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