Domain

Explanation

Speaker

• Dr. Dhanada Mishra has recently left the position of Manager of Rocla Technology in Sydney Australia to take up a Professorship in the Department of Civil Engineering at the Kalinga Institute of Industrial Technology in India. He has a Ph. D. in Civil Engineering Materials specialising in Engineered Cementitious Composites from University of Michigan. He has worked in commercial research, development and consulting for the Associated Cement Companies (ACC) in Bombay, India and Rocla Industries in Australia.
• In his role as a consulting engineer with ACC, Dhanada Mishra was involved in a number of investigation and repair/rehabilitation of major concrete structures including nuclear power stations, cement plants, dams and thermal power plants. In his role as Manager, Product Development at Rocla Technology, Dhanada Mishra has been involved in the development of a number of key precast concrete products utilising Geopolymers and Engineered Cementitious Composites. This involved concept development, concept proofing, prototyping and commercialisation.
• Dhanada Mishra is a member of the American Concrete Institute and the ECC Technology Network. He has contributed to the activities of Australian Standards Committees and Concrete Pipe Association of Australasia.

Abstract

• Fly Ash is a by-product of the combustion of pulverised coal. It is collected at coal powered thermal power plants by electro-static precipitator from the flu gases. It was estimated that 600 million tonnes of fly ash would have been produced world wide in year 2000, most of which would have ended up in land-fills polluting the environment. Flyash is classed as a pozzolan with the potential to react with lime and moisture to form calcium-silicate hydrate CSH and other cementitious compounds. The pozzolanicity of flyash varies from source to source depending on the chemical composition (class F or class C), particle size distribution, level of impurity (Loss of Ignition) etc. Its low cost (mainly stemming from additional processing cost at the source and the transport cost), abundant availability, pozzolanic properties and proven benefits in terms of enhanced durability of concrete has resulted in the recent research being mainly focused in the increased use of flyash in concrete. Use of flyash (either as a replacement of Portland cement or as an additive), is on the increase in the building and construction industry worldwide. The pioneering work of V. M. Malhotra in Canada in recent years in high volume flyash cement concrete has demonstrated the potential for extensive use of flyash in concrete in the future.
• Alkali activated cements have been actively researched in the countries of former Soviet Union as part of the need to find a way to deal with the massive problems associated with disposal of industrial by-products such as flyash and slag. The work of Krivenko and Glukhovsky in Ukraine has resulted in many significant structural and non-structural applications (e.g., coatings, repairs, thermal resistance, lightweight construction etc.) of such concrete materials based on non-Portland cement binders. Recently Davidovits has studied the alkali activated cements family of materials in a different context and has discovered what is essentially an extended family of materials that he has termed 'Geo-polymers' or 'Inorganic Polymers'. Geopolymers are composed of molecular chains similar to organic polymers except that the monomers in this case are composed of 'Al', 'Si' and 'O' atoms or ions. It is claimed that in the Alkali activated cement system, the introduction of Ca ions creates mixed geopolymer phases with calcium silicate hydrates. Depending on the Al:Si ratio available in the feedstock and a whole host of other variables, geopolymer materials can range from the low tech fly ash based alkali activated concrete to the high tech carbon fiber reinforced composites developed by Davidovits and Balaguru for aerospace applications.
• Fly ash being a rich source of Al and Si, presents a great opportunities for the future. On the one hand as a major environmental problem, it needs a viable disposal option, while on the other its use can potentially reap rich dividend. The presentation begins with some of the fundamental aspects of fly ash and ends by looking at some of the exciting potential applications for what was otherwise a problematic waste material.

Rocla

• Patented technologies: decades old
• Engineering technology changes slower than IT: hence needs product succession phase

1. Material changes
2. Design & modeling
3. Prototyping: full-scale, testing & derive properties
4. Production & manufacturing
5. Business & commercialisation

• Products: spun concrete pipes, poles with compaction by spinning moulds & spraying concrete

Contents

• Background
• Geopolymers: inorganic polymers
• Environmental implications: future influences
• Conclusions

Fly ash

• Introduction:

1. Fly ash: inorganic residue by electro-static precipitator from combustion by-products
2. 2000: 600million tonnes of fly ash: 80% of unused are dumped into landfills
3. Pozzolanic: siliceous (Si) aluminous (Al) materials à water & lime à cementitious materials
4. Uses: road bases, concrete blocks, bricks

Fly ash basics

• Fly ash particle: cenospheres à hollow spheres
• As impounded materials in dams’ bulk: slurry à water à evaporates à not sustainable
• Classification:

1. ASTM C618
2. Mehta’s for siliceous mineral admixtures

• PSD:

1. Average fly ash: 20microns (slighter larger than cement)
2. Average cement: 10microns
3. Sieving for classification
4. R&D: to decrease fly ash particles to 5-10microns

• Properties: vary according to combustion attributes, wastes/fuels burnt, locations of plants & types/composition, etc.

1. Workability
2. Less water needed: Lower hydration heat, cracking, shrinkage
3. But lower initial compressive strength development: but eventually same strength
4. More durable: less permeable, less sulfate attack, less ASR, less steel corrosion

Fly ash current applications

• Free binders: 100% fly ash, no cement at all
• Blended cement: clinker (less 8% clay), inter-ground (less 30%)
• Mineral admixture: less 30%
• Special binders: alkali activated
• Bricks & blocks
• Aerated concrete & foamed concrete
• Lightweight aggregates
• Ceramics
• Soil stabilization
• Waste neutralization & waste encapsulation
• Landfills
• Alkali-activated flyash concrete
• Enhanced flyash concrete: lower LOI, modify PSD for specific uses
• Hazardous waste immobilization
• Geopolymers

Geopolymers

• Polymers:

1. Organic polymers: chains of organic monomers from C, O, H, Cl & N
2. Inorganic polymers: chains of monomers from Si, Al, O (no carbon, hence lower inflammable)

• Sol-gel silica technology: Si-O- chains
• Geopolymer: extend sol-gel silica technology to introduce Ca ions creates mixed geopolymer phases with calcium silicate hydrates (CSH)
• Geopolymer pioneer: Davidovits
• Alkaline solution à Dissolve: Si, Al à monomers à heat à condensation: gel, polymer
• Geopolymer process:

1. Dissolution: dissolve Si, Al
2. Condensation of polymerization: geopolymer
3. Long-term gradual crystallization: abt. 100 years

• Geopolymer mineral structures very close to nature: durable by deduction
• Ancient civilizations used volcanic ash, activated by lime for mortar & binders for cast concrete blocks
• Concrete hydration:

1. Hydration products: CSH not natural à inherently unstable à react with time
2. End-products: problems

• Evidence of geopolymers:

1. Reactants: feedstocks or sources for Si & Al in amorphous or glassy phases à Class F fly ash, GGBF slag, metakaolin
2. Al/Si ratio:

• 1:1: bricks
• 2:1: cements, concretes
• 3:1: moulds, fibre-reinforced composites
• >3:1: sealants, adhesives
• (20-35):1: fire & heat-resistance for fibre materials & composites for aerospace

Hypothesis

• Modeling of polymerization process:

1. Dissolution
2. Condensation
3. Polycondensation

• Problems:

1. 2 types of end-products:
2. Geopolymers: good, lower CO₂ & pollution
3. Zeolites: bad
4. Glassy states: not the commonly-known crystalline states à requires R&D
5. Magnetic resonance imaging: to see particles

• End-products:

1. Primary product: rigid chains or nets of geopolymers
2. By-products: more water & Na ions & unreacted silicon hydroxide

• Competing processes:

1. Geopolymers & zeolites
2. Al ions must be readily dissolvable, or else slow hydration with crystallization to form zeolites

• Geopolymers:

1. Not hydrate
2. Uses all alkaline OH
3. Water like enzymes: water à reacts: geopolymers & zeolites à releases water

• Consequences of geopolymers:

1. No bound water: more heat resistance, lower shrinkage
2. Less free alkaline OH: more acid resistance, no long-term alkaline attack

• Material properties of low-tech concrete:

1. More strength
2. Higher rate of strength gain
3. More heat & fire resistance
4. More acid resistance
5. More waste encapsulation

Environmental implications

• Cements:

1. 1995: 1.4 billion (2 billion: 2000) tonnes
2. 1 ton Portland cement produces 1 ton of CO₂ gas: 50% from calcinations of limestones & 50% from coal combustion
3. Major producers: East Asia, South-East Asia, Europe

• Fly ash disposal:

1. 49%: landfills
2. 41%: surface impoundments (e.g. dams)
3. 10%: old quarries à more groundwater pollution

• Benefits from using fly ash:

1. Similar costs as cements: fly ash still needs activator by NAOH (industrial by-products from paper-making etc.)
2. Lower pollution
3. Less CO₂
4. Less waste materials for feedstocks
5. Encapsulate waste hazards & solidify toxic wastes: react directly with wastes
6. Cannot burn or emit toxic fumes during fires

Conclusions

• More strategic implications
• Future trends: less cement; more waste usage; profitably utilize fly ash
• Less CO₂; less pollution