Use of PPC in construction
Use of PPC in construction

Use of PPC in construction

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 says
Anil Kumar Pillai, DGM - Tech Service at The Ramco Cements
.

The concept of blending materials in concrete to enhance durability is quite old and many of the old structures which have survived the ravages of time is a testimony to the fact that blending cementitious materials has enhanced the life of the concrete structure. The concrete structure in extreme exposure conditions will have to use blending materials to ensure that the concrete is able to resist various chemical attacks on it. Therefore, it is well accepted in the construction industry that flyash in concrete acts as a pozzolana and hence is being increasingly used in concrete worldwide.

ASTM 618 - 94a describes pozzolana 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.

Pozzolanas 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 pozzolana 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 precipitator 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.

Flyash: The source of silica in concrete
The concept of using of ash in concrete is not new. It´s usage can be traced to the Romans who chose concrete as a construction material which was used in monuments such as the Pantheon in Rome and marine structures. Many of the Roman underwater concrete structures have endured the saltwater environment of the sea. On examining the proportion of the concrete by an engineer - Marcus Vitruvius Pollio who later became Emperor Augustus - around 30 BC it was found that the major ingredient that played a vital role was the volcanic ash which the Romans combined with lime to form mortar. These structures have withstood various attacks for more than around 2,000 years. The historical findings was a part of the study by a research team from the University of California, Berkeley who have examined the fine-scale structure of Roman concrete to arrive at this conclusion.

The first reference to the idea of utilising coal flyash in concrete was by McMillan and Powers in 1934 and in subsequent research (Davis et al., 1935, 1937). In the late 1940s, 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. These structures are still in excellent condition after some 50 years.

When coal burns in a power station furnace between 1,250 and 1,600 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) 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) 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) works show (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 per cent is interground with clinker or ground separately, the water requirement of concrete is markedly reduced. 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 optimising the packing of particles.

We should note that silica has to be amorphous, that is glassy because crystalline silica has very low reactivity. 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. 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 Portland Puzzolana Cement (PPC) is lower and delays the removal of shutters. However, in practice it is seen that formwork are removed within the usual time limits. This is because PPCs produced nowadays have compressive strength higher than the minimum codal requirements as stated in IS 1489 (Specification for PPC). 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 per cent 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 lime content as high as 24 per cent. High lime ash has some cementitious properties of its own because its lime will combine with silica and alumina portions of the ash, and 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 of 12 per cent residue on 45 micrometer sieve which is a convenient basis of classification of size. The main requirements of ASTM C 618 - 94a are a minimum content of 70 per cent silica, alumina and ferric oxide taken all together, a maximum SO3 content of 5 per cent, a maximum loss on ignition of 6 per cent and a maximum alkali content of 1.5 per cent. 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 per cent 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 colour 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 pozzolana in cement, cement mortar and concrete lays down the physical requirements as follows:

  • Fineness - specific surface in m2/kg by Blaine´s Permeability method to be not less than 320 m2/kg
  • 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 Pozzolana 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 PPC whichever is lower meets all other requirements.

Results
IS 1489 (Part 1) :1991: PPC - Specification - Part 1 Flyash based outlines the following advantages of PPC:

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

Naik and Ramme has presented the following results based on the research conducted at precast /prestressed concrete plant with Class C flyash:
a.Cement replacement of up to 30 per cent with flyash increases early strength compared with no flyash
b.As the amount of flyash increases the water required for the same workability in the mix decreases
c.Flyash use improves the workability of the mix and thus allows a decrease in the amount of water used.
d.For the same workability, the water to cementitious ratio decreased significantly as the flyash content increases from 0-30 per cent 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 PPC, silica fume and flyash offers significant advantages over binary blends and even greater enhancements over Plain Portland Cement.

Thomas et al has emphasised on the following:
a.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.
b.In terms of durability blends are vastly superior to plain Portland cement concrete.
c.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 per cent silica fume with 20 to 30 per cent high CaO shows satisfactory performance in both Alkali Silica reaction and sulphate expansion tests.
d.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:

  • 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 based on the Paste Mix with various proportions {Cement + Flyash/(Cement + Flyash) = 0,20,40,60 ,100 per cent + Copolymer SuperPlasticiser or Napthalene SuperPlasticiser/(Cement + Flyash) = 0.15 ;0.25;0.50;1.0 2.0,3.0,4.0 } reveals the following:
a.Presence of Flyash is beneficial increasing the fluidity of the pastes; it seems that action of Super Plasticiser is not limited to the Cement Particles, acting on the flyash particles too; the rheological behaviour of the mixes made with Flyash contents greater than 60 per cent are different from the others
b.CoPolymer SuperPlasticiser is more efficient in flyash Particles than Napthalene SuperPlasticiser

Observations
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.

Usage of flyash enhances durability in concrete structures. There are tests which helps 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. 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, etc.

In view of enhancement in durability many major projects have been utilising flyash in concrete. At Kuala Lumpur City Centre, there are 450 m high twin towers made of RCC core and 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. (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 20th 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 PPC requires about 1.5 tonnes of limestone and considerable amounts of both fossil fuel and electrical energy. Flyash is and will remain the major supplementary cementing material for years to come.

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 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

  • 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
  • Newman, John; Choo, Ban Seng Editors; Advanced Concrete Technology Constituent Materials Elsevier, Butterworth Heineman
  • Newman John, Choo Sang as quoted in the book Advanced Concrete Technology òRoy 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 òDhir RK, 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
  • Cabrera JG and Plowman C (1987) Hydration and microstructure of high flyash content concrete conference on Concrete dams, London
  • Monk, M.G (1983) Portland - flyash cement: A comparison between intergrinding and blending: Concrete research , 35, 124 , September 131-141
  • Neville A.M , Properties of concrete , Fourth Edition, pp83
  • Naik.R. Tarun and Ramme W Bruce (1990), High Early Strength Concrete Containing Large Quantities of FlyAsh, August 1990, ASCE - MTD Conference, Denver, Colorado
  • Thomas M.D.A, Shehata M.H, Shashiprakash SG, Hopkins DS, Cail K, Use of ternary cementitious systems containing silica fume and flyash in concrete, Cement and Concrete Research 29 (1999) 1207-1214
  • Camoes A, Influence of Mineral Admixtures in the Fresh Behavior of SuperPlasticized Concrete Mixes
  • Malhotra VM (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
  • Schmidt Michael, Midden dorf Bernhard, Vellmer Carstein, Geisenhansleuke Carsten, "Blended Cements" from the book "Innovations in Portland Cement manufacturing PCA".

ABOUT THE RAMCO CEMENTS
It is the flagship company of the RAMCO group, one of the leading cement manufacturers in India with a production capacity of 16.5 mtpa. Ramco manufacturers different types of Portland Pozzolona Cements through it various plants. It is the only Indian cement manufacturer to win the Four Leaves Award instituted by Centre for Science & Environment. The research wing of the cement major - 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.

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 Free
w/c
Slump af-
ter 30 min

1 d

3 d

7 d

28 d
M20 310 620 500 780 176 - 0.55 82mm 10 16 20 28
M25 340 600 500 770 180 - 0.5 102 mm 11 17 25 35

Table 2: Chloride ion penetrability based on charge passed

Charge Passed

(coulombs)

Chloride Ion

Penetrability

> 4,000 High
2,000-4,000 Moderate
1,000-2,000 Low
100-1,000 Very Low
<100 Negligible
Source: ASTM C 1202 - 94

Table 3: Worldwide cement production and CO2 emissions

Year

Cement

Production,

Billion tonne

Total CO2

Emissions,

Billion tonne

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 4: Worldwide production of cement, flyash and other supplementary cementing materials (Million tonne)


Cement

Flyash

Others

Current Production 2,800 900 20
Anticipated Year 2020 4,000 2,000 >100
>100 Source: Sustainability Issues and Concrete Technology - Dr VM Malhotra

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