Lec 6 Flashcards
What is microstructure
The type, amount, size, shape, and distribution of phases present in a solid constitute
What is the limit resolution of the unaided human eye
Approximately 1/5 milimeter (200 niumeter)
What is the use of microstructure
for the microscopically magnified portion of a macrostructure.
The microstructure-property relationship in concrete so far
the microstructure would be helpful before discussing the factors influencing the important engineering properties of concrete, such as strength, elasticity, shrinkage, creep, and cracking, and durability.
Tell 3 examples of microstructures of the hydrated cement paste
- Solids (C-S-H, CH, ettringite)
- Voids (Entrapped air, entrained air, capillary pores)
- Water (Capillary water, adsorbed water, interlayer water)
https://ibb.co/JzzDYNh
https://ibb.co/tckXv1T
- Voids
- Fly ash
In solid, microstructural inhomogeneities can lead to
Serious effects on strength and other related mechanical properties, which controlled by microstructural extremes
https://ibb.co/B2qw2yY
It is a model pf a well-hydrated Portland cement paste
A: aggregation of poorly crystalline C-S-H particle which have at least one colloidal dimension
H: hexagonal such as CH = C4AH19 = C4ASH18
C: capillary cavities
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- Interparticle spacing between C-S-H sheets
- Capillary voids
- Hexagonal crystals of Ca(OH)2 or low sulfate
- Aggregation of CXS-H particles
- Entrained sir bubble
- Max. Spacing of entrained air for durability to frost action
- Entrapped air void
https://ibb.co/LSC2fD1
- Ettringite
2.Monosulfate hydrate
https://ibb.co/Ldk4tzB
- Fresh
- After 1 hr
- After several hours
- After days
- After weeks
- Unhydrated cement
- Hydrates (mostly C-S-H)
- Calcite crystals
https://ibb.co/S5ZksKh
C-S-H
CH
C-(A,F)-H
Monosulfate
Porosity
Ettringite
Solids in the microstructure
C-S-H gel
CH
Calcium sulfoaluminates
Hydrates
Unhydrated clinker grains
Explain C-S-H gel
• The calcium silicate hydrate phase makes up 50 to 60 percent of the volume of solids.
• the most important phase determining the properties of the paste
• C-S-H is not a well-defined compound; the C/S ratio varies between 1.5 and 2.0.
• Continuous matrix; intrinsic porosity; amorphous
• Is an irregular and extremely fine substance. it so-called colloid (gel) is because it is in a colloidal configuration with a specific weight of 2.3-2.6.
• Generally CaO / SiO2 value is 1.5-2.0 or higher, and the appearance is needle-like.
• When chemical changes occur during the hydration reaction, it may be caused by electronic bond, and then form continuous matrix of silicate polymerization.
• The content of C-S-H is large, which is helpful to the strength and durability of the cement paste.
Explain CH (Calcium Hydroxide)
• Constitute 20 to 25 percent of the volume of solids.
• Is a compound with a definite stoichiometry, Ca(OH)2.
• It tends to form large crystals with a distinctive hexagonal-prism morphology.
• The strength-contributing potential is limited as a result of considerably lower surface area
• hexagon column crystal with a specific weight of 2.24
• In the third stage (acceleration) of C3S hydration, CH forms crystalline nucleus in the pore space, and CH can grow into free space. If the space is blocked by another CH crystal, it may stop growing or turn to another direction.
• CH can fill pores to increase strength, but it has limitations on long-term effects. Due to its crystallization and growth pattern, it tends to dissipate shear forces, so that the general failure surface is broken by the edge of CH to form a weak surface.
• CH easily dissolves and precipitates on the surface of the cement paste, forming efflorescence (白華), it generates pores, which will reduce the durability.
Explain calcium sulfoaluminates hydrates
• occupy 15 to 20 percent of the solid; small crystal (1 ~ 10μm)
• during the early stages of hydration the sulfate/alumina ionic ratio of the solution phase generally favors the formation of trisulfate hydrate, C6AS3H32, also called ettringite, which forms needle- shaped prismatic crystals.
• ettringite eventually transforms to the monosulfate hydrate
• makes the concrete vulnerable to sulfate attack.
https://ibb.co/L6FFZLG
https://ibb.co/bmqzpBS
- Ettringite needles
- CH
- Unhydrated cores
- C-S-H
- CH
- Capillary pores
- Ettringite
- CH
9.Monosulfoaluminate
Explain unhydrated clinker grains
Depending on the particle size distribution and the degrees of hydration
Conform to the size range 1 to 50 μm
Voids in microstructures
- C-S-H layers
- Porosity capillary;gel
- Capillary voids
- Air voids: Much larger than capillary pore → bad to strength/durability
https://ibb.co/qCjVhD6
- Interlayer water
- Capillary water
- Physically adsorbed water
What is HCP
Hydrated cement paste
Explain capillary water
It exists in porosity with a pore size of more than 50 Å and can be considered as volumetric water, but without attraction to solid surfaces.
The behavior:
1. Water with a pore size greater than 500 Å is called free water(自由水),
2. Capillary water with a pore size of 50-500 Å , lose of this water will cause the paste to shrink.
Explain adsorbed water
this water is very close to the surface of the solid substance of the paste and adsorbed on it.
When the relative humidity RH drops to 30%, most of the adsorbed water in the paste will be lost, and the loss of adsorbed water is the main cause of the shrinkage of the harden cement paste.
Explain interlayer water (colloidal water)
it is combined with the structure of C-S-H colloid, interlayer water has been considered to exist between layered C-S-H.
Due to the monomolecular water layer strongly held by the hydrogen bond, this layer of water will only be lost in a strongly dry state with a relative humidity of less than 11%.The C-S-H colloid shrinks, when the interlayer water is lost.
Explain chemically combined water
Will not be lost due to drying. It can only evaporate when the paste is subjected to high temperatures to break the bond.
What are microstructure-property relationships in the HCP
Strength, dimensional stability, durability
What properties affect strength
C-S-H colloid (continuous matrix) main factor controlling concrete strength (fc’)
Porosity, major factor to weaken the strength
CH and Calcium sulfoaluminates hydrates, filling the porosity the durability issue can even weaken the strength
What properties affect dimensional stability
C-S-H : Interlayer water will only be lost in a strongly dry state with a relative humidity of less than 11%.The C-S-H colloid shrinks, when the interlayer water is lost.
Porosity: major factor
CH and Calcium sulfoaluminates: little effect, response like elastic material
What properties affect durability
C-S-H: most durable due to its low solubility and water impermeable
• Porosity: dominate the permeability and influence durability
• CH: High solubility and increase porosity → decrease durability
• sulfoaluminates hydrates : major factor for sulfate attack
