As major town centre site, liaison and community interaction and involvement were important and Considerate Contractors scores were very high.

Responsible sourcing was also important - sub-contractors and suppliers were questioned about the environmental credentials of materials used. During the construction, sustainably sourced materials (including a high content of recycled material) will be used wherever possible and all major building elements have a Green Guide 'A' rating.

Concrete was supplemented with ground granulated blast-furnace slag (GGBS) where slabs were not required to be major load bearing, so reducing the CO2 emissions associated with concrete. Local stone was used, bricks came from neighbouring Devon and concrete blocks were sourced from St Austell. Slightly damaged blocks were used for areas of the building where finishes allowed, saving some 60 skips worth of construction waste. The plans specified Western Red Cedar timber from South West England, rather than Canada.

A green transport plan has been created and a strong pedestrian link is incorporated in the design to encourage movement throughout the town centre. 

All jobs associated with the project have been advertised locally, and a local training and education programme has been operated, with weekly visits for GCSE, ONC and HNC construction students. Apprentices have also been employed across the site.

3.1.6 Project Team Details:

Client: White River Developments Ltd,

Architect: Chetwoods Architects,

Developer: Sir Robert McAlpine Ltd,

Building Services: Hulley & Kirkwood,

PM/QS: HCD Management,

BREEAM Assessors: GBSPM Ltd/TPS Consult.

3.2 Radcot Weir Project on Carbon Reduction

3.2.1 Background:

The Radcot Weir project is one of five sites in the Paddle and Rymer package. The main construction work at Radcot involves removing the existing paddle and Rymer structure and replacing it with a dipping radial gated weir. This will benefit operators of the weir through the removal of H&S risk, easier operation and standardisation of the weir. In addition the project will provide a new bypass channel to allow upstream and downstream movement of fish and provide recreational use for canoeists.

3.2.2 Reducing the Carbon Footprint:

Using the EX construction Carbon Calculator, the total carbon footprint of the Radcot weir project is- 600 tonnes. The most significant contributions are from concrete and steel. The project team by looking at material selection have saved around 50 tonnes of CO2 during the first phase of works.

3.2.2.1 Use of Granular Ground Blast Furnace Slag (ggbs) in Concrete:

50% of ggbs replacement was used in the base and 70% in the rest of the structure. This saved around 40tonnes of CO2 (a saving of over 60% when compared to a CEM1 concrete).

3.2.2.2 Re-Use of Material:

The old structure was demolished and then crushed onsite, this material was then used underneath the building instead of a primary aggregate saving both the cost and carbon of disposal of the material and the import of primary aggregate. This saved around 5.2 tonnes of CO2, resulted in 40 less lorry movement and saved £10 K.

3.2.2.3 Avoiding Waste:

The team are casting the coping stones for the structure on site by using the left over concrete pours which would normally be wasted. The coping stones are being produced over time meaning that no extra concrete is ordered for their production saving around 1.2 tonnes of CO2 and around £2-3 K

3.2.3 Additional benefits:

The big win from the project was the use of ggbs as a replacement in the concrete, this also has many other benefits apart from the saving in CO2 including lighter colour (desirable in this case), lower early thermal cracking, higher strength development over time and increased durability and increased workability. A potential disadvantage was noted as slower early strength development/ longer striking time which can increase construction programme hence only 50% replacement being used in the base, but in this case the concrete typically achieved strength of 30N/mm2 at seven days.

These reductions as from only half of the project, the second phase which starts next year is estimated to save at least an equivalent amount resulting in a final saving of around 100 tonnes which is almost 20% of the estimated project footprint as well as potentially £25K in costs. For future projects, the team is looking at saving further CO2 by potentially reducing the criteria for crack control steel reinforcement.

3.2.4 Project Team Details:

Company: Environment Agency

Prepared by: Gary Haley

Date of Issue: November 2010

Chapter 4: Conclusions and Recommendation

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