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Flyash in Concrete: Quality and Ecological Perspective

Anil Kumar Pillai, AGM Tech Services, Madras Cements Ltd

Madras Cements Ltd, the flagship company of the RAMCO group has a capacity of over 12 million tonnes per annum. Ramco Supergrade, the Portland Pozzolona Cement manufactured by RAMCO through its various Plants is being supplied to major projects. Alathiyur Plant of RAMCO group is the only cement Plant in the country who has received Four Leaves Award instituted by Centre for Science & Environment. The research wing of Madras Cements Ltd, Ramco Research Development Centre (RRDC), is actively involved in works relating to quality of cement and concrete. Number of trials using PPC for various concrete grades has been carried out in RRDC.

Abstract

Global warming, climate change, environmental concerns, sustainability issues are receiving greater attention now. Adoption of latest technologies in the construction industry and judicious concrete mix proportions using blending materials will reduce the emission of greenhouse gases. Apart from fulfilling the requirements of quality Concrete should also fulfill the benchmark from environment sustainability perspective.

Flyash is being used in concrete either as an additive to concrete or as Portland Pozzolona Cement. The use of flyash has enhanced properties of concrete in both fresh and hardened state. All measures to enhance durability of concrete structures would also be sustainable. There are number of literature available in the areas of usage of flyash in concrete and number of concrete mixes have been used at sites incorporating flyash .Literatures and the results of concrete properties have supported the hypothesis that flyash in concrete enhances durability . This paper attempts to give an overview of the beneficial aspects of flyash usage in concrete through available literature and site mixes.

Introduction

Flyash in concrete acts as a pozzolona and hence is being increasingly used in concrete worldwide. ASTM 618 - 94 a describes pozzolona as a siliceous or siliceous and aluminous material which in itself possesses little or no cementitious value but will in finely divided form and in the presence of moisture chemically react with calcium hydroxide at ordinary temperatures to form compounds possessing cementitious properties.

Pozzolonas must be finely divided in order to expose a large surface area to the alkali solutions for the reactions to proceed.Flyash from a coal fired power station is a pozzolona that results in low permeability concrete which is more durable and able to resist the ingress of deleterious chemicals. Flyash is generally collected through a electrostatic precipiatator or mechanical means.

The first Calcium silicate cements were produced by the Greeks and Romans who discovered that volcanic ash if finely ground and mixed with lime and water produced a hardened mortar which was resistant to weathering. The reaction is known as the pozzolanic reaction and is the basis of the contribution made to strength and concrete performance by materials such as flyash, microsilica and metakaolin in modern concrete.

Benefits of using Flyash in Concrete

The first reference to the idea of utilizing coal flyash in concrete was by Mc Millan and Powers in 1934 and in subsequent research (Davis et al., 1935, 1937).[v] In the late 1940’s UK research was carried out (Fulton and Marshall, 1956) which led to the construction of Lednock, Clatworthy and Lubreoch Dams during the 1950s with flyash as partial cementitious material.[vi] These structures are still in excellent condition, after some 50 years.

When coal burns in a power station furnace between 1250 and 1600 C the incombustible material coalesce to form glassy droplets of silica ( SiO2 ) , alumina ( Al2O3) , iron oxide( Fe2O3) and other minor constituents .When flyash is added to concrete the pozzolanic reaction occurs between the silica glass ( SiO2) and Calcium hydroxide ( Ca(OH)2) OR LIME ; this is a byproduct of the hydration of Portland Cement . The hydration products produced fill the interstitial pores reducing the permeability of the matrix. Roy( 1987) [vii] states 'the reaction products are highly complex involving phase solubility , synergetic accelerating and retarding effects of multiphase multiparticle materials and the surface effects at solid liquid interface' . The reaction products formed differ from the products found in Portland cement - only concretes. A very much finer pore structure is produced with time presuming there is access to water to maintain the hydration process.

Dhir et al.( 1986) [viii] have also demonstrated that the addition of flyash improves the dispersion of the Portland cement particles , improving their reactivity .

Pozzolanic reaction enhances the durability in concrete structures due to the reaction of Calcium hydroxide and silica which is as follows:

Calcium hydroxide + Silica = Tricalcium Silicate +water

Cabrera and Plowman(1981) [ix] works shows (i) Depletion of calcium hydroxide (lime) with time and how this reaction affects the long term gain in strength of flyash concrete compared to PC concrete control.(ii) The reaction takes place both within pores of cement paste and on the surface of flyash particles . When relatively coarse flyash i.e 45 micrometer residue >12 percent is interground with clinker or ground separately the water requirement of concrete is markedly reduced according to Monk [x] Monk also states that (i) Flyash particles less than 50 micrometer are generally spherical and the larger sizes tends to be more irregular (ii) Flyash spheres appear to act as "ball bearings" within the concrete reducing the amount of water required for a given workability (iii) The spherical particles confer significant benefits to the fluidity of concrete in a plastic state by optimizing the packing of particles.

We should note that silica has to be amorphous that is glassy because crystalline silica has very low reactivity [xi] .

The flyash particles are spherical ( influencing the water requirement) and a very high fineness : vast majority of particles have a diameter between less than 1 micrometer and 100 micrometer and the specific surface of flyash is usually between 250 and 600 m2/kg using the blaine method [xii]. Flyash should react with calcium hydroxide generated during the hydration of cement for which specific surface is the major criteria.

It's perceived that compressive strength of PPC is lower and delays the removel of shutters. However in practice it is seen that formwork are removed within the usual time limits. This is because PPC's produced nowadays are having compressive strength higher than the minimum codal requirements as stated in IS 1489 (Specification for Portland Pozzolona Cement). Therefore if proper mix proportions are followed at site then the concrete so produced will have the desired compressive strength.

Classification of flyash

American Classification of flyash as given in ASTM C 618-94a is based on the type of coal from which the ash originates. The most common flyash derives form bituminous coal which is mainly siliceous and is known as Class F flyash. Sub - bituminous coal and Lignite results in high lime ash known as Class C flyash. The pozzolanic activity of Class F flyash is in no doubt but it is essential that it has constant fineness and constant carbon content. The two are often interdependent because the carbon particles tend to be coarser. Modern boiler Plants produce flyash with a carbon content of about 3 percent but much higher values are encountered in flyash from older plants. The carbon content is assumed to be equal to the loss on ignition although the latter includes also any combined water or fixed carbon dioxide present.

Class C flyash is high lime ash originating from lignite coal. Such ash may occasionally have a lime content as high as 24%. High lime ash has some cementitious properties of its own because its lime will combine with silica and alumina portions of the ash there will less of these compounds to react with lime liberated by the hydration of cement .The carbon content is low, fineness is high and colour is light . However MgO content can be high and some of MgO and lime can lead to deleterious expansion.

British Standard BS 3892: Part 1: 1993 specifies a maximum 12 percent residue on 45 micro meter sieve which is a convenient basis of classification of size.

The main requirements of ASTM C 618 - 94a are : a minimum content of 70 percent silica , alumina and ferric oxide taken all together , a maximum SO3 content of 5 percent , a maximum loss on ignition of 6 percent and a maximum alkali content of 1.5 percent . The latter value is applicable only when the flyash is to be used with reactive aggregate.

British Standard BS 3892 : part 1 : 1993 specifies a maximum content of SO3 of 2.5 percent and some other requirements. A limitation on MgO content is no longer specified because it exists in non reactive form.

It should be noted that flyash may affect the color of the resulting concrete ,the carbon in the ash making it darker; this may be of importance from the standpoint of appearance especially when concretes with and without flyash are placed side by side.

Bureau of Indian Standards IS 3812 ( Part1 ) : 2003 , Pulverized Fuel Ash - Specification PART1 for use as pozzolona in cement, cement mortar and concrete lays down the physical requirements as follows :

a. Fineness - specific surface in m²/kg by Blaine's Permeability method to be not less than 320 m²/kg

b. Lime Reactivity - Average Compressive strength in N/mm² to be not less than 4.5 N/mm²

IS 3812 also permits the usage of flyash with fineness 250 m²/kg in manufacture of Portland Pozzolona cement by intergrinding it with Portland cement clinker if the flyash when ground to fineness of 320 m²/kg or to the fineness of the resultant Portland Pozzolona cement whichever is lower meets all other requirements.

4. Research Results from Literature of concrete using flyash and other blending materials:

IS 1489 (Part 1):1991: Portland Pozzolona Cement - Specification - Part 1 Flyash based outlines the following advantages of Portland Pozzolona Cement:

Produces less heat of hydration
Offers greater resistance to the attack of aggressive waters than normal Portland cement
Reduces the leaching of calcium hydroxide liberated during the setting and hydration of cement

Naik and Ramme have presented the following results based on the research conducted at precast/prestressed Concrete Plant with Class C flyash:

Cement replacement of upto 30 percent with flyash increases early strength compared with no flyash
As the amount of flyash increases the water required for the same workability in the mix decreases
Flyash use improves the workability of the mix and thus allows a decrease in the amount of water used.
For the same workability the water to cementitious ratio decreased significantly as the flyash content increases from zero to 30 percent replacement of cement

Various materials could be blended with cement to get the desired performance in concrete. Thomas et. al has concluded in their paper that ternary cementitious blends of Portland Cement , silica fume and flyash offers significant advantages over binary blends and even greater enhancements over Plain Portland Cement .

Thomas et al has emphasized on the following:

Combination of silica fume and low CaO flyash is complementary; the silica fume improves the early age performance of concrete with flyash continuously refining the properties of hardened concrete as it matures.
In terms of durability blends are vastly superior to plain Portland cement concrete.
Shortfall of high CaO flyash in terms of controlling Alkali Silica Reaction and sulphate resistance can be complemented for by the incorporation of relatively small quantities of silica fume. Combination of 3-5% silica fume with 20 to 30 % high CaO shows satisfactory performance in both Alkali Silica reaction and sulphate expansion tests.
Silica fume while imparting significant contributions to concrete strength and chemical resistance can create increase in water demand , placing difficulties and plastic shrinkage problems in concrete if not properly used and can present handling difficulties in the raw state . Most flyashes regardless of composition tend to reduce water demand of concrete and increase its resistance to fluid flow and the ionic diffusion. The beneficial effects of flyash on permeability and diffusivity tend to become more apparent with time (Maturity) especially in case of more slowly reacting low CaO flyashes.

Combination of silica fume and flyash in a ternary cement system should result in a number of synergistic effects which are as follows [xvi]:

Silica fume compensates for low early strength of Concrete with low CaO flyash
Flyash increases long term strength development of silica fume concrete
Flyash offsets increased water demand of silica fume
Silica fume reduces the normally high levels of high CaO flyash required for sulphate resistance and ASR prevention
Very high resistance to chloride ion penetration can be obtained with ternary blends
Low CaO flyash compensates for higher heat release from Type 10SF cement
The relatively low cost of flyash offsets the increased cost of silica fume .

Study made by Camoes.A [xvii] based on the Paste Mix with various reveals the following:

Presence of Flyash is beneficial increasing the fluidity of the pastes; it seems that action of SuperPlasticiser is not limited to the Cement Particles, acting on the flyash particles too; the rheological behavior of the mixes made with Flyash contents greater than 60% are different from the others.
CoPolymer SuperPlasticiser is more efficient in Flyash Particles than Napthalene SuperPlasticiser

Observations on few site mix concrete trials using flyash & PPC - flyash based

Referring Table 1; when concrete is made using flyash based PPC there is an improvement in workability. Only when slump retention is for a longer time then we need to resort to the use of admixture with PPC.

The mix proportions for trials as shown in table needs to be changed based on the quality of ingredients and method of mixing and placing concrete.

The Indian Standard code for PPC i.e IS 1489 states that the minimum compressive strength at 3 days, 7days and 28 days is 16 MPa, 22 MPa and 33 MPa respectively. However PPC cements manufactured in the country are much above these minimum values and in most of the cases the stipulated minimum 28 day compressive strength of 33MPa is achieved within 7 days. Therefore PPC is being used in large quantities for major concreting works across the country.

Table 1: Mix proportions (in kg) using PPC: to be changed based on site conditions

Grade of Concrete	Cement(PPC)	River sand	12 mm	20mm	Water	Admixture
M20	310	620	500	780	176	-
M25	340	600	500	770	180	-

Free w/c	Slump after 30 minutes	1 d	3 d	7 d	28 d
0.55	82mm	10	16	20	28
0.5	102 mm	11	17	25	35

Fig 1 compares compressive strengths of concrete with various percentages of flyash. When water / (cement + flyash) ratio is reduced, even with higher flyash percentages, rate of gain in compressive strengths is increased.

Usage of flyash enhances durability in concrete structures. There are tests which help in evaluating concrete durability. ASTM C1202 -94 (Standard Test Method for Electrical Indication of Concretes ability to resist Chloride Ion Penetration) gives a summary of the method for determination of electrical conductance of concrete to provide a rapid indication of its resistance to the penetration of chloride ions .The method consists of monitoring the amount of electrical current passed through 51 mm thick slices of 102 mm nominal diameter cores or cylinders during a 6 h period. A potential difference of 60 v dc is maintained across the ends of the specimen, one of which is immersed in a sodium chloride solution, the other in a sodium hydroxide solution. The total charge passed, in coulombs, has been found to be related to the resistance of the specimen to chloride ion penetration.

Permeability of chloride ions is an essential step for concrete exposed to marine atmosphere as this parameter indicates the quality of concrete in terms of durability .

Chloride Ion Penetrability Based on Charge Passed

Charge Passed ( coulombs)	Chloride Ion Penetrability
> 4,000	High
2000-4000	Moderate
1000-2000	Low
100-1000	Very Low
<100	Negligible

Factors which are known to affect Chloride ion penetration include: water cement ratio, the presence of polymeric admixture, sample age, air void system, aggregate type, degree of consolidation and type of curing

In view of enhancement in durability many major projects have been utilizing flyash in concrete. At Kuala Lumpur City Centre, there are 450 m high twin towers made of RCC core & columns and composite steel /concrete deck floors. Flyash was used in concrete for a grade of M80. Concrete Admixture was used to produce 200 mm slump of concrete (Concrete by Dr S.C. Maiti , Ex Joint Director , National Council for Cement & Building Materials, New Delhi and Raji k Agarwal, MD, Marketing & Transit ( India) Pvt Ltd , New Delhi)

Environmental impact and Sustainable development

The materials and the construction practice in the 20^th century developed primarily to meet the need for high speed construction have generally proven harmful to the durability of concrete structures exposed to severe environmental conditions. Each tonne of Portland Cement requires approximately 1.5 tonnes of Limestone and considerable amounts of both fossil fuel and electrical energy

Table 2 : Worldwide Cement Production and CO₂ Emissions

Year	Cement Production , Billion tones	Total CO₂Emissions Billion tones	CO₂ Contribution by Cement Industry , %
1995	1.4	21.6	7
2008	2.8	36.5	7.7

Source: Sustainability Issues and Concrete Technology - Dr V.M. Malhotra

Table 3: Worldwide Production of Cement , Flyash and other Supplementary Cementing Materials ( Million tones)

	Cement	Flyash	Others
Current Production	2800	900	20
Anticipated Year 2020	4000	2000	>100

Source: Sustainability Issues and Concrete Technology - Dr V.M. Malhotra

Flyash is, and will remain the major supplementary cementing material for years to come. [i]

Conclusions

Usage of flyash in concrete and cement has become inevitable not only from ecological perspective but also from quality perspective. Flyash usage is inevitable from the perspective of durability in concrete structures. Many major structures and ready mix concrete units have been using flyash. Hence awareness programs amongst the Engineers on the usage of flyash with emphasis on enhanced durability will help in removing misconceptions on the usage of PPC. Blended cement was being increasingly used in European countries for quite a long time. Blended Cements accounts for about two thirds of the total European cement consumption of about 175 million metric tones in the year 2000 alone.

References

1,2-Lewis Robert, Sear Lindon , Wainwright Peter , Ryle Ray , "Cementitious Additions" from the book Advanced Concrete Technology Constituent Materials edited by Newman , John; Choo , Ban Seng Editors Elsevier , Butterworth Heineman

3,4-Newman, John; Choo , Ban Seng Editors ; Advanced Concrete Technology Constituent Materials- Elsevier , Butterworth Heineman

5,6-Newman John , Choo Sang as quoted in the book Advanced Concrete Technology

7-Roy D.M ( 1987) Hydration of blended cements containing slag, flyash or silica fume, Sir Frederick Lea Memorial Lecture 29 April - 1 May 1987 Institute of Concrete Technology Annual Symposium

8-Dhir.R.K , Munday .J.G.L and OngL.T(1986) Investigations of the engineering properties of OPC/Pulverised fly ash concrete - deformation properties The Structural Engineer 64B,No2 ,36-42

9-Cabrera ,J.G and Plowman C ( 1987) Hydration and microstructure of high flyash content concrete conference on Concrete dams, London

10-Monk , M.G ( 1983) Portland - flyash cement : A comparison between intergrinding and blending : Concrete research , 35,124 , September 131-141

11,12,13-Neville.A.M ,Properties of concrete, Fourth Edition ,pp83

14-Naik.R.Tarun and Ramme .W.Bruce ( 1990 ), High Early Strength Concrete Containing Large Quantities of FlyAsh , August 1990 , ASCE - MTD Conference , Denver , Colorado

15,16- Thomas M.D.A , Shehata .M.H , Shashiprakash .S.G, Hopkins .D.S , Cail.K , Use of ternary cementitious systems containing silica fume and flyash in concrete , Cement and Concrete Research 29(1999) 1207-1214

17-Camoes .A , Influence of Mineral Admixtures in the Fresh Behavior of SuperPlasticized Concrete Mixes

18, 19, 20-Malhotra. V.M ( 2012), "Sustainability Issues and Concrete Technology", Proceedings of the International Workshop on Sustainability and Advances in Concrete Technology ( SACT 2012) , NIT - Kozhikode ,07-08 May 2012

21- Schmidt Michael , Midden dorf Bernhard , Vellmer Carstein , Geisenhansleuke Carsten," Blended Cements" from the book " Innovations in Portland Cement manufacturing PCA

About the Author

Anil Kumar Pillai

Assistant General Manager - Technical Services, Madras Cements Ltd

Academic Qualifications

BE(Hons) Civil Engineering from BITS, Pilani
EMBA ( Marketing Mangement) from S.P.Jain Institute of Management & Research Mumbai
MBA( Operations Management) from IGNOU , New Delhi
PGDCM (Post Graduate Diploma in Construction Management)

Professional Bodies Affiliation

Member of Institution of Engineers
Life Member of All India Management Association

Representing Madras Cements Ltd as an Alternate member on BIS Committee of Cement and Concrete Sectional Committee CED 2

Experience

1. Associated with cement Division of Larsen & Toubro limited in their operations in South India and Srilanka

2. Worked on Major construction projects in Delhi and UP for Trafalgar House construction India . Associated with bridge projects, Piling and Diaphragm wall projects major amongst the were those of Parliament Library project, NDMC (Ne Delhi Municipal Council) , UPPWD etc

3. Also involved in Management teaching having associated with SCMS - Kochi for teaching Post graduate students of Management

Papers presented ,Workshops and Training Programs conducted

Conducted number of Seminars for Civil Engineers across the country on Cement and Concrete Technology
Presented papers on Marketing Management at well known Institutions like IIM - Kozhikode and at University of Wollongong in Dubai
Conducted training programs related to Marketing , Cement /Concrete to Executives and students .

Major Area of Interest

Cement , Concrete Technology , Marketing Research , Consumer Behaviour

Email: akp@madrascements.co.in

Alternate Email: anilpills@yahoo.com

Mobile : 9710 433436 Tel: No. 044 28116050