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Throughout history, the use of concrete as a building material has contributed significantly to the built environment. Enduring examples of various forms of concrete can be found as far back as the early Egyptian civilisation. Significant building remnants still exist from the Roman civilization, which used concretes made from naturally occurring volcanic ash pozzolans, mixed with water, sand and stone. Now concrete is being used in the construction of durable bridges, roads, water supply, hospitals, churches, houses and commercial buildings, to give people a social foundation, a thriving economy, and serviceable facilities for many years. In the modern era, the properties of concrete were refined in the late 1800s, with the introduction of a patented manufacturing process for portland cement. While it has ancient roots, concrete, as we know it today, is a modern and highly advanced building material. In the last 150 years, concrete has become one of the most widely used building materials on earth.
Figure . Tranditional tools used for concrete mixing.
Concrete is one of the most widely used construction materials in the world. However, the
production of portland cement, an essential material in concrete, leads to the release of significant amount of CO2, a greenhouse gas. One ton of portland cement clinker production is said to creates approximately one ton of CO2 and other greenhouse gases. Environmental issues are playing an important role in the sustainable development of the cement and concrete industry. For example, if we run out of limestone, as it is predicted to happen in some places, then we cannot produce portland cement; and, therefore, we cannot produce concrete and all the employment associated with the concrete industry goes out-of-business. A sustainable concrete structure is one that is constructed so that the total environmental impact during its entire life cycle is minimal. Concrete is a sustainable material because it has a very low inherent energy requirement and is produced to order as needed with very little waste. It is made from some of the most plentiful resources on earth and has a very high thermal mass. It can be made with recycled materials and is completely recyclable. Sustainable design and construction of structures have a small impact on the environment. Use of "green" materials embodies low energy costs. Their use must have high durability and low maintenance leading to sustainable construction materials. High performance cements and concrete can reduce the amount of cementitious materials and total volume of concrete required. Concrete must keep evolving to satisfy the increasing demands of all its users. Reuse of post-consumer wastes and industrial byproducts in concrete is necessary to produce even "greener" concrete. "Greener" concrete also improves air quality, minimizes solid wastes, and leads to sustainable cement and concrete industry.
What is Sustainable Concrete?
Concrete is a very environmentally friendly material. Concrete has been used for over 2,000 years. Concrete is best known for its long-lasting and dependable nature. However, additional ways that concrete contributes to social progress, economic growth, and environmental protection are often overlooked. Concrete structures are superior in energy performance. They provide flexibility in design as well as affordability, and are environmentally more responsible than steel or aluminum structures.
Entire geographical regions are running out of limestone resource to produce cement. Major metropolitan areas are running out of sources of aggregates for making concrete. Sustainability requires that engineers consider a building's "lifecycle" cost extended over the useful lifetime. This includes the building construction, maintenance, demolition, and recycling [ACI 2004].
A sustainable concrete structure is one that is constructed so that the total societal impact during its entire life cycle, including during its use, is minimal. Designing for sustainability means accounting in the design and also the short-term and long-term consequences of the societal impact. Therefore, durability is the key issue. New generation of admixtures/additives are needed to improve durability. To build in a sustainable manner and conduct scheduled & appropriate building maintenance are the keys that represent the "new construction ideology" of this generation. In particular, to build in a sustainable manner means to focus attention on physical, environmental, and technological resources, problems related to human health, energy conservation of new and existing buildings, and control of construction technologies and methods.
Environmental Issues with Concrete
The production of portland cement releases CO2 and other greenhouse gases (GHGs) into the atmosphere. Total CO2 emissions worldwide were 21 billion tons in 2002, Table 1.
Environmental issues associated with the CO2 emissions from the production of portland cement, energy demand (six-million BTU of energy needed per ton of cement production), resource conservation consideration, and economic impact due to the high cost of portland cement manufacturing plants demand that supplementary cementing materials in general and fly ash in particular be used in increasing quantities to replace portland cement in concrete [Malhotra 1997, 2004]. Fly ash is a by-product of the combustion of pulverized coal in thermal power plants. The dust collection system removes the fly ash, as a fine particulate residue from the combustion gases before they are discharged in the atmosphere. For each ton of portland cement clinker, 3 to 20 lbs. of NOx are released into the atmosphere. In 2000, the worldwide cement clinker production was approximately 1.6 billion tons [Malhotra 2004]. Longer lasting concrete structures reduce energy needs for maintenance and reconstruction. Concrete is a locally available material; therefore, transportation cost to the project site is reduced. Light colored concrete walls reduce interior lighting requirements. Permeable concrete pavement and interlocking concrete pavers can be used to reduce runoff and allow water to return to the water table. Therefore, concrete is, in many ways, environmentally friendly material. As good engineers, we must use more of it [Malhotra 2004]. In view of the energy and greenhouse gas emission concerns in the manufacturing of Portland cement, it is imperative that either new environmentally friendly cement-manufacturing technologies be developed or substitute materials be found to replace a major part of the portland cement for use in the concrete industry [Malhotra 2004].
Energy consumption is the biggest environmental concern with cement and concrete production. Cement production is one of the most energy intensive of all industrial manufacturing processes. Including direct fuel use for mining and transporting raw materials, cement production takes about six million BTU's for every ton of cement. The industry's heavy reliance on coal leads to especially high emission levels of CO2, nitrous oxide, and sulphur, among other pollutants. A sizeable portion of the electricity used is also generated from coal.
What types of materials are being used to make sustainable concrete?
Coal combustion products (CCPs)
It is important to develop recycling technology for high-volume applications of coal combustion products (CCPs) generated by using both conventional and clean-coal technologies. Many different types of CCPs are produced; for example, fly ash, bottom ash, cyclone-boiler slag, and clean coal ash. In general some of these CCPs can be used as a supplementary cementitious materials and the use of portland cement, therefore, can be reduced. The production of CCPs in USA is about 120 million tons per year in 2004. Cyclone-boiler slag is 100% recycled. Overall recycling rate of all CCPs is about 40%.
Figure . Fly ash is a byproduct of coal burning power plants.
Today's use of other pozzolans, such as rice-husk ash, wood ash, GGBFS, silica fume, and other similar pozzolanic materials such as volcanic ash, natural pozzolans, diatomite (diatomaceous earth), calcined clay/shale, metakaolin, very fine clean-coal ash (microash), limestone powder, and fine glass can reduce the use of manufactured portland cement, and make concrete more durable, as well as reduce GHG emissions. Chemical composition of ASTM Type I portland cement and selected pozzolans is given in Table 2.
Recycled- Aggregate Concrete
Recycled-aggregate concrete (RAC) for structural use can be prepared by completely substituting natural aggregate, in order to achieve the same strength class as the reference concrete, manufactured by using only natural aggregates. This is obviously a frustration, since a large stream of recycled aggregates to allow for full substitution of natural aggregates is not available. However, it is useful to prove that to manufacture structural concrete by partly substituting natural with recycled aggregates by up to fifty percent is indeed feasible. In any case, if the adoption of a very low water to cement ratio implies unsustainably high amounts of cement in the concrete mixture, recycled-aggregate concrete may also be manufactured by using a water-reducing admixture in order to lower both water and cement dosage, or even by adding fly ash as a partial fine aggregate replacement and by using a super plasticizer to achieve the required workability.
High-volume fly ash recycled aggregate concrete (HVFA-RAC) can be manufactured with a water to cement ratio of 0.60, by simultaneously adding to the mixture as much fly ash as cement, and substituting the fine aggregate fraction. Thus, water to cementitious material ratio of 0.30 is obtained enabling the concrete to reach the required strength class (Table 3). This procedure is essential for designing an environmentally-friendly concrete. All the concretes can be prepared maintaining the same fluid consistency by proper addition of an appropriate class of a super plasticizer.
SUSTAINABLE CONCRETE SOLUTIONS
Concrete is a strong, durable, low environmental impact, building material. It is the cornerstone for building construction and infrastructure that can put future generations on the road towards a sustainable future [Cement Association of Canada 2004]. Benefits of concrete construction are many, for example [Cement Association of Canada 2004]: concrete buildings - reduce maintenance and energy use; concrete highways - reduce fuel consumed by heavily loaded trucks; insulating concrete homes - reduce energy usage by 40% or more; fly ash, cement kiln dust, or cement-based solidification/stabilization and in-situ treatment of waste for brownfield redevelopment; and, agriculture waste containment - reduces odor and prevents groundwater contamination. The concrete industry must show leadership and resolve, and make contribution to the sustainable development of the industry in the 21 century by adopting new technologies to reduce emission of the greenhousegases, and thus contribute towards meeting the goals and objectives set at the 1997 Kyoto Protocol. The manufacturing of portland cement is one such industry [Malhotra 2004].
6 PORTLAND CEMENT
Portland cement is not environmentally very friendly material. As good engineers, we must reduce its use in concrete [Malhotra 2004]; and, we must use more blended cements, especially with chemical admixtures. Clinker production is the most energy-intensive stage in cement production, accounting for over 90% of total energy use, and virtually all of the fuel use.
Processing of raw materials in large kilns produces portland cement clinker. These kiln systems evaporate the inherent water in the raw materials blended to manufacture the clinker, calcine the carbonate constituents (calcinations), and form cement minerals (clinkerization) [Worrell & Galtisky 2004].
6.1 Blended cements
The production of blended cements involves the intergrinding of clinker with one or more additives; e.g., fly ash,bnb granulated blast furnace slag, silica fume, volcanic ash, in various proportions. The use of blended cements is a particularly attractive efficiency option since the intergrinding of clinker with other additives not only allows for a reduction in the energy used (and reduced GHG emissions) in clinker production, but also directly corresponds to a reduction in carbon dioxide emissions in calcinations as well. Blended cement has been used for many decades around the world [Worrell & Galtisky 2004].
6.2 Concrete and the use of blended cements
Although it is most common to make use of supplementary cementing materials (SCM) in the replacement of cement in the concrete mixture, blended cement is produced at the grinding stage of cement production where fly ash, blast furnace slag, or silica fume are added to the cement itself. The advantages include expanded production capacity, reduced CO2 emissions, reduced fuel consumption and close monitoring of the quality of SCMs [Cement Association of Canada 2004]. "Kyoto Protocol (UN Pact of 1997, requires to reduce GHGs, including CO2)." It is now ratified. USA has not ratified it. "The Russian Government approval allowed it to come into force worldwide." By 2012, emissions must be cut below 1990 levels (in Japan by 6.0 + 7.6 = 13.6% by 2012) [The Daily Yomiuri 2004]. In Japan "(Per) householdâ€¦5,000 yen green tax" per year is planned (starting April 2005). This includes "3,600 yen in tax per ton of carbon." "The revenue would be used to implement policies to achieve the requirements of Kyoto Protocol." A survey released (on Oct. 21, 2004) showed that 61% of those polled are in favor of the environmental tax." [The Japan Times 2004]. Rate of CO2 emission and global warming is shown in Figure 1. In last 2 yrs. CO2 has increased at a higher rate than expected [Corinaldesi & Moriconi 2004b].
6.3 Foundry by-products
Foundry by-products include foundry sand, core butts, abrasives, and cupola slag. Cores are used in making desired cavity/shapes in a sand mold in which molten metal is cast/poured. Cores are primarily composed of silica sand with small percentages of either organic or inorganic binders.
The most important conclusion drawn appears to be that the compressive strength of the recycled aggregate concrete can be improved to equal or even exceed that of natural-aggregate concrete by adding fly ash to the mixture as a fine aggregate replacement. In this way, a given strength class value, as required for a wide range of common uses, can be reached through both natural-aggregate concrete and recycled-aggregate concrete with fly ash, by adequately decreasing the water to cement ratio with the aid of a superplasticizer in order to maintain the workability.
Concrete manufactured by using recycled aggregate and fly ash shows no deleterious effect on the durability of reinforced concrete, with some improvement for some cases. From an economical point of view, if only the traditional costs are taken into account, recycledaggregate concrete with fly ash could be less attractive than natural-aggregate concrete. However, if the eco-balanced costs are considered, the exact opposite would be valid. Moreover, the fine fraction with particle size up to 5 mm, when reused as aggregate for mortars, allowed excellent bond strengths between mortar and bricks, in spite of a lower mechanical performance of the mortar itself. Also the masonry rubble can be profitably treated and reused for preparing mortars. Even for the fine fraction produced during the recycling process, that is the concrete-rubble powder, an excellent reuse was found, as filler in self-compacting concrete. The attempt to improve the quality of the recycled aggregates for new concretes by reusing in different ways the most detrimental fractions, i.e., the material coming from masonry rubble and the finest recycled materials, allowed to achieve surprising and unexpected performances for mortars and selfcompacting concretes. Other industrial wastes, such as GRP waste powder, can prove useful to be re-used in cementitious products, by improving some durability aspects.
"The concrete industry will be called upon to serve the two pressing needs of human society; namely, protection of the environment and meeting the infrastructural requirement for increasing industrialization and urbanization of the world. Also due to large size, the concrete industry is unquestionably the ideal medium for the economic and safe use of millions of tons of industrial byproducts such as fly ash and slag due to their highly pozzolanic and cementitious properties. It is obvious that large-scale cement replacement (60 - 70 %) in concrete with these industrial by-products will be advantageous from the standpoint of cost economy, energy efficiency, durability, and overall ecological profile of concrete. Therefore, in the future, the use of by-product supplementary cementing materials ought to be made mandatory" [Malhotra 2004].