nano concrete - possibilities & challenges
TRANSCRIPT
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Nano-Concrete: Possibilities and Challenges
P.N.Balaguru
Rutgers University Ken Chong and Jorn Larsen-Basse National Science Foundation, USA
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Nano cement composites
• Manufacturing of cement
• Admixtures
• Fillers (aggregates)
• Fibers
• Fabrication technique
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Nano cement composites
• Opportunities
• Challenges
• Basics
• Summary
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Synthesis of Nano-cement
• Grinding (NSF-REU U of Delaware)
• Four times the surface area
• Rougher surfaces
• Strength about same
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Grinding
• Mechanical limitations
• Hydration of cement particles due to moisture present in the atmosphere
• Grinding under controlled environment, Low humidity
• Agglomeration of particles
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Chemical Synthesis
• Shows more promise
• Storage
• Very low humidity
• Non-reactive mediums
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Admixtures
• Mineral
• Chemical
• Pozzolans
• Water reducers
• For nano cement, nano silica fume, nano glass particles
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Aggregates
• Ground sand
• Nano or micro ?
• Titanium oxide
• Zinc oxide
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Fillers
• Reduce shrinkage
• Larger/smaller than cement particles
• Larger- more volume fraction
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Fibers
• Nano carbon tubes
• Carbon whiskers
• Short carbon fibers, 7 microns
• Fiber tows
• Fabrics
• Silicon carbide whiskers
• Glass fibers
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Fibers
• Woollastinite
• Metallic fibers
• Ceramic fibers (Nextel) - high temperature applications
• Polymeric fibers, flexible membranes
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Fabrication
• Casting may not be feasible
• Extrusion
• Pulltrusion
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Fabrication Techniques
• Vacuum bagging
• Curing under pressure and high temperature
• Better quality control
• Better mechanical properties
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Products
• Micro meter thick sheets
• Bars
• Tubes
• Laminations
• Coating formulations
• Crack fillers
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Applications
• Electronics
• High strength/ high temperature composites
• Nano meters thick coating to protect electronic components
• Repair of cracks in existing structures
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Applications
• Sensors
• Laminates to protect against terrorism
• Sleeves for cables in bridges
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Nano coatings
• Coatings to reduce corrosion
• Coatings to reduce ingress of harmful chemicals
• Coating to change electrical properties
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Opportunities
• Can be used as an inorganic adhesive with carbon fibers.
• Micron size cement particles are not conducive for use with 7 micron diameter carbon fibers.
• Fire resistant. Will not emit any voc
• Composites can be attached to parent concrete substrate using a compatible adhesive.
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Opportunities
• It will be also very competitive with current inorganic composites because they have to be processed at high temperature
• Could be used instead of organic polymers in Fiber Reinforced Polymers (FRP) systems
• Will be compatible with micro steel meshes
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Challenges
• Heat of hydration
• Special organic and inorganic additives need to be developed to control the setting and heat of hydration
• Even though this is a risky and tough venture, the authors believe that the risk is worth taking
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Challenges
• manufacture nano size cement particles
• Chemical vapor deposition shows promise
• Separation of smaller particles in micro-cement
• Other avenue is high tech grinding
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Basic Questions
• Is the influence of water-cement ratio same for nano cement?
• Will the strength and strain capacity remain same?
• Is it possible to use metallic nano fibers? Will it be possible to dry process the
cement-filler-fiber mix and cure using stream impregnation?
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Basic Questions
• In fiber composites will the influence of fiber volume content remain same ?
• For: strength
• Stiffness
• Electric conductivity
• Thermal conductivity
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Summary
• Large amount of funds and effort are being utilized to develop nano technology. Even though cement and concrete may constitute only a small part of this overall effort, it could pay enormous dividends in the areas of technological breakthroughs and economic benefits.
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Summary
• Current efforts are focused on understanding cement particle hydration, nano size silica and super plasticizer additions and sensors. Unique opportunity exists for the development of nano-cement that can lead to major long standing contributions.
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Basics of Hydration
• Three major solid components of hydrated cement paste are: Calcium Silicate Hydrate (CSH), Calcium Hydroxide crystals (CH or portlandite) and Calcium Sulfo-aluminates (CS or ettringite). CSH occupies about 50 to 60 percent of the volume where as CH and CS occupies 20 to 25 percent and 15 to 20 percent respectively.
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Basics of Hydration
The size of CSH sheet is less than 2 nm and the space between the sheets vary from 0.5 to 2.5 nm. Aggregation of poorly crystalline CSH particles could occupy 1 to 100 nm. Inter-particle spacing within an aggregation vary from 0.5 to 3 nm.
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Basics of Hydration
• CH products are typically large with a width of about 1000 nm.
• CS has needle type structure and is unstable.
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Basics of Hydration
Size of capillary voids range from 10 to 1000 nm. However in well hydrated paste with a low water-cement ration the pore size is typically less than 100 nm.
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Basics of Hydration
• C3A generates the most heat and C2S generates the least amount of heat.
• Heat of hydration has two peaks, one occurs during the dissolution stage and the second occurs during the formation of compounds
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Basics of Hydration
Aluminates hydrate much faster than silicates. Silicates, which make up about 75 percent of cement plays a dominant role on strength development.
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Basics of Hydration
Of the two mechanisms of hydration through-solution hydration is more suitable for nano cements. In this mechanism, complete dissolution of anhydrous compounds to their ionic constituents and eventual precipitation of hydrates are assumed to take place.