Please enable JavaScript.
Coggle requires JavaScript to display documents.
CIVL320: Midterm 1 Review (Cement Manufacturing (Producing concrete…
CIVL320: Midterm 1 Review
Aggregates
To minimize void spaces and therefore amount of hcp required, aggregates need to have a continuous range of particle sizes
Classification
Particle size
Fine Aggregate: Passes through a 4.75mm sieve
Nominal maximum aggregate size: ~95% of aggregate passes through
Grading
Particle size distribution -> Weight proportion of different sized particles
Specific Weight
HEAVY WEIGHT: SG > 3
Magnetite, Hematite, Iron
Used for shielding applications
NORMAL WEIGHT: 2.5 <= SG <= 3.0
LIGHT WEIGHT: SG < 2.3
Shale, clay, pumice
Wet lightweight aggregates used to produce insulating self-curing concrete
Particle Shape
Rounded
Less interlocking, increased workability
Angular
More interlocking, more friction and therefore higher strength
Moisture Content & Interaction with Water
Aggregate pores can retain, absorb and release water according to their porosity, environmental conditions and surrounding materials
If moisture content < absorption capacity, the aggregate will absorb water from the surrounding concrete
If moisture content > absorption capacity, aggregate will release water into the surrounding concrete
If pores in the aggregate are interconnected, higher porosity
Lower porosity is generally desired
Less workability
Higher bulk density
Higher durability
Higher strength
Higher abrasion resistance
Higher elastic modulus
Aggregates are inert fillers
Bonding between cement paste and aggregates strongly affects tensile strength of concrete
Mechanical interlocking
Higher friction
Capillary action
Electrostatic condition on the surface
Durability is affected by unstable volume changes, mechanical deterioration and chemical degradation
Freeze-thaw cycles
Alkali-reactive aggregates give rise to abnormal expansion and map cracking
Materials finer than 75 micrometer increase water requirements
Clay coating reduces bond strength between cement paste and aggregates
Organic impurities delay setting time and reduce strength
Cement Manufacturing
Producing concrete
Molds are oiled thoroughly to avoid strong bonding that could damage concrete during demolding
Dry materials are blended
Water is added and mixed
Fresh concrete is poured
Concrete is vibrated to help with homogeneity and to let excess air out
Excess air would generate more voids
Supplementary Cementitious Materials
Materials that when added to cement contribute to the concrete's hardened properties through hydraulic or pozzolanic activity
High strength concrete has more SCMs (fly ash, silica fume)
Concrete Curing
Necessary to avoid rapid water evaporation
Rapid water loss gives rise to plastic shrinkage, which generates cracks in early age concrete
Protection is added at the surface through impermeable surfaces
Additional water is supplied to the water to maintain the w/c ratio and to avoid drying shrinkage
Cement Manufacturing
Raw Materials
CaCO3, SiO2
Raw materials are prepared
Raw materials are heated in a rotary kiln
Clinkers produced
Clinkers are cooled and ground with gypsum
Clinker compounds
C3S
C4AF
C2S
C3A
Main reaction: Calcium silicates + water -> CSH, CH
Hydration Products
CSH
CSH gel is the main strength imparting compound
CH
Platey CH crystals do not impart any strength, water soluble
Ettringite needles
Found in high concentrations along with CH crystals at interfacial transition zones
Hydration Characteristics
Reactivity of Compounds
C3A > C3S > C4AF > C2S
Fineness controls rate of reaction
Increased fineness = increased rate of reaction
Stages of Heat Evolution
1 - Rapid heat evolution
2 - Dormant/incumbent period
3 + 4 - C3S and C3A hydration ---> Most heat evolved
Heat, Shrinkage & Cracking
Concrete and cement paste are vulnerable to cracking
Thermal gradients generate deformation due to curvature
Water movement and surface tension in pores of cement paste generate stress
Admixtures
Materials added to cement immediately after or during the mixing process.
Mineral Admixtures
SCMs are added to improve workability, strength and durability
SCMs combine with calcium hydroxide produced during the hydration reaction, forming calcium silicate hydrates
Higher CSH content
Lower Ca(OH)2 content
More durable and resistant to leaching
SCMs
Natural Pozzolans
Pumicite, metakaoline
Industrial By-products
Fly Ash, Silica Fume, Blast Furnace Slag, Wood/rice husk ash
Fly Ash
Reacts with CH to produce more CSH
Slow reaction rate, strength develops slowly, long term strength improved
Reduces permeability, increases durability
Spherical particle shape, better workability, reduced water demand
Reduces heat of hydration
Silica Fume (Super Pozzolan)
Achieves very high strength, low permeability, improves cohesiveness and higher chemical resistance
Applications: Shotcrete for mine and excavation support, chemical tanks, high strength concrete
Blast Furnace Slag
Needs to be quenched and ground to be hydraulically active
Slow reaction rate, strength develops slowly
Reduces permeability, durability increases
Chemical Admixtures
Mainly organic salts, some hydrophobic and others hydrophilic
Plasticizers
Provide workability aid
Inexpensive, but contain impurities
Usually long chain polymers
Plasticizing Action
Polymer molecules are surface active and get adsorbed on to the surface of cement grains, creating a net negative charge and mutual repulsion. This results in steric hindrance and releases entrapped water particles, increasing lubrication
Accelerators
Increase rate of hardening
Enhanced early age strength
Can reduce curing time for concrete in cold weather
CaCl2 gets involved in hydration reaction, acting as a catalyst
Increased vulnerability to corrosion of embedded steel; cannot be used with prestressed or reinforced concrete!
Retarders
Delays setting time
Can be used to combat rapid heat evaporation in hot weather or control large pours
Slows heat of hydration evolved
Extends the time that concrete remains workable
Superplasticizers
Used to improve workability at a much larger scale
Steric hindrance and mutual repulsion of cement grains due to adsorption
Air Entraining Agents
Organic materials that entrain air bubbles in cement paste
Act as air-water interface in the cement paste
Provides freeze-thaw resistance
Ensures the development of correct void spacing, size and amount by creating a system of uniformly dispersed air voids