ENHANCING THE USAGE OF FLY-ASH IN CEMENT ANDCONCRETE THROUGH MECHANICAL ACTIVATION FORSUSTAINABLE CONSTRUCTION SOLUTIONS

Satyanarayan P. Arya

Ardor Cement Machinery

Introduction:

In India, the generation of electricity is overwhelmingly depending on the combustion of high ash coalin Thermal Power Plants (TPP). The present availability of fly ash is already exceed 120Mt/yr. and thefigure is likely to increase manifolds in future, posing serious disposal and ecological problems inaddition to occupying large tracks of scarce cultivable lands. Fly ash is considered as industrial waste ingeneral and raw material in cement industry for production of Portland Pozolona Cement (PPC). Flyash can be transformed into property enhancer to cement through mechanical process to enhance theuse of fly ash. For last two decades classification of fly ash through air-separator is done in fly ashindustry but this leaves 50 to 60% coarse fly ash of very low reactivity, unusable in construction. Flyash is economical to use by replacement of cement and this is done through mechanical activation of flyash which improve lime reactivity of whole quantity of fly ash two to three time.

Out of the current generation of 120 Mt/yr. fly ash from Thermal Power Plants (TPP) in India only 65Mt/yr. of fly ash is being presently used in cement manufacturing, construction industry and otherindustries. Thus leaving a vast scope for maximizing its use. The reaction of fly ash with calciumhydroxide results in the formation of strength giving compounds namely calcium silicate hydrates. Ifthe fly ash is not reactive towards lime, the PPC is to be treated as diluted Ordinary Portland cement(OPC), which is not desirable. The fineness of fly ash, particle shape & structure has direct relationshipwith its lime reactivity. Fly ash addition to OPC is limited for its low lime reactivity. Solution to theproblem of disposal of large quantity of fly ash generated in TPP lies in converting it to value addedproduct which perform as property-enhancer to OPC. It is well established in cement industry that fly-ash having lime reactivity in the range of 7 to 12Mpa can be added 35% to OPC to produce PortlandPozolona Cement (PPC) of grade-53.

Material & Methods:

Fly ash is valuable ingredient in PPC but its intrinsic properties are not fully utilized in manufacturingof PPC due to poor grinding of fly ash with clinker in existing grinding systems. Inter-grinding of thefly ash with clinker is prevailing practice in cement industry. Fly ash with high glass content isfavorable for manufacturing of PPC. Glass phase mainly depends upon chemical composition andcombustion temperature of coal. Indian fly ashes generally have glass content in the range of 16 to 40%compared to fly ash samples from USA, Japan and France where the glass content is as high as 80%.Grinding of fly ash is considered a good solution by the scientists but its commercial viability has beenin question for decades. In recent times cement industry and fly ash industry have tried various methodsof grinding the fly ash. The desired fineness is achieved by the pulverization in various types of

grinding mills currently available in the market. Mechanical activation is being performed in a newlydesigned Multi Roller Mill (MRM) in India to make the fly ash grinding cost-effective compared toBall mill, Vibratory mill, Attrition mill and Tower mill.

Size reduction of fly ash particles through grinding process is termed as Mechanical Activation of flyash. This is proven through various investigations & research work done across the globe on improvingthe lime reactivity of fly ash through mechanical activation. Also there is substantial increase in theStrength Activity Index (SAI) of fly ash through mechanical activation.

For achieving lime reactivity of fly ash in the range of 7 to 12Mpa, it is necessary to grind it to thefineness of 4000 to 6000 sq.cm./g Blaine values which is close to particle size D50-10 microns and thiscan be achieved by grinding the fly ash in separate grinding mill. It is important to note that process ofgrinding the fly ash make the difference. Fly ash ground in Ball Mill and Eccentric Vibratory Mill(EVM) have different characteristics and behavior. In ball mill particle size reduction is by attritionwhile in in EVM (mill) it is impact force applied for size reduction. Different processes generatedifferent particle shape and structure. Lime reactivity conversion factor depends on particle size,particle shape and structure hence process of grinding have important role in improving the limereactivity & Strength Activity Index.

Addition of fly-ash in PPC grade-53 is limited to 20-30% for low lime reactivity of fly-ash. In ReadyMix Concrete (RMC) and other applications in construction industry the present use of unprocessed flyash is limited to 10 to 15% with cement and this can be increased two to three fold by the use ofmechanically activated fly ash of lime reactivity 7 to 12Mpa. Lime reactivity 7-12Mpa of fly ash isattained through mechanical activation. There is potential to double the consumption of fly ash incement industry & construction industry through mechanical activation of fly ash. Fly ash as receivedfrom TPP has spherical particles of size 1 to 350 microns and these particles have non-reactive hardlayer on outer surface which act as dissolution barrier to reduce the lime reactivity. Average limereactivity of fly ash is found 4.5 to 6 Mpa and it is the limiting factor for the addition of fly-ash incement, RMC and other applications in civil constructions. Fly ash is converted to value added productthrough mechanical activation process. Fly ash grinding in the new mill-MRM has shown increase inpozolonic reactivity. Fly ash of class F (ASTM C 168) from TPP in Gujarat, India was ground in MRMfor investigating the change in lime reactivity. Also utilization of a typically processed fly ash in OPChas shown increased compressive strength in all ages as compared to strengths achieved for OPC withunprocessed fly ash.

Mechanical activation of fly ash does three important changes in physical structure of fly ash particlesto increase its lime reactivity. a- Particles, above 15 microns size are broken to produce amorphousparticles. b- Dissolution barrier from outer surface of spherical particles is substantially removed. c-Average particle size of fly ash is reduced to improve density & filler effect of fly ash.

Physical Properties of Raw Fly Ash:

a- Particles are spherical in shape and structure is crystalline. b- Particles have vitreous body and size isvarying from 1 micron to 350 microns. c- Density, 2.2 to 2.4 g/c.cm. d- High flow-ability. e- Particlestend to agglomerate.

Physical Properties of ground Fly Ash (mechanically activated):

a- Particles are amorphous in shape & have no crystalline structure. b- Particles of 15 microns andabove size are broken to smaller size of irregular shape. c- Particles below 15 microns size get groundand dissolution barrier is reduced. d- There is 5 to 10% increase in density of fly ash. e- Particlestendency to agglomerate is reduced.

Grinding process of fly ash (mechanical activation):

Fly ash is ground with other ingredients in cement industry and this process is called combinedgrinding. Intrinsic properties of fly ash are not fully utilized in cement manufacturing due to typicalphysical properties of fly ash which make it difficult to grind. Poor grinding of fly ash result in low limereactivity.Separate grinding of fly ash enables good control on Particle size Distribution (PSD). Fly ashparticle size of 5 to 15 microns is ideal to achieve good filler effect and faster dissolution in alkaliswhiles the process of cement hydration. Fly ash grinding in Multi Roller Mill (MRM) is economical forits typical grinding process in which intense shearing forces work on particles at high speed. For highspeed of grinding mill and intense shearing forces acting on fly ash particles, required particle sizereduction is achieved in short time. Crushed fly ash particles are amorphous and structure is non-crystalline.

Performance of mechanically activated fly ash in cement mortar & concrete:

a-Reduction in water requirement in cement mortar and concrete. b- Improvement in lime reactivityenables more addition of fly ash in Ordinary Portland cement. c- Rise in early strength and long termstrength of cement mortar and concrete. Strength Activity Index is found 110 to 125%.d-Hard &denseconcrete is formed to improve strength and reduce permeability. e- Substantial improvement inworkability of concrete. f- Heat generation is reduced in cement hydration process.

Test results of fly ash ground in Multi Roller Mill:

Substantial improvement in lime reactivity and change in PSD is recorded. (Table1& 2)

Table 1- Lime reactivity test report

Fly ash type LimeReactivity

Blaine Particlestructure

Particleshape

Residue on325 mesh

ParticleSize

class F (ASTMC168)

4.79Mpa 286Sq.m./kg

Crystalline Spherical 35% D50 at 45microns

Mechanicallyactivated

12.50Mpa 640Sq.m./kg

Amorphous Irregular 4% D50 at 9microns

Table 2- Particle size distribution (PSD)

Fly ash type 0-5microns

5-10microns

10-15microns

15-25microns

25-45microns

Above 45microns

Class F (ASTM C168) 9% 8% 8% 12% 18% 45%Mechanically activated 32% 31% 14% 5% 14% 4%