A Comparative Study on Specialized and Environmental Impact Evaluation of Concrete Products

Concrete is a material used in building construction, consisting of a hard, chemically inert particular substance, known as an aggregate (usually made from different types of sand and gravel), that is bonded together by cement and water. The Assyrians and Babylonians used clay as the bonding substance or cement. The Egyptians used lime and gypsum cement. In 1756, British engineer, John Smeaton, made the first modern concrete (hydraulic cement) by adding pebbles as a coarse aggregate and mixing powered brick into the cement. In 1824, English inventor, Joseph Aspdin, invented Portland Cement, clay together. The burning process changed the chemical properties of the materials and Joseph Aspdin created stronger cement than that which uses plain crushed limestone would produce. Cement production requires a source of calcium (usually limestone) and a source of silicon (such as clay or sand). Small amounts of bauxite and iron ore are added to provide specific properties. These raw materials are finely ground and mixed, then fell into a rotary cement kiln, which is the largest piece of moving industrial equipment in the world. The kiln is a long, sloping cylinder with zones that get progressively hotter up to about 1,480°C. The kiln rotates slowly to mix the contents moving through it (Fig. 1). In the kiln, the raw materials undergo complex chemical and physical changes required to make them able to react together through hydration. The most common type of cement kiln today is a dry process kiln, in which the ingredients are mixed dry. Many older kilns use the wet process.

The first important reaction to occur is the calcining of limestone (calcium carbonate) into lime (calcium oxide) and carbon dioxide, which occurs in the lower-temperature portions of the kiln-up to about 900°C. The second reaction is the bonding of calcium oxide and silicates to form dicalcium and tricalcium silicates. Small amounts of tricalcium aluminate and tetracalcium aluminoferrite are also formed. The relative proportions of these four principal compounds determine the key properties of the resultant Portland cement and the type classification (Type I, Type II, etc.). These reactions occur at very high temperatures with the ingredients in molten form. As the new compounds cool, they solidify into solid pellet form called clinker. The clinker is then grounded to a fine powder, a small amount of gypsum is added, and the finished cement is bagged or shipped bulk to ready mix concrete plants. However, environmental performances of concrete have only recently become a subject of concern. They are often improved by the incorporation of recycled Environmental impacts of cement production have been studied (Josa et al., 2007; Pade and Guimaraes, 2007; Huntzinger and Eatmon, 2009) because it requires high energy consumption and induces important greenhouse gas emissions. Whereas developments of new constituents of cement accelerate, cement concrete production processes are only prone to minor evolutions. However, the influence of the process characteristics on the concrete properties (and consequently on the concrete composition needed for a given certified characteristics) are widely recognized, either for high performance or common concretes. In this article, a method to compare environmental and economical consequences of different technological solutions on the basis of the influence on the cement consumption needed for a given concrete performance is presented. Case study is the choice of dry batch or central mixed concrete production systems. These two main processes of ready mix concrete production are compared, with the aimto advance in quantifying the difference in energy consumptions and greenhouse gas emissions.

It is proposed that environmental properties of building products be divided into 2 classes: basic and energy raw materials. Each of these would have two classification options. Basic raw materials are either termed recycled or natural while energy raw materials can be renewable or non-renewable. In order to limit the number of environmental parameters, the harmful emissions can be characterised and weighed as follows. The suggested procedure is mainly based on other references (Anon 1995) with more detailed reasoning presented elsewhere (Häkkinen and Kronlöf 1994). The characterisation and weighing of emissions having a potential affect on the ozone depletion or on nutrification are not dealt with. This is because house building does not significantly effect the nutrification with reference to its volume and because the Finnish building product industry has abandoned the use of aerosol propellants, which potentially affect the volume of the ozone layer. In the following analysis the ecotoxical and toxically compounds are also excluded because generally accepted methods for dealing with these emissions are still missing. Table 1 shows the computation rules for classified environmental effects.

An environmental impact assessment was performed using LIME whose evaluations are based on Japanese weather and the country’s geographical conditions. The LIME method sets forth four objects of protection consisting of human health, public assets, biodiversity and primary production capacity, which have unique indexes consisting of DALY (Disability-Adjusted Life Year, unit: year), YEN (Japanese monetary unit, unit: yen), EINES (Expected Increase Numbers of Extinct Species, unit: species) and NPP (Net Primary Productivity, unit: t/ha/year), respectively (Kawai et al. 2005a). The degree of environmental impact can be evaluated with these four indexes and furthermore with a single index that is an integrated index of these four indexes. In this case study, the manufacturing of materials, transportation of materials, construction, waste treatment, material recycling and change of land use were estimated. As environmental impact, the uses of oil, coal, natural gas, purchased electricity, nonmetallic minerals and iron and the emissions of CO2, SOX, NOX and particulate matter were estimated. To evaluate the revegetation on the hollow blocks in Case-1, the followings are assumed based on categories of land use in LIME & 3: 1200 m2 of “forest” were changed to 360 m2 of “other groves” and 840 m2 of “road,” and the land uses were maintained for 50 years. Regarding the construction works, the road construction work was adopted as the EINES damage factors of works, in this case study.

cement (converted into lime through calcination). Common sources of silicon include clay, sand, and shale. Certain waste products, such as fly, can also be used as a silicon source. The iron and aluminum can be provided as iron ore and bauxite, but recycled metals can also be used. Finally, about 5% of cement by weight is gypsum, a common calcium- and sulfur-based mineral. It takes 1,455…1,597 kg of raw materials to produce one ton of finished cement, according to the Environmental Research Group at the University of British Columbia (UBC). The water, sand and gravel or crushed stone used in concrete production in addition to cement are also abundant. With all of these raw materials, the distance and quality of the sources have a big impact on transportation energy use, water use for washing, and dust generation. Some aggregates that have been used in concrete production have turned out to be sources of radon gas. The worse problems were when uranium mine tailings were used as concrete aggregate, but some natural stone also emits radon. Fly ash is a fine, glass-like powder recovered from gases created by coal-fired electric power generation. Power plants produce millions of tons of fly ash annually, which is usually dumped in landfills. Fly ash is an inexpensive replacement for Portland cement used in concrete, while it actually improves strength, segregation, and ease of pumping of the concrete. Fly ash is also used as an ingredient in brick, block, paving, and structural fills. The use of fly ash from coal-fired power plants is beneficial in two ways: it can help with our solid waste problems, and it reduces overall energy use. While fly ash is sometimes used as a source of silica in cement production, a more common use is in concrete mixture as a substitute for some of the cement. Fly ash, or pozzolan, can readily be substituted for 15…35% of the cement in concrete mixes, and for some applications fly ash content can be up to 70%. Fly ash today accounts for about 9% of the cement mix in concrete. Fly ash reacts with any free lime left after the hydration to form calcium silicate hydrate, which is similar to the tricalcium and dicalcium silicates formed in cement curing. Through this process, fly ash increases concrete strength, improves sulfate resistance, decreases permeability, reduces the water ratio required, and improves the pumpability and workability of the concrete. Fly ash is widely used in Europe as a major ingredient in autoclaved cellular concrete (ACC); in the United States, North American Cellular Concrete is developing this technology. to certify that concrete characteristics are respected in the most variable conditions. They should take into account that significant deviations in strength and flowability occurs during concrete production. These deviations come from:

• Constituents physical and chemical variability; most often the main effect is those of cement, but water, aggregates and admixture may also have significant effect;

• Process characteristics variation, like temperature, loading sequence, mixing time, transport duration.

• Mix-composition variability.

Several industrial studies e Day (1995) is one of the few published e indicate that the key variability reason is the water content inconstancy, largely before other fluctuations like chemical activity of cement or sand’s fines content. Main types of production process : Concrete production consists of mixing aggregates (sand and gravels), cement (and sometimes other fine elements), water and admixtures, and then molding the obtained mixture. Ready mixed concrete is manufactured in a concrete plant and transported to the building site by the use of truck mounted mixer. Two main types of production systems are used for ready mixed concrete production. In the so called “central mixed concrete plant”, ingredients are poured into a fixed mixer and mixed generally less than 1 min. The resulting mixture is poured into the truck mounted revolving mixer which performs only an additional mixing during transport. A concrete mixing truck volume is usually about 6e9 m3 and several batches are needed to fill it, as typical mixer’s volume is 1 or 3 m3. Each batch respects the mix-design composition. However, current practice is to estimate water content in the mixed batch, for instance by using power consumption evolution. The plant operator uses this information to adjust the added water proportioning in the following batches. This method allows adjusting water content in the truck.

The assessment of environmental burdens of concrete is based on the materials and energy flows reported by the Finnish material producers. The inventory principles followed in this study are in accordance with the ISO-proposal. In concrete manufacturing processes, including limestone quarrying, burning and grinding of clinker, extraction, excavation and crushing of stone materials, concrete manufacturing and transportation of raw materials and products, the environmental burdens which result are:

• using land,

• using energy and also from process emissions.

Figure 2 shows the concrete manufacturing process from the procurement of raw materials until demolition and recycling of the product. The environmental profile for a 1-kg concrete product (hollow-core slab) is shown in Table 2, excluding assembling, using, demolition and recycling.

Table 2. Environmental profile of steel reinforced concrete hollow-core slab.

energy use in the transportation and manufacturing processes.

Cement and concrete are vital components in building construction today. Concrete has many environmental advantages, including durability, longevity, heat storage capability, and chemical inertia. In many situations concrete is superior to other materials such as wood and steel. But cement production is very energy intensive – cement is among the most energy intensive materials used in the construction industry and major contributor to CO2 in the atmosphere. In spite of recent evolutions in control and dosage systems, concrete production processes tolerate obvious inaccuracies, particularly concerning the water dosage. Aggregates are naturally moistened and their water content has to be monitored to assess total water proportion into the mixture. At this stage, the study is limited to the ready mix concrete production. The cement consumption is compared between a dry batch plant and a central mixed plant. These two main types of ready mixed concrete manufacturing processes have been compared from technical, environmental and economical point of views, on the basis of identical strength of the final product (hardened concrete). Comparison is performed on the basis of an annual production of 40,000 m3 yr_1. The additional mixer in a wet batch concrete plant is a 2 m3 mixer. To perform environmental and economical comparisons, a new environmental indicator, called EADT (Environmental Amortization Duration Time) was created. The environmental assessment was performed taking into account the variability range of Life Cycle Inventories data. The EADT for released CO2 and particles as well as energy consumption, are found below 6 years, in the whole range of tested environmental data. The NOx and SO2 EADT results are found sensitive to the releases of the steel plant. The financial amortization duration time corresponding to the purchase of a mixer, is found under 2 years. Although the chosen application case is simplified, these results encourage further research.

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