1.0 What is GREEN concrete?

Most people associate GREEN concrete with concrete that is colored with pigment. However, it is also referred which has not yet hardened. But in the context of this topic, green concrete is taken to mean environmentally friendly concrete. This means concrete that uses less energy in its production & produces less carbon dioxide than normal concrete is green concrete. Engineers and architects have choices of the material and products they use to design projects – when it comes to a building frame the choice is typically between concrete, steel and wood. Material choice depends on several factors including first cost, life cycle cost and performance for a specific application. Due to growing interest in sustainable development, engineers and architects are motivated more than ever before to choose materials that are more sustainable. However, such choice is not as straight forward as selecting an energy star rated appliance or a vehicle providing high fuel mileage.

Engineers and architects can compare materials and choose one that is more sustainable or specify a construction material in such a way as to minimize environmental impact? Recent focus on climate change and the impact of greenhouse gas emissions on our environment has caused many to focus on CO2 emissions as the most critical environmental impact indicator. Life Cycle Assessment (LCA) is the parameter; the construction industry should look into. LCA considers materials over the course of their entire life cycle including material extraction, manufacturing, construction, operations, and finally reuse/recycling.

Concrete is one of the world’s most widely used structural construction material. High quality concrete that meets specification requires a new standard of process control and materials optimization. Increasingly, concrete is being recognized for its strong environmental benefits in support of creative and effective sustainable development. Concrete has substantial sustainability benefits. The main ingredient in concrete is cement and it consists of Limestone (Calcium Carbonate CaCO3). During manufacture of cement, its ingredients are heated to about 800 – 10000C. During this process the Carbon Dioxide is driven off. Approximately 1kg of cement releases about 900gms of Carbon Dioxide into the at

2.0 Features of Green Concrete:

Where does the Carbon Dioxide come from in concrete?

Cement production accounts for more than 6% of all CO2 emission which is a major factor in the world global warming (Greenhouse gas). India is the third largest cement producer in the World and one of the largest consumers of cement per capita in the world. Rough figures are that India consumes about 1.2 Ton/year/capita, while as World average is 0.6 Ton/year/capita.
There have been a number of efforts about reducing the CO2 emissions from concrete primarily through the use of lower amounts of cement and higher amounts of supplementary cementitious material (SCM) such as fly ash, blast furnace slag etc. CO2 emissions from 1 ton of concrete produced vary between 0.05 to 0.13 tons. 95% of all CO2 emissions from a cubic meter of concrete is from cement manufacturing. It is important to reduce CO2 emissions through the greater use of SCM.

i) Cement:

Most of CO2 in concrete is from the cement manufacturing process. A typical cubic meter of concrete contains about 10% cement by weight. Out of all ingredients, cement gives out most carbon dioxide. The reaction in the process of Cement manufacture is:
CaCO3 = CaO + CO2

ii) Aggregate:

Use of virgin aggregates contributes about 1% of all CO2 emissions from a typical cubic meter of concrete. Therefore, the use of alternate aggregate is desirable. The use of local and recycled aggregates is desirable as it can reduce transportation and fuel cost and support sustainable development.

iii) Resources:

The growing shortage of natural aggregate and sand is another aspect the construction industry must consider. While this may not appear to be a priority topic, pressure from environmentalist and conservationists worldwide will continue to encourage both legislators and construction engineers to look for viable alternatives to natural resources. Use of recycled materials like aggregate, water is some ingredient which should be encouraged since fresh resources are becoming increasingly scarce.

Green Concrete:

Obtaining the most suitable mix based on the specification or suggesting improvements in the mix is to assist with the most suitable concrete for the project. The concrete which can fall in the category of green must have the following characteristics.

  • Optimizes use of available materials
  • Better Performance
  • Enhanced cohesion workability / consistency
  • Reduced shrinkage / creep.
  • Durability – Better service life of concrete
  • Reduced carbon footprint
  • No increase in cost
  • LEED India Certification

Green concrete mix is designed with the principle of “Particle-Packing Optimization” to meet requirements of plastic and hardened properties.

3.0 Materials for Green Concrete:

Green construction materials are composed of renewable, rather than non-renewable resources. Green materials are environmentally responsible because impacts are considered over the life of the product. Depending upon project-specific goals, green materials may involve an evaluation of one or more of the following criteria.

  • Locally available: Construction materials, components, and systems found locally or regionally, saving energy and resources in transportation to the project site.
  • Salvaged, re-furnished, or re-manufactured: Includes saving a material from disposal and renovating, repairing, restoring, or generally improving the appearance, performance, quality, functionality, or value of a product.
  • Reusable or recyclable: Select materials that can be easily dismantled and reused or recycled at the end of their useful life.

Recycled materials that the Industry has found to perform favorably as substitutes for conventional materials include: fly ash, granulated blast furnace slag, recycled concrete, demolition waste, microsilica, etc. Generation and use of recycled materials varies from place to place and from time to time depending on the location and construction activity as well as type of construction projects at a given site. Following materials can be considered in this category and are discussed here.
a. Recycled Demolition Waste Aggregate
b. Recycled Concrete Aggregate
c. Blast furnace Slag
d. Manufactured Sand
e. Glass Aggregate
f. Fly ash
They are divided in cement, cementitious material, coarse and fine aggregate. Their definitions are as usual.

4.0 Coarse Aggregate:

Aggregate contents have direct and far-reaching effect on both the quality and cost of concrete. Unlike water and cement, which do not alter in any particular characteristic except in the quantity in which they are used, the aggregate component is infinitely variable in terms of shape, size and grading etc. With coarse aggregates graded infractions between 5mm and 40mm, differences in particle shape and surface texture affect the bulk void content and frictional properties of concrete. Generally the requirement of course aggregate in concrete is more than 50% as shown in figure 1. Similarly sand required is about 30%. They contribute in large quantity so its availability and effect on environment must be carefully examined. Following source of coarse aggregate are discussed: Figure 1 – Distribution of Ingredients in a typical Concrete Mix

  • Fresh Local Aggregate
  • Recycled Demolition Waste Aggregate
  • Recycled Concrete Material (RCM)
  • Blast Furnace Slag (BFS)

A) Fresh Local Aggregate:

Many places there are stone quarry available. Though these may not be of high quality stone like granite, basalt, Dolomite etc. but they may be of little lower quality. These can be used in making concrete with the help of appropriate mix design – may be for lower characteristic strength. A typical local aggregate in sand stone is shown in figure 2.
                                                                                      Figure 2 – Typical shape of local sand stone aggregate

B) Recycled Demolition Waste Aggregate:

Construction industry produces huge waste called demolition waste or MALWA. It is estimated that per capita waste generation (including Municipal waste) generally range from 0.4 to 0.8 Kg per day per person. A typical waste dump is shown in figure 3. The waste contributes to greenhouse gas emissions and thus waste prevention and/or its recycling will reduce greenhouse gases and methane gas emissions etc.
Figure 3 – A typical Waste Dump on the Road sides
Therefore, for sustainability of resources, it is necessary that all waste must be scientifically managed. Waste Management is Collection, Transport, Processing, Recycling or disposal of waste materials. When analyzed, a typical waste product distribution in any solid waste dump is shown in figure 4. This waste distribution shows that there is about 50% demolition waste in the dump. In order to have sustainability of resources this demolition waste must be recycled and used.

Figure 3 – A typical Waste Dump on the Road sides
Therefore, for sustainability of resources, it is necessary that all waste must be scientifically managed. Waste Management is Collection, Transport, Processing, Recycling or disposal of waste materials. When analyzed, a typical waste product distribution in any solid waste dump is shown in figure 4. This waste distribution shows that there is about 50% demolition waste in the dump. In order to have sustainability of resources this demolition waste must be recycled and used.

                                                                        Figure 4- Typical Distribution of material in waste Dump lying on road side

Recycled Aggregate is produced from Broken Building Part called MALWA. Such demolition waste – MALWA, can be converted to Course aggregate. The Demolition waste can be broken into the pieces of approximately 20 & 10 mm size with the help of light crusher. Typical shape & color of recycled aggregate is shown in figure 5. This type of processing the waste will make the system Sustainable.
The physical and chemical properties of such recycled aggregate must be determined before use as given in chapter III. The properties of recycled aggregates will vary from place to place and from time to time. For example, the specific gravity of recycled aggregate may be less than fresh conventional aggregate because it may have mixture of materials.

                                                                                     Figure 5 – Typical shape and color of Recycled aggregate

Use In General Purpose Concrete:

If properties of sample recycled aggregate are suitable then such aggregate can used in concrete mix. The concrete mix design can be done as of general method described earlier. The shape of slump cone of concrete with recycled aggregate is generally similar to conventional aggregate. The failure pattern of cubes / fracture mechanism is also similar to conventional cubes as shown in figure 6.

If properties of sample recycled aggregate are suitable then such aggregate can used in concrete mix. The concrete mix design can be done as of general method described earlier. The shape of slump cone of concrete with recycled aggregate is generally similar to conventional aggregate. The failure pattern of cubes / fracture mechanism is also similar to conventional cubes as shown in figure 6.

C) Recycled Concrete Material (RCM) 

Recycled Concrete Material (RCM), also known as crushed concrete is similar to demolition waste. It is a reclaimed Concrete material. Primary sources of RCM are demolition of existing concrete pavement, building slabs & foundations, bridge structures, curb and gutter and from commercial/private facilities. This material is crushed by mechanical means into manageable fragments. The resulting material is in the form of Coarse Aggregate. Comprised of highly angular conglomerates of crushed quality aggregate and hardened cement, RCM is rougher and more absorbent than its virgin constituents.

Crushed concrete’s physical characteristics make it a viable substitute for coarse aggregate however, its physical and chemical properties must be determined before use as given in chapter III. The properties of recycled aggregates will vary from place to place and from time to time. Such aggregate can be used in concrete mix or in Highways concrete construction similar to demolition waste aggregate concrete.
Figure 6 – Failure Pattern of Crushed Bricks made with Recycled Aggregate

D) Blast Furnace Slag (BFS):

In India more than 10 million tones of Blast Furnace Slag is produced every year and it is increasing with the increase in steel production. Blast furnace slag is a waste product from the manufacture of pig iron and obtained through rapid cooling by water or quenching molten slag. Iron ore, as well as scrap iron, is reduced to a molten state by burning coke fuel with fluxing agents of limestone and/or dolomite. Blast furnace slag is a nonmetallic co-product produced in the process of steel production. BFS forms when slagging agents (e.g., iron ore, coke ash, and limestone) are added to the iron ore to remove impurities. In the process of reducing iron ore to iron, a molten slag forms as a nonmetallic liquid (consisting primarily of silicates and alumino silicates of calcium and other bases) that floats on top of the molten iron. The molten slag is then separated from the liquid metal and cooled. Different forms of slag product are obtained depending on the method used to cool the molten slag and subsequent processing: BFS consists primarily of silicates, aluminates, silicates, and calcium-alumina-silicates. Air-Cooled Blast Furnace Slag (ACBFS), one of various slag products, is available when the liquid slag is allowed to cool under atmospheric conditions.
Crushed Air-Cooled Blast Furnace Slag may be broken down as typical aggregate with the help of processing equipment to meet gradation specifications. Thus, blast furnace slag can be available as an aggregate as construction materials and acceptable as coarse or fine aggregate for use in green Concrete. A typical shape of ACBFS coarse aggregate is shown in figure 7.
                                                                                         Figure 7 – A typical shape of ACBFS Coarse aggregate

5.0 Fine Aggregate (Sand):

Following source of Fine aggregate are normally used. Some are discussed here:

  • Fresh River Sand
  • Manufactured Sand
  • Recycled Glass aggregate.
  • Blast Furnace Slag (BFS)

A) Manufactured Sand For Concrete: 

Sand is generally obtained from river bed. However, sand can also be manufactured / produced after crushing stone from rocks. This process is similar to getting crushed coarse aggregate. Infect after crushing rock stone for coarse aggregate and sieving it on set of sieves between 40 – 6 mm size, the remaining portion passing through 6 mm is called stone dust. This can also be said to be a bi-product of manufacturing coarse aggregate. Such product / stone dust is generally in cubical form and depend on the type of rock being crushed and can be called manufactured sand. Cubical sand manufactured from crushed rock is the most desirable fine material for concrete production. It is generally accepted that particle shape depends on the rock type, breakage energy and the type of crusher used. It is also generally accepted that the crushers most successful at producing non-flaky aggregates are autogenous (rock on rock) and vertical-shaft impactor. If it is produced simultaneously, it saves energy and cost, providing further economies in the overall production cost. Here, fracture in rock generally takes place along the rock’s natural grain, producing the characteristic cubical shape and surface texture.

Optimum Shape:

The optimum shape of sand is cubical as shown in figure 8. Similarly, an even gradation of the total coarse aggregate fraction is desirable so that the smaller particles of sand can fit between the larger particles, thereby minimizing the voids. Well shaped aggregates also minimize the incidence and degree of segregation.
Figure 8 – Shape of Manufactured Sand (Enlarged view through Microscope)

Natural Sand Vs Manufactured Sand:

Natural sand often contains undesirable minerals and clays, and the effect of these materials on both the fresh and the hardened concrete can be extremely harmful. For example, the effect of clay particles in fresh concrete is obvious, as the particles absorb disproportionate volume of water and hence swell to many times their original size. This swelling occupies a volume in the cement paste in its fresh state. When it hardens, the clay particles contract and leave minute voids which in turn increase the shrinkage and permeability. This in turn reduces the concrete’s chemical resistance and compressive strength. Other undesirable materials, ranging from basic chlorides to harmful chemicals, can exist in such fine material fraction. The use of manufactured sand, however, reduces the risk of impurities.
It has been proven that about 20kg of cement can be saved for every cubic meter of concrete that is made by replacing a poorly shaped aggregate with a cubical aggregate. In addition, both compressive strength and flexural strength are improved by using cubical aggregates, which also increases workability and reduces bleeding and shrinkage. The impact of the physical characteristics of the sand used in the concrete mix is even greater than that of the coarse aggregate fractions, both in the concrete’s plastic and hardened states

Recycled Glass Aggregate:

Glass is formed by super cooling a molten mixture of sand (silicon dioxide), soda ash (sodium carbonate), and/or limestone to form a rigid physical state. Glass aggregate is a waste product of recycled mixed glass from manufacturing and post consumer waste. Glass aggregate, also known as glass cullet, is 100 percent crushed material that is generally angular, flat and elongated in shape. This fragmented material comes in variety of colors or colorless. The size varies depending on the chemical composition and method of production / crushing.
When glass is properly crushed, this material exhibits fineness modulus & coefficient of permeability similar to sand. It has very low water absorption. High angularity of this material, compared to rounded sand, enhances the stability of concrete mixes. Such material can be easily used in concrete construction as fine aggregate and give a better cohesive mix which will save on the consumption of cement.

Blast Furnace Slag (BFS):

Blast furnace slag is described above under coarse aggregate. Here if blast furnace slag may be broken down as typical fine aggregate also with the help of processing equipment to meet gradation specifications. Thus it can be available as fine aggregate also as construction materials and acceptable for use in green Concrete. A typical shape of ACBFS fine aggregate is shown in figure 9.
Figure 9 – A typical shape of ACBFS fine aggregate

6.0 Cementitious materials – Fly Ash:

Fly ash is a by-product produced during the operation of coal-fired power plants. The finely divided particles from the exhaust gases are collected in electrostatic precipitators. These particles are called Fly ash. Gray to black represents increasing percentages of carbon, while tan color is indicative of lime and/or calcium content.
Fly ash particles are very smooth and quite spherical in shape. These particles range from 1 to 150 m in diameter. A typical shape of fly ash particles is seen in figure 10. Based on its composition, fly ash is classified into two groups: ASTM Class C or high calcium fly ash and ASTM Class F or low calcium fly ash are the two categories of fly ash.
Figure 10 – Fly ash particles at 2,000x magnification (seen through Electron Microscope)

Use of Fly ash & Economic Impact:

Fly ash can be used as part replacement of Cement in Concrete. Finer the fly ash, better is its reactivity and lesser is its water requirement. Fly ash particles finer than 10 microns get adsorbed on cement particles giving a negative charge causing dispersion of cement particle flocks, thereby releasing the water trapped within the cement particle flocks and improves workability.

Advantages Of Using Fly Ash in Concrete:

  • Utilization of fly ash as a part replacement of cement or as a mineral admixture in concrete saves on cement and hence the emission of CO2.
  • Use of good quality fly ash in concrete has shown remarkable improvement in durability of concrete, especially in aggressive environment.
  • Some of the technical benefits of the use of fly ash in Green Concrete are:
    a) Higher ultimate strength
    b) Increased durability
    c) Improved workability
    d) Reduced bleeding
    e) Increased resistance to alkali-silica reactivity.
    f) Reduced shrinkage.

7.0 Green Concrete Mix Design:

The concrete mix design method for such concrete is the same as for conventional concrete. However, the constituent materials shown in figure 11 must pack themselves in such a manner that they occupy minimum volume or give minimum voids in concrete. In figure 11 all individual material has large voids. For getting a dense or impervious green concrete, all such voids must be packed with smaller particles of next type of material.
This can be done by seeing the slump test of dry all – in – aggregates and other materials. They must stand in natural angle of repose as seen in figure 12.
Figure 11 – Shape of packing of material for making concrete in isolated configuration

7.1 Green Concrete Mix Design Objectives: 

  • Optimizes void space between aggregates by optimizing particle proportions and packing of materials. This makes more effective use of the cement binder.
  • Aggregates replace excess cement paste to give improved stability, less shrinkage and increase in strength & durability.
  • Less cement also generates less heat of hydration.
  • The slump of the concrete and its flow are a function of the shape & the quantity of the predominant size of the aggregate in the mix.
  • Use of more fine aggregate gives higher slump & flow. So the optimum proportions of coarse & fine aggregate must be critically found to have the best and dense concrete in both fresh & hardened stage of concrete.

Figure 12 – Slump test of all in combined aggregate

7.2 Advantage Of Green Concrete: 

It will give enhanced cohesion so user friendly – easier to place, compact & finish concrete. It can be seen in concrete slump given in figure 13. Some other advantages of such mix are:

  • Optimized mix designs mean easier handling, better consistency and easier finishing
  • Reduction in shrinkage & creep
  • Green Concrete uses local and recycled materials in concrete.
  • The heat of hydration of green concrete is significantly lower than traditional concrete
  • This result in a lower temperature rise in large concrete pours which is a distinct advantage for green concrete.

Improved engineering properties:

  • Mix can result in a reduced paste volume within the concrete structure resulting in a higher level of protection against concrete deterioration.
  • Higher strength per kilogram of cement
  • Increased durability & lower permeability
  • More aggregates typically mean higher Modulus of elasticity.

Concrete stiffness or MOE is an important property of concrete in a reinforced concrete structure.
Figure 13 – Typical spread of mix – note the aggregate distribution & that it does not bleed

8.0 Geopolymer Green Concrete:

8.1 What Is Geopolymer? 

The term ‘Geopolymer’ was first introduced by Davidovits, a French technologist. It is a mineral polymer resulting from geochemistry. The Geopolymerisation process comprises of a chemical reaction under highly alkaline conditions on Al-Si minerals in slag or fly ash yielding polymeric Si-O-Al-O bonds. Geopolymer is used as the binder, instead of Cement paste, to produce concrete. In this process it does not produce CO2 like in production of Portland cement. This process of polymer concrete can be compared with conventional concrete as below:

Geopolymer Concrete: 

Low calcium flyash + Alkali solution + water = Polymerrisation gets hard product on curing

Conventional concrete: 

Conventional Cement + water = Hydration process gets hard product on curing
The Geopolymer paste binds the coarse and fine aggregates and other un-reacted materials together to form Geopolymer Concrete. The production of Geopolymer Concrete is similar to that of Portland Cement Concrete. In both types of concrete, the aggregates occupy the largest volume. It is truly a Green Concrete. The structural model of Geopolymeric material is still under investigation. One of the ‘visualized mechanism’ of Geopolymerisation’ is ‘dissolution, transportation and poly condensation’, which takes place through an exothermic process.
The Silicon and aluminum 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 un-reacted materials. They are defined as:

  • These are also termed – ‘Alkali Activators’
  • These can be single or a combination of several materials
  • A combination of Sodium or Potassium Silicate and Sodium or Potassium hydroxide has been widely used
  • The ‘Alkali Activator – to – base material’ (i.e., flyash or slag) ratio is generally in the range of 0.25 to 0.35
  • Hydroxide solutions of Concentration 8 M to 16 M are generally used. Chemical composition of Geopolymers is similar to Zeolites, but shows an amorphous micro – structure.

8.2 Properties of fresh Geopolymer Concrete Mix:

a) Workability Of Geopolymer Concrete Mix:

  • Fresh fly ash based Geopolymer concrete, in general, has good consistency and is glossy in appearance. Generally it is cohesive as seen from figure 14.
  • Similar to Portland Cement Concrete, water content in the mix influences the workability, as measured by conventional slump test
  • The mix is aminable for vibration
  • Generally such concretes can be workable upto about 1 1/2 hrs.

b) Curing Of Geopolymer Concrete: 

Curing is generally carried out at elevated temperature in the range of 50 to 80OC. Adequate humidity has to be ensured otherwise the product/element is to be insulated to preserve water in the mix.

  • Curing is done generally at elevated temperatures (ensuring humidity), right from the time water and activation source material are added to the base material.
  • Curing at elevated temperatures (ensuring humidity or insulation) after about 12 hours when water and activation source material are added to base material.
  • Later curing is done at normal temperatures in traditional way. Duration of curing is generally for a short period; say about 3 to 5 days. Generally higher the curing temperature, higher is the compressive strength achieved

Figure 14 – Shape of Geopolymer Concrete mix Slump Cone

8.3 Properties of Hardened Geopolymer Concrete:
a. Strength Of Geopolymer Concrete:

  • The Strength of Geopolymer Concrete depends on the nature of source materials and the curing temperature.
  • At present Geopolymer Concrete upto 90 MPa Strength has been developed.
  • Geopolymer concrete made from calcined source materials such as metakaolin (calcined kaolin), flyash, slag etc. reach higher strength when compared to those made from non-calcined materials such as Kaolin.
  • The alkali activator used for Geopolymerisation also has a dominant influence on strength.

b. Stress – Strain Relations:

  • The general Stress – Stress curves of Geopolymer concrete, indicate that the Stress – Strain relations are similar to that of Portland Cement Concrete.
  • The Stress – Strain relations of geopolymer concrete can also be predicted using equations developed for Portland Cement Concrete.

c. Long Term Properties:

  • Experiments on typical mixes have shown that fly ash based polymer concrete undergoes low creep.

8.4 Applications Geopolymer Concrete:

Such concrete can be used where cement concrete is normally used. The structural applications are mainly in precast components and in- situ normal strength concrete components.