ACBM
VIDEO:
Hydration of Cement
ACBM
Cement Hydration
• Water + Cement
chemi cal reaction
(Cement grains dissolve, di ffuse and precipitate)
• Reaction is exothermic (heat released)
• Heat signature can be i mportant to
characterize material development
ACBM
Rate of Heat
Evolution
Heat of Hydration
Final set
hydrolysis
I
C3S reacts
III
nucleation
dissolution
II
IV
diffusion control
V
Initial set
Time
Stage I
Rapid Heat Evolution
(<15 mins)
Stage II
Dormant Period
Important for transportation
(2-4 hrs)
Stage III
Accelerating Stage
Begins with initial set
(4-8 hrs)
Stage IV
Deceleration Stage
No longer workable
(12-24 hrs)
Stage V
Steady State
ACBM
Typical Setting Times for Portland
Cements
Vicat apparatus
(Gebhardt 1995 and PCA 1996).
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Conceptual View of Hydration
Initial
During
Hydration
w/c>0.32
100% Hydration
w/c < 0.32
100% Hydration
Portland Cement Grain (Un hydrated)
Water Filled Capillary pores
C-S-H
Note: Calcium Hydroxide and Calcium Sulfoaluminates not shown
• For complete hydration 1 g of PC requi res 0.32 - 0.36 g of water
(i.e., equal volumes)
ACBM
Structure of H ydrated Cement Paste
Calcium Silicate
Hydrate (C-S-H)
• 50-67% of solids volume
(major product)
• C/S ranges f rom 1.5~2
• Morphology - poorly
crystalline to reticular
network
• Resembles tobermorite
(naturally occurrin g mineral)
Image is from Paul Stutzman, 2003
ACBM
Structure of H ydrated Cement Paste
Calcium Hydroxide - (CH, Portlandite)
•
•
•
•
20-25% of solids volume
Known stoichiometry Ca(OH) 2
Morphology - hexagonal crystal plates
Adverse effect on durabilit y
C-SH
C 3S
CH
10 mm
Image is from Jennings, 2003
ACBM
Structure of H ydrated Cement Paste
Calcium Sulfoaluminates (ettringite)
•
•
•
•
15-20% of solids volume
Minor role in structural behavior
Hexagonal plate crystal
Ettringite (Aft) formation
Unhydrated clinker grains
• Cores of larger particles
• Morphology - resembles clinker
ACBM
Relative Volume of Major
Compounds in Hydrated Paste s
Function of Time
Locher, Richartz, and Sprung 1976
Function of Degree of Hydration
Tennis and Jennings 2000
ACBM
Water In H ydrated Cement Past e
X
X
X X
OO X
X
X X
X X X O O X
X
XX X
X XX X OO O
X OO
OO
X X X
X X X X X
X
X
X
O O OO
O
O
O
O O OO O
O
X
O OO
X X XX
O
X
X X
O
X X X
O O
O O OO O O
O OO
X
X X
X X
O
O O
X
X X X
O
O X
O O
O O O
X
O
O OOO X X X
OO
X
**After Feldman and Serada 1970
Interlayer water (X)
- Associated with C-S-H structure
- Lost on drying below 11 %
•Capillary water
- Large voids
- Free water, pores > 50 nm
- Capillary tension, pores 5-50 nm
Adsorbed water (O)
- Held close to solid surface
- Up to approx imately 6 layers
Chemicall y combined w ater
- Integral part that is not lost on heating
ACBM
Voids in H ydrated Cement Past e
Interlayer space in C-S-H (gel void) à nm level
• Small voids - probably 5-25Å
Capillary voids à mm level
• Popularly called porosity
Air voids à mm level
• Generally round
• Entrapped air (as large as 3mm)
• Entrained air (50 -200mm)
Capillary
Porosity
Hydration
Products
Unhydrated
Grain
ACBM
Dimensions of Solids & Pores
Interparticle
Spacing Between
C-S-H Sheets
Hexagonal
Ca(OH) 2 Crystals
Entrapped Air
Entrained Air
Capillary Void
Aggregation of
C-S-H Particle
0.001mm 0.01mm 0.1mm
**After Monterio and Mehta Fig 2-7
1mm
10mm
Max Air Spacing
100mm
1mm
10mm
ACBM
Volume of Products
Effect of Time
Consider 100 cm3 of cement with w/c = 0.63 (by wt.)
Total Volume of Paste (cc)
VWater
VCementrCement w 100cc(3.14g / cc )
=
=
0.63 = 200cc
r Water
c
1.0g / cc
300
Capillary
Pores
200
Hydration
Product
100
0
Initial
Day 7
50% hyd.
Day 28
75% hyd.
Day 365
100% hyd.
Anhydrous
Cement
ACBM
Volume of Products
Effect of w /c Ratio
Capillary Pores
37%
300
30%
22%
Hydration Product
11%
200
0%
100
Strength
Total Volume of Paste (cc)
Consider 100 cm 3 of cement with 100% hydration
Complete Reaction = 200 cm 3
0
0.32
w/c 0.7
Vol. 320
0.6
288
0.5
257
0.4
225
0.32
200
w/c
Conclusion: lower w/c has lower porosity, greater strength
ACBM
Calculation of Volume Changes
Empirical equations deri ved by T.C. Powers from experiments
Evaporable Water - Lost at 105°C
Capillary and gel pores (interlayer pores)
Non-Evaporable Water (wn) - Lost at 1000°C
Approximate measure of combined water
Capillary
pores
Increasing Hydration
Gel pores
Evaporable
Water
Hydration
products (gel)
Total ‘Solid
Volume’
Unhydrated cement
ACBM
Calculation of Volume Changes
Assumption: 1 g of cement requires 0.36 g of w ater for
complete hydration ( 1 g of cement = 0.32 cm 3)
Volume of gel: Vg = 0.68a cm 3/g, where a = % hydration
Capillary Porosit y: VCP = w/c - 0.36a
= 0.63 - 0.36*1 = 0.27 cm 3/g
(if w/c = 0.63 and a = 1)
Volume of CP = CP*rcement = 0.27*(100)*3.15 @ 85 cm 3
(for 100 cc of cement)
Gel Space ratio = Volume of gel/Space ava ilable for gel
= 0.68a/(w/c + 0.32a)
Note: because of space requirements minimum w /c = 0.42
Solid space = 1 - capillary porosity
ACBM
Porosity, Gel Space,
Strength, and Permeability
Typical Capillary
Porosity
Typical Capillary
Porosity
Hydration
Capillary Porosity
120
20
80
10
40
0
1.0
0.9
0.8 0.7
0.6
0
0.5 0.4
Solid/Space Ratio (1-P)
Water/Cement Ratio
30
100% 75%
0.3
0.6
-12 )
0.45
(cm/secx10
0.3
Permeability Coefficient
(x10 3 psi)
Compressive Strength
0
50% 25%
0.4
0.5
0.6
0.7
0
0.1 0.2 0.3
0.4 0.5 0.6
Capillary Porosity, Vol. Fraction P
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•
•
•
•
•
Ways to Measure Porosity
Electrical Measurements
Image Analysis
Nuclear Magnetic Resonance (NMR)
Weight Loss
Computer Simulations ( Virtual Cement and Concrete
Testing Laboratory-VCCTL)
• Mercury Intrusion Porosimetry
ACBM
Mercury Intrusion Porosimetry
(MIP)
Penetration Volume (cc/g)
Hydration
Pore Volume
Pore Diameter
0.5
28 Days
AGE
0.4
90 Days
INCREASING w/c
0.3
365 Days
0.2
0.1
0
5000 2000 1000 500
200 100
Pore Diameter A
ACBM
Chemically
Combined Water
C
B
Adsorbed and
Interlayer
Water
A
Free
Water
Shrinkage
Water Loss
Water and Shrinkage
C
A
B
100%
Relative Humidity
Loss of Water
Note: Relationship between shrinkage and water loss is
not unique, it depends on age and degree of hydration
ACBM
VIDEO:
Microstructure of Concrete
Images from Paul Strutman, 2003
Clinker, HF-etch, 300 mm
FW
0.45 w/c, 7 d 30 mm FW
0.45 w/c, 1 d, 50 mm FW
ACBM
Key Ideas
• How long are the stages of the hydrati on
reaction?
• What is the relative sizes of C-S-H
crystals, capillary voids, entrained air and
entrapped ai r?
• What effect does w/c have on the
microstructure … and other properti es?