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Some Observations relatingKinetics, Chemistry, and ProductStructure of Hydrating CementPaste – Reaction Mechanisms
Hamlin M. JenningsMSE and CEE
Northwestern University
July 09Kinetics Summit
Mechanism of reaction
• Must explain kinetics– Divided into several periods
• Must explain thermodynamics– Equilibrium
• Must explain location of reaction• Must explain morphology
Required Steps in Reaction
• Dissolution• Diffusion• Precipitation• Growth• Location, morphology, and rate of
growth of product(s) must beconsidered
Rate of hydration (kinetics)Traditional view
Kinetic description – 5 periods
Acceleratory Period
Deceleratory Period
Rate of hydration (kinetics)
Time (hours)
Rat
e of
reac
tion
Induction (dormant)
2
NucleationAnd
GrowthDiffusion
10 24
From a mechanistic view there are 3 periods:
There is an induction periodunder some conditions
0
10
20
30
40
0 2 4 6 8
Time (days)
1% sugar
0% sugar
Rate
of heat evolu
tion
(J/(
h*g
initia
l cem
ent)
)
Garci Juenger and Jennings CCR 2002
Hydration of C3Stwo products form
C3S + (3 ! x + y)H"Cx ! S! Hy + (3 ! x)CH
X = 1.7Y = 4
- Volume of solids increases- Total volume decreases (Chemical shrinkage)
C3S +72.5
5.3H95.4
167.9 166.9
C1.7SH4 + 124
1.3CH42.9Phase volume:
Total:
Why is there an inductionperiod?
• Protective layer forms and is later disrupted– For
• Physical evidence• Equilibrium and if so with what?
– Against• Other Physical evidence• No obvious reason for disruption
Hypothesis #1
Why is there an inductionperiod?
Induction period X C3S
Equilibrium in C3SBoth under-saturationand over-saturationreturn to phase lineImplies equilibriumwith a protective layer
0.1
1
10
100
1000
0 5 10 15 20 25 30
[SiO
2µ
M
[CaO] (mM)
Aqueous Phase
C-S-H+
Aqueous Phase
S
M
After Jennings et al ICCC Sweden 1997
The concentration onM is stable for weeks
H.M. Jennings, “queous Solubility Relationships for Two Typesof Calcium Silicate Hydrate,” Journal of the American Ceramic
Society, 69 [8] 614‑618 (1986).
But is layer protective? SEMdry (two products form)
2 hrs 4 hrs
0.2 µm 0.2 µm
Layer
• Layer prevents high concentrations inaqueous phase– Equilibrium is established quickly
• Inconsistent physical evidence
Why is there an inductionperiod?
• Delayed nucleation (Le Chatelier)– For
• Ca++ concentration• Kinetics – nucleation and growth
– Against• What is the seed (CH or C-S-H)? nothing works
Hypothesis #2
Why is there an inductionperiod?
[Ca]
Time
Supersaturation in calcium at early times
End of the Induction Period
CH saturation
Retarders poison the precipitation
CCaO (mM)
0 5 10 15 20 25
CSiO
2 (m
M)
1
10
100
1000
A
C'
C''
C
Ca(OH)2
Phase diagram: equilibrium but some supersaturation
Curves C, C', C" and A representa spectrum of C-S-H structures
Tobermorite-like- short chain length -no Ca-OH
5 A
Jennite-like-long chain length-more Ca-OH
5 A
A
Silicate polymerization is keyto variations
Chen, J.J., J.J. Thomas, H.F.W. Taylor, and H.M. Jennings,Cem. Concr. Res., 2004. 34(9): p. 1499-1519.
C-S-H nor CH acceleratemuch
Gartner and Gaidis Materials Science of Concrete I (1989)
Acceleration period
• Nucleation and growth has dominatedmodeling– Avrami: transformation throughout volume– Boundary: transformation starts at boundary
• Diffusion control -- very little argument
Late period
Location and Morphology
• One valiant attempt in the 1979’s– Reverse silicate garden -- membrane forms
and breaks from osmotic pressure resultingin the formation of needles
• Double et al.• Birchall et al.
• Otherwise not much, with confusionover morphologies such as Hadleygrains
What is new?
• C-S-H is colloid– Detailed model of density and pore structure– Packing and morphology change with time which
explains many morphological variations• Kinetics described exactly by boundary N+G• Seed -- C-S-H can work well -- nucleates in
volume
What is new: C-S-H is gel
1 um
Wet TEM taken atImperial College London(1980)
Small particles after Powers
Micrographs of shrinkageHuge deformation on drying
Wet
Dry3 day .5 w/c
Fig 4
Surface area, density -- colloid model
HYPOTHESIS: So what is themechanism controlling early
rate?• Layer, thermodynamically separating
particle from aqueous exists• Normal hydration starts when nuclei
form in the layer -- N+G controls rate• Autocatalytic growth of product into
pore space -- both CH and C-S-H
Must explain kineticsPicture of self stimulation
= kinetics (middle)
Must explain retarders: Sugar
• Thomas and Birchall showed that sugarpoisons C-S-H
• Under normal conditions nucleationoccurs in layer where some degree ofsupersaturation exists
Delayed addition ofsugar greatlyreducedeffectiveness =nuclei formed withinlayer --
H.M. Jennings, H. Taleb, G. Frohnsdorff, and J.R.Clifton), Proceedings of the 8th InternationalCongress on the Chemistry of Cement, Rio de Janeiro,Brazil, III 239‑243, (1986)
Must explain acceleratorsSeed from soluble salts - dispersed with active surface
(Jeffrey J. Thomas, Hamlin Jennings, and Jeffrey J. Chen), Journal
of Physical Chemistry C, 113, 4327-4334 (2009).
Must explain locationSchematic of N + G
(Jeffrey J. Thomas, Hamlin Jennings, and Jeffrey J. Chen), Journal of Physical Chemistry C, 113,
4327-4334 (2009).
Figure 3: SEM micrographs of hydrated paste made without C -S-H seed (left) and with 2% C -S-H seed by mass
of C3S (right), after [ Error! Bookmark not defined. ]. Both pastes are 28 d old and were made at w/c = 0.5. Black
is capillary porosity, grey is hydration product, and white is unreacted C 3S. Note the much lower amount of
capillary porosity in th e seeded paste at right.
Surface area m easured by
small angle neutron scattering
(m2/cm
3), one year old paste of
white Portland cement paste ,
w/c=0.5
Compressive strength (kN/m2)
16 day old paste of ordinary
Portland cement paste ,
w/c=0.5 (from [ 1])
No additives 123 35
Additive 143.5 40
[1] Millea, J. The Effects of Calcium Silicate Hydrate Seed on the Compressive Strength of
Portland Cement Past e, Senior Thesis, Northwestern University, Evanston, 2006.
Thomas, Jennings, Chen, Physical Chemistry C, 2009
First principlescontrol ofmicrostructure
Must explain morphologySlow and fast drying = very open
packing at early time
3 Day Old - rapid dry
3 Day Old - 18 day dry Fonsica and Jennings submitted
Seed activates Slag whensoluabilized
Late Reaction
• Possibly not diffusion control• Rate controlling step is difficulty in
finding active sites or, equitantly thenucleation process just continues onslowly
Consumption of active siteson particles
Not diffusion: D2O “Effects of Deuterium Oxide and Mixing on theEarly Hydration Kinetics of Tricalcium Silicate,” (J.J. Thomas and H.M. Jennings),Chemistry of Materials, 11, 1907-1914 (1999).
Summary: Reaction kinetics /Mechnaism and control of microstructure
• Nuclei must form from some degree ofsupersaturation -- normally within layer
• Nuclei once formed stimulate newproduct when surface is available --supersaturation not required
• Formation of nuclei can be poisoned butactive surface can not
Particle dissolves
If seed with activesurface
Stimulates newCH and C-S-Hproduct growth
Growth continuesuntil active sitesexhaust
If no seed
Nuclei form onsurface of cementin layer
Supersaturation –likely in layer
Reaction Steps
Overview
• Dispersed seed accelerates - diffusion intopores does not seem to be rate limiting
• Sugar retards -- poison formation of nuclei– Sugar does not prevent autocatalytic growth– Seed, if formed and dispersed, trumps sugar
• Seed accelerates activated slag• But seed must have active surface --
prehydration does not work well becausemuch surface is not available
Monolayer ofwater IGP
Interlayerwater
C-S-H dry 2.85 g/cm3
AB
C
Fig 1D
Globule
Model of particles and poreCM: Colloid
Deformation mapping
Dry to 50% rh Dry to 5% rh
Total shrinkage is sum of shrinking and restraining phasesC.M. Neubauer and H.M. Jennings, J. Mater. Sci. 35, 5741 (2000)
ESEM of Wet Samples
0.5 Hours 8 Hours
0
40
80
120
160
0
100
200
300
0 10 20 30 40 50 60 70
Surfa
ce A
rea
(m2 /c
c)
Heat EvolvedSANS Surface Area
Heat Evolved (Joules)
Hydration Time (hours)
Surface area development and heat evolution*
OPC Paste, 20ºC
*J.J. Thomas, H.M. Jennings and A.J. Allen, Cem. Concr. Res. 28, pp. 897-905 (1998).
SANS, LOI, and N2LD early
Mikhail and Abo-El-Enein (1972)
Aging 125 days
Two densities
1um
N+G in two areas
• On surface of particles– See Jeff Thomas
• In volume between particles