Upload
others
View
7
Download
0
Embed Size (px)
Citation preview
A C
BM
VIDEO:Hydration of Cement
A C
BM Cement Hydration
• Water + Cement chemi cal reaction(Cement grains dissolve, di ffuse and precipitate)
• Reaction is exothermic (heat released)
• Heat signature can be important to characterize material development
A C
BM Heat of HydrationR
ate
of H
eat
Evol
utio
n
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
I
II
III IV
Vnucleationdissolution
hydrolysisC3S reacts
diffusion control
Initial set
Final set
A C
BM
VicatVicat apparatusapparatus
Typical Setting Times for Portland Cements
(Gebhardt 1995 and PCA 1996).
A C
BM Conceptual View of Hydration
Portland Cement Grain (Unhydrated)Water Filled Capillary poresC-S-HNote: Calcium Hydroxide and Calcium Sulfoaluminates not shown
Initial During
Hydration w/c>0.32
100% Hydration w/c < 0.32
100% Hydration
• For complete hydration 1 g of PC requi res 0.32 - 0.36 g of water (i.e., equal volumes)
A C
BM Structure of Hydrated 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
A C
BM Structure of Hydrated 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-S-H
CH
C3S
10 µm
Image is from Jennings, 2003
A C
BM Structure of Hydrated 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
A C
BM Relative Volume of Major Compounds in Hydrated Pastes
Function of Time Function of Degree of Hydration
Locher, Richartz, and Sprung 1976 Tennis and Jennings 2000
A C
BM Water In Hydrated Cement Past e
Interlayer water (X)- Associated with C-S-H structure- Lost on drying below 11 %
XXX
XX
XX
XX
XX
O OO
OO
XX
XX
XX
XO
OO
OO
OO
O
OO
XX
XX
XX
X
X
X
OOO
OO
OO
OO
O
OO
O
O
OO
O
O OO
OO
O
XX
XXX
X
XX
X
XX
XXX
XOO
OO
XX
X
X
X
XXX O O OO
OO
OOO
O
O OO
XX
XX
XX
X
X
OO
OO
Adsorbed water (O)- Held close to solid surface
- Up to approx imately 6 layers
•Capillary water- Large voids - Free water, pores > 50 nm- Capillary tension, pores 5-50 nm
Chemically combined water- Integral part that is not lost on heating**After Feldman and Serada 1970
A C
BM Voids in Hydrated Cement Past e
Interlayer space in C-S-H (gel void) à nm level• Small voids - probably 5-25Å
Capillary voids à µm level• Popularly called porosity
Air voids à mm level• Generally round• Entrapped air (as large as 3mm)• Entrained air (50 -200µm)
UnhydratedGrain
Hydration Products
Capillary Porosity
A C
BM Dimensions of Solids & Pores
0.001µm 0.01µm 0.1µm 1µm 10µm 100µm 1mm 10mm
Entrapped Air
Entrained Air
Max Air SpacingAggregation of C-S-H Particle
Capillary Void
Interparticle Spacing Between C-S-H Sheets
Hexagonal Ca(OH)2 Crystals
**After Monterio and Mehta Fig 2-7
A C
BM Volume of Products Effect of Time
Consider 100 cm3 of cement with w/c = 0.63 (by wt.)
( ) cc20063.0cc/g0.1
cc/g14.3cc100cwVV
Water
CementCementWater ==
ρρ
=
Tota
lVol
ume
of P
aste
(cc)
Initial
300
200
100
0
Day 7 50% hyd.
Day 28 75% hyd.
Day 365 100% hyd.
Capillary Pores
Hydration Product
Anhydrous Cement
A C
BM Volume of Products Effect of w/c Ratio
Consider 100 cm 3 of cement with 100% hydrationComplete Reaction = 200 cm 3
Conclusion: lower w/c has lower porosity, greater strength
Tota
l Vol
ume
of P
aste
(cc)
w/c
300
200
100
0
Vol.0.7320
37%
0.6288
30%
0.5257
22%
0.4225
11%
0.32200
0%
Capillary Pores
Hydration Product
Stre
ngth
w/c0.32
A C
BM Calculation of Volume Changes
Empirical equations deri ved by T.C. Powers from experimentsEvaporable Water - Lost at 105°C
Capillary and gel pores (interlayer pores)Non-Evaporable Water (wn) - Lost at 1000°C
Approximate measure of combined water
Unhydrated cement
Hydration products (gel)
Capillary pores
Gel pores
Increasing Hydration
Total ‘Solid Volume ’
Evaporable Water
A C
BM 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 = 0.68α cm3/g, where α = % hydrationCapillary Porosit y: VCP = w/c - 0.36α
= 0.63 - 0.36*1 = 0.27 cm 3/g(if w/c = 0.63 and α = 1) Volume of CP = CP*ρcement = 0.27*(100)*3.15 ≅ 85 cm3
Gel Space ratio = Volume of gel/Space ava ilable for gel= 0.68α/(w/c + 0.32α)
Note: because of space requirements minimum w /c = 0.42
(for 100 cc of cement)
Solid space = 1 - capillary porosity
A C
BM Porosity, Gel Space, Strength, and Permeability
1.0 0.9 0.8 0.7 0.6 0.5 0.4
Solid/Space Ratio (1-P)
Com
pres
sive
Stre
ngth
(x10
3ps
i)
30
20
10
0 Perm
eabi
lity
Coe
ffici
ent
(cm
/sec
x10
-12)120
80
40
0
Typical Capillary Porosity
0 0.1 0.2 0.3 0.4 0.5 0.6
Capillary Porosity, Vol. Fraction P
Wat
er/C
emen
t Rat
io
0.3
0.4
0.5
0.6
0.7
100% 75% 50% 25%
Typical Capillary PorosityHydration
0 0.3 0.45 0.6Capillary Porosity
A C
BM 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
A C
BMMercury Intrusion Porosimetry
(MIP)
Hydration Pore Volume Pore Diameter
Pene
tratio
n Vo
lum
e (c
c/g) 0.5
0.4
0.3
0.2
0.1
0
Pore Diameter A5000 2000 1000 500 200 100
28 Days
90 Days365 Days
AGEINCREASING w/c
A C
BM Water and Shrinkage
Relative Humidity
Wat
er L
oss Chemically
Combined WaterAdsorbed and Interlayer Water
Free Water
100%
Note: Relationship between shrinkage and water loss is not unique, it depends on age and degree of hydration
Loss of WaterSh
rinka
ge
A
B
C
A B
C
A C
BM VIDEO:Microstructure of Concrete
0.45 w/c, 7 d 30 µm FW
0.45 w/c, 1 d, 50 µm FW
Clinker, HF-etch, 300 µm FW
Images from Paul Strutman, 2003
A C
BM 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 air?
• What effect does w/c have on the microstructure … and other properti es?