Question

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The cement and concrete industry is a key link in the circular economy. Alternative fuels/raw materials in cement kilns, utilising recycled aggregates and cementitious industrial by-products in concrete and producing durable concrete structures assist in the circular economy. Example of fly ash production is shown in Figure 1. Discuss the use of any THREE new binders, waste materials and recycled materials (as ingredients) in concrete production and their suitable applications. Produce a ‘white paper’ (not more than 5 pages) and it should include the following:

(i) Identified 3 ingredients and a brief description of their production.

(ii) How these materials are used to produce certain grades of concrete.

(iii) Energy savings and carbon dioxide emissions of all ingredients.

(iv) Link with circular economy.

Answer 1

Sol:

Management and treatment of industrial solid waste and municipal waste has been gaining importance. Waste material can be utilized as fuel in cement kilns, as raw material for cement clinker, as mineral addition to cement, or as granular material in cement mortar and concrete. Depending on the properties of the waste, it can be used in the cement and concrete industry without treatment or after treatment. Although vast amounts of waste material can be consumed in the production of cement clinker and blended cement, that consumption can be increased substantially if it is used as aggregate in cement mortar and concrete. Using waste material in this way can solve problems of shortage of aggregate in construction sites and reduce environmental problems related to aggregate mining and waste disposal.

For effective use of waste in the construction industry, a combined chemical, civil, material and environmental engineering approach is necessary. Careful characterization of waste is necessary before it is used in construction. Treatment of some waste materials is necessary to improve their quality before being incorporated into concrete. Environmental assessment of waste and concrete containing waste is also necessary for proper evaluation of waste materials as a constituent in concrete. Slump, bulk density, unit weight, and air content; mechanical properties such as strength and elasticity modulus, and durability properties such as water absorption, shrinkage, carbonation and chloride resistance of fresh and hardened concrete containing waste materials, are normally evaluated. Some waste materials can improve certain concrete properties such as fire behaviour, thermal insulation and cracking, and therefore some reports are available on these topics too. But the durability performance of concrete containing waste materials has not been evaluated to the same extent and therefore significant research is necessary on this topic. As different waste materials can change the.microstructure of mortar and concrete and the hydration chemistry of cement, more work is necessary to understand the aggregate-cement interaction in concrete and to develop more robust waste aggregate-based concrete technology

1. USE OF PLASTIC WASTE IN CONCRETE

Huge amounts of plastic solid waste are generated all over the world by the consumption of packaged products. Dumping plastic waste in landfills is not possible due to its slow degradation rate and bulky nature. The high calorific value of plastic waste can be used for incineration or other high temperature processes, but their combustion produces dangerous gases that could be harmful to human health. Although these alternatives are feasible, recycling waste plastic to produce new materials, such as concrete, appears to be the best solution for disposing of it since it is both cheaper and has ecological advantages.

The incorporation of plastic in the form of aggregate or fiber lowers the slump value of fresh concrete because of the sharp edges and angular particle size of plastic aggregate However, an increase in slump value due to incorporation of this material has also been reported . Concrete mixes containing plastic aggregates will have more free water as plastic aggregates neither absorb nor add any water to the concrete mix and therefore this increases slump

The incorporation of plastic as aggregate decreases the modulus of elasticity and the compressive, splitting-tensile, and flexural strengths of the resulting concrete The factors that may be responsible for low compressive and flexural strength of concrete containing plastic are:

1. the decrease in bond strength between the surface of the plastic waste and the cement paste;

2. the hydrophobic nature of plastic waste, which can inhibit cement hydration reaction by restricting water movement. In a recent study it was found that the incorporation of plastic up to a certain level did not influence the compressive and flexural strength of concrete . However, curing conditions were slightly different from those of the majority of the other studies and that might have had some effect on the properties.

Concrete containing waste plastic showed no propagation of micro cracks and improved toughness compared with conventional concrete . However, toughness indices of concrete containing PET fiber decrease with longer curing time due to degradation of the fiber by alkaline hydrolysis . The incorporation of plastic aggregate in plain concrete improves thermal insulation performance, which is dependent on the shape of the plastic aggregate, too .

So we can say that waste plastics can be used as replacement for fine aggregates in concrete for the production of lightweight concrete. Addition of granulated waste plastics effects  the compressive strength and density of concrete. Portland cement was mixed with the aggregates to produce the concrete composites. Grade 20 concrete design strength of mix ratio 1: 2.3: 3.5 and 0.65 w/c ratio was used for the experiment. Five weight fractions 0%, 5%, 10%, 15%, 20%, and 30% of granular plastic waste were used to replace the fine aggregate in the batching.In this way we high compressive strength concrete is produced

Cement manufacturing is highly energy- and emissions-intensive because of the extreme heat required to produce it. Producing a ton of cement requires 4.7 million BTU of energy, equivalent to about 400 pounds of coal, and generates nearly a ton of CO2. Given its high emissions and critical importance to society, cement is an obvious place to look to reduce greenhouse gas emissions.

Circular economy solutions for plastics include: producing plastics from alternative non-fossil fuel feedstocks; using plastic wastes as a resource; redesigning plastic manufacturing processes and products to enhance longevity, reusability and waste prevention; collaboration between businesses and consumers to encourage recycling and increase the value of plastic products; encouraging sustainable business models which promote plastic products as services, and encourage sharing and leasing; developing robust information platforms to aid circular solutions; and adopting fiscal and regulatory measures to support the circular economy.

To mitigate the adverse effects of the current mainly linear plastics production and use model, plastics production from renewable sources needs to increase to reduce dependence on fossil fuels significantly. Production processes and products should be redesigned to improve longevity, reusability, recyclability, as well as to prevent waste and chemical pollution. Sustainable business models that promote products as services, facilitate sharing and leasing of plastic products, and increase reuse should also be encouraged. Plastics at the end of life should increasingly be recycled into new products to significantly reduce the volume of plastics leaking into the environment.

Use plastic waste as a resource The capture and recovery of plastic waste for remanufacturing into new value products has been widely demonstrated, for example, for making bricks and composites, in road construction, for furniture, as well as for making clothes and footwear. Plastic waste has also been converted to liquid fuel78 and has been burned as fuel in a waste-to-energy cycle, though there are downsides to the latter80. Through chemical recycling, the petrochemical components of plastic polymers can also be recovered for use in producing new plastics, or for Recycling including through remanufacturing, chemical recycling, organic recycling, and conversion to new products Consumer use based on sustainable business models Production of only essential plastics using renewable resources Redesigned plastic production and products Energy recovery Leakages Fossil fuel base Renewable alternative sources Reuse through models such as refurbishing, sharing, leasing, redistribution and symbiosis Plastics and the circular economy the production of other chemicals, or as an alternative fuel. For example, a recent study successfully developed plastics that can be chemically recycled and reused infinitely. Studies also suggest that polyethylene plastic, a significant proportion of manufactured plastics globally, can be broken down by bacteria and caterpillars, highlighting opportunities for biobased recycling of waste plastics .

2. MUNICIPAL SOLID WASTE INCINERATION (MSWI) ASH

The MSW incineration process generates various types of waste, the major ones being bottom ash and fly ash. These ashes normally contain a large amount of toxic constituents and are therefore hazardous in nature . The characteristics of ash are influenced by incineration parameters like furnace type and temperature, capacity, and waste input. MSWI ash is generally recycled in concrete after treatment or after solidification/stabilization in monolithic form, because it contains harmful components . The presence of harmful constituents like elemental Al, water soluble salts of chlorides and sulfates and organic compounds in MSWI ash, is the main problem with using this ash in concrete . Among the techniques used to improve MSWI ash properties in concrete are: combustion to remove organic compounds; water leaching or leaching at basic pH using chemical solutions to remove aluminium, soluble matter content, toxic elements, etc. , and solidification by cement or other chemicals to immobilize toxic components of MSWI ashes. Weathering may be a method of treating MSWI ashes because atmospheric carbonation of MSWI ash can immobilize some toxic elements, too . The evaluation of physical-chemical, mineralogical and environmental characteristics of waste and mechanical and durability-related properties, along with the environmnetal suitability of concrete, is therefore necessary for the proper utilization of MSWI ashes in concrete mixes.

As MSWI waste is hazardous in nature (like most industrial waste) a different methodology is needed to properly evaluate these materials. Present research results suggest that MSWI ashes should only be used in non-structural concrete. Evaluation of durability properties and long term mechanical behaviour of MSWI ash-containing concrete is needed to properly use this material. Further work is necessary to understand the effect of the chlorides and sulfates in MSWI ashes on the behaviour of concrete containing these ashes. A set of technical requirements for MSWI ashes used in concrete and for concrete containing MSWI ashes is needed, along with recommendations and regulations on the industrial production of concrete and other building products containing MSWI ash

COAL ASH AS AN AGGREGATE IN CONCRETE

Though a significant number of studies are available on the properties and use of fly ash as a mineral addition in normal Portland cement, not much literature exists on the use of fly ash and coal bottom ash (CBA) as a granular additive in concrete. The CBA falls into the furnace bottom in modern large thermal power plants and constitutes about 20% of the total ash content of the coal fed into the boilers. The properties of bottom ash depend on the coal type, pulverizing system, combustion conditions, temperature, type of furnace, minerals in the coal, and milling system. It is collected from the bottom of the combustion chamber in a solid granular form.

The workability of concrete mixes containing fly-ash as partial substitution of sand is better than that of concrete prepared without fly ash. The water requirement of concrete mixtures containing fly-ash as partial substitution of sand is slightly higher than for concrete prepared without fly ash. The rate and volume of water loss due to bleeding from concrete containing fly ash are similar to those of concrete prepared without fly ash. The setting of the concrete mix is delayed due to the incorporation of fly-ash in concrete as partial replacement of fine aggregateThe incorporation of fly ash as sand replacement improves strength properties and abrasion resistance of concrete, due to the pozzolanic action of flyash, which improves interfacial bond between paste and aggregates

Earlier fly ash is being used in concrete with ordinary Portland cement. Optimization of fly ash in concrete by using Portland slag cement have obviously good impact on lowering the cost of concrete as well as consumption of by product and waste material fly ash and slag. Portland slag cement is already blended cement in which slag and clinker are grinded together by adding 65% slag .On one hand using the blended cement in concrete and on other hand using fly ash additionally will great effort to reduce the CO2 emission. Optimized the quantity of fly ash in concrete of grade M15, M20, M25,M30,M35,M40. Portland slag cement confirming IS:455-1989 and high lime fly ash which is categorised as class C fly ash by ASTM, because of its high calcium content ,is used for the each trial mixes of concrete .High lime fly ash will act as binder material along with cement, also it’s pozzolanic nature improves the durability of the concrete. In each mix for each grade of concrete , w/c ratio is kept lower side regarding the concern of maximum w/c ratio criteria as per IS 456-2000.The physical and chemical properties most importantly, its fineness, residue and lime content imparts vital role in water cement ratio and strength parameters in concrete

In trial mixes of each grade, 06 trials are being performed. First trial of each grade is containing the quantity of Portland slag cement, which is being used by mass concrete users. On the basis of quantity of Portland slag cement used in 1st trial, different percentage of fly ash is added on further trials. The trials with maximum quantity of fly ash, which achieve the target strength for a particular grade, is the optimized percentage of fly ash for that grade of concrete. As from the results, the optimized quantity of fly ash is 40% for M15 & M20,25% for M25,20% for M25 & M30 and 10% for M40.It is clear from the results, as we move for the higher grade optimized percentage goes to lower.

The optimised quantity of fly ash will differ if the physical and chemical properties of fly ash and PSC cement differs, like different slag content of PSC, different fineness and lime content of fly ash. Also optimized percentage will differ if water cementitious selection for any grade will differ. Even if all the changeable parameter is varying the optimised percentage will vary in range of 0- 5%.This work on optimization of fly ash in concrete by using the blended cement as PSC is helpful to lower the cost of concrete, CO2 emissions but not on the cost of strength and durability parameter

The benefits of using Fly ash as a replacement for clinker in cement production have been demonstrated in several studies and research. Maximising its use seems an acceptable and feasible path to lead the industry into a more sustainable direction. As discussed in previous sections, a considerable reduction of CO2 emissions can be produced when UPs improve the quality of FA (Fly ash) under certain conditions. When the energy required to modify FA is above 3.98 GJ/tonne, emissions generated in the upgrading process surpass the savings produced by the replacement of clinker. Considering that there are technologies available for upgrading which perform at maximum values of 1 GJ/tonneFA, it is necessary to encourage stakeholders to make use of these options, which have been shown to produce savings in CO2 emissions. In future research, it is also necessary to address other important obstacles for which upgrading processes haven’t been widely used and commercialised, considering the significant benefits associated with them. It is also necessary to emphasise that whilst other, more efficient technologies for reducing CO2 emissions have been partially developed, use of SCMs is highly viable alternative and should be maximised. Nevertheless, this paper can be considered the initial point for further research on the topic of upgrading other materials to be used as SCM, in which economic, social and other environmental dimensions can be considered.

In today’s scenario vital research on concrete technology is pursuing. To lower the cost impact and to save our environment is prime motivation; necessarily clinker factor should be minimized to reduce the CO2 emissions. The future scope of using the blended cement in concrete manufacturing shows the way. It will contain clinker ,slag and fly ash in an effective proportion to fulfil the requirement

3.  USE OF RUBBER TIRE AS AGGREGATE OR FIBRE OR FILLER IN CONCRETE

The disposal of rubber tire waste has become a serious problem for modern civilization because of the huge amounts of tires that are discarded, which are non-biodegradable in nature. The accumulation of used tires at landfill sites also presents the threat of uncontrolled fires which would produce a complex mixture of chemicals that could contaminate soil and vegetation and also provide breeding grounds for rats, mice, vermin and mosquitoes.

Innovative solutions to meet the challenge of the tire disposal problem are urgently needed because of the rapid depletion of available sites for waste disposal. Tire rubber can be used in asphaltic concrete mixtures, in incinerators to produce steam, to produce different plastic and rubber products, as a fuel for cement kiln, as feedstock for making carbon black, and as artificial reefs in marine environment, to name some of the attractive utilization options There is extensive literature, including excellent reviews, on the use of tire rubber as coarse or fine aggregate or as a filler material for the preparation of different types of concrete . Some results of concrete containing rubber tire waste particles as aggregate or fiber or filler material are highlighted here:

• Increasing incorporation of rubber as aggregate, fiber or filler in concrete lowers the workability and bleeding of concrete; concrete containing rubber as aggregate has acceptable workability in terms of ease of handling, placement, and finishing ;

• The incorporation of rubber increases the water requirement of the resulting cement mortar composition; because of the low specific gravity of rubber aggregates the resulting concrete containing rubber has lower unit weight than conventional concrete ;

• The air content of concrete that contains rubber is higher than that of conventional concrete, which is due to the non-polar nature of rubber, and its tendency to trap air in its rough surfaces and to attract air due to its water repellent nature ;

• The incorporation of rubber granules and shreds into cement mortar reduces plastic shrinkage cracking and also delays the onset of cracking in comparison to control mortar; incorporation of rubber in concrete increases free shrinkage length changes ;

• The incorporation of rubber decreases the compressive and splitting tensile strength and elastic modulus and increases the flexural strength of concrete , a decrease in the flexural strength of cement mortar and concrete containing rubber tire as aggregate is also reported ; the compressive strength of concrete containing coarse rubber aggregate is lower than that of the concrete containing fine rubber aggregate ; however, the opposite trend for the compressive strength behaviour of concrete containing coarse and fine rubber particles is also reported; the addition of silica fume to normal Portland cement improves the strength properties of concrete containing rubber tire aggregate for the same unit weight, the strength properties of concrete containing expanded rubber aggregate are better than those of concrete containing compact rubber aggregate ;

• The use of rubber as aggregate in concrete containing magnesium oxychloride cement gives higher strength and better rubber particle-cement paste binding characteristics than those exhibited by the Portland cement based concrete containing rubber particles

Because of the improvement of some specific properties such as ductility, energy absorption efficiency, shrinkage and cracking associated with hardening and curing, concrete containing rubber as aggregate can be used for various purposes . However, a major drawback of using rubber in concrete is the deleterious effect of rubber on the strength behaviour of concrete, and therefore further research is necessary to improve the strength and related behaviour of concrete containing rubber.

Tire derived aggregate (TDA) has been proposed as a possible lightweight replacement for mineral aggregate in concrete. The role played by the amount of TDA replacing coarse aggregate as well as different treatment and additives in concrete on its properties is examined. Conventional concrete (without TDA) and concrete containing TDA are compared by examining their compressive strength based on ASTM C39, workability based on ASTM C143, splitting tensile strength based on ASTM C496, modulus of rupture (flexural strength) based on ASTM C78, and bond stress based on ASTM C234. Results indicate that while replacement of coarse aggregates with TDA results in reduction in strength, it may be mitigated with addition of silica fume to obtain the desired strength. The greatest benefit of using TDA is in the development of a higher ductile product while utilizing recycled TDA.

Small amounts of waste tires (TDA) in the range of 7.5% to 10% can be used in concrete with a target compressive strength of up to 4000 psi (≈28 MPa) but strength enhancing materials like silica fume need to be used. As TDA increase, the compressive strength drops. At 7.5% of TDA replacing coarse aggregates, this drop is found to be approximately 10% compared to the control concrete if silica fume is added into the mixture. However, this amount of strength can also be achieved without using strength enhancing materials (silica fume) if the top TDA size is lowered from 2 in to 1 in.

Using TDA to substitute for mineral aggregates lowers the modulus of elasticity of concrete by about 20% but increases the concrete toughness and ductility. However silica fume, as much as it increases compressive strength and consistency, has a negative effect to ductility. Workability of fresh concrete with TDA is slightly better as it has a slump of averagely 1 in higher compared to the control concrete. However if a stress of 3000 psi is applied to each material, the concrete without TDA deforms elastically to a maximum of 0.001 in/in while the concrete with TDA would deform elastically to a minimum of 0.00125 in/in. Therefore concrete with TDA would deform elastically 20% more compared with concrete without TDA.

TDA lowers the modulus of rupture of concrete but it increases displacement up to 50% (improved concrete deformation) during loading. The splitting tensile strength improves by 12.7% with introduction of TDA into concrete. The bond strength of the TDA concrete is not significantly different from that of the control concrete but TDA improves postcracking behavior of the concrete as noted from the pull-out tests.

Increasing amount of waste tyres represents a global problem as tyres can pose a risk to the environment if not treated properly. Recycling of waste tyre rubbers in civil engineering as aggregate in cement concrete can be an effective environmental and economic approach. The recycled tire rubber proved to be an excellent aggregate to use in the concrete. It was observed that its compressive strength was reduced by only 14% (28 days), in comparison to the conventional concrete, reaching 48 MPa for the mixture with higher resistance. The concrete compositions were found to be lighter and a reduced interface was observed between the rubber and cement matrix after the chemical treatment. The rubberized concrete can support construction sustainability, minimize the consumption of natural resources by using an industrial residue and produce a material with special features.

4. INDUSTRIAL SLAG

Slag is a partially vitreous by-product of smelting ore to separate the metal fraction from the worthless fraction. Although they can be regarded as a mixture of metal oxides slags can, however, contain metal sulfides and metal atoms in the elemental form. Several waste materials are nowadays vitrified to produce slag too.

• Slag cement is the hydraulic binder of high strength, consisting of fine ground slag with a predominance in the composition of CaO, SiO2 , Al2O3 (the total content of up to 95%) and alkaline activator curing (baking soda, liquid glass, etc) . Upon receipt of slag cement granulated slag – blast, electrotermometria, aluminum potassium and non-ferrous metallurgy (smelting). The necessary conditions of the possibility of using slag is the presence of glassy phases that interact with the alkali in the process of hardening and high specific surface area of at least 300 m2 /kg.
• With increasing content in the slag of small particles increases the rate of hardening and strength of the binder by increasing the number of defects in the structures and formations on the surface, with a large reserve of excess surface energy. As the alkaline component is most often used caustic and soda ash, potash, soluble sodium silicate, and alkaline industrial wastes, which allows obtaining significant amounts of slag binders. The optimum content of alkaline compounds in the binder is 2-5% by weight of the slag.
• Compared with calcium compounds to the high activity of compounds of alkali metals allows to obtain rapid hardening, high-strength binders. The presence of alkalis intensificare destruction and hydrolytic dissolution of slag glass, formation of alkaline hydroalumination and creating an environment that promotes education and high resistance low-basic calcium hydrosilicates. Poor solubility of tumors, the stability of the structure in time determines the durability of slag-alkaline stone.
• When using slag cement in concrete production, the resulting structure of cement stone is much less capillary pores than that of concrete with ordinary Portland cement. The difference between this binder of Portland cement is in high specifications of water resistance and frost resistance, as well as in low rates of shrinkage and creep.

Blast-furnace slag cement obviates the need for crushing and burning which are required in the manufacture of ordinary cement, thereby reducing fuel consumption by 43% and CO2 emissions by 41%.