Lecture 2: Cement Hydration - CoMSIRU

Cement Chemistry for Engineers, Cape Town 31st January 2013

Lecture 2: Cement Hydration

Nature of hydrates

Thermodynamic prediction of hydrate assemblages

Evolution of microstructure

Reaction between water and cement: Increasing solid volume, replaces water

Cement grain

water hydrates

Transformation from fluid paste to solid

NB cement grains are polymineralic

alite

belite

aluminate ferrite

Hydrate Phases

Alite, C3S + H C-S-H + CH

Belite, C2S + H C-S-H + CH

Aluminate,C3A + 3C$+ H AFt (ettringite)

AFt + 2C3A + H AFm

Ferrite, C4AF

Limestone CaCO3

Silicates

Aluminates

Basic Reactions

C-S-H

48%

CH

14%anhyd

3%pores

16%ett (AFt)

4%AFm

11% other

4%

C2S

15%

C3A

8%ferrite

7%

C3S

70%

Phases present – 14 month paste from Taylor

calcium hydroxide

Hydrated lime

portlandite

Ca(OH)2

CH

crystalline

Hexagonal

morphology

hydrated calcium silicate

C-S-H

Nano crystalline

multiple morphologies

~ 15-25% of hydrated paste

~ 50-65% of hydrated paste

Hydration of calcium silicates : C3S and C2S

C-S-H

• “atomic” structure and composition

• “meso” structure

• “microstructure / morphology”

C/S =

0.8 to 1.5 for synthetic

preparations

1.7-2 PC pastes

0,00

0,50

1,00

1,50

2,00

2,50

0,00 10,00 20,00 30,00 40,00

CaO mmol/l

C/S

Flint and Wells at

30°C

Taylor at 17-20°C

Lecoq 20°C

Thordvaldson 25°C

(+expé sursat)

Source Nonat, Taylor conf

Definite phase with

reproducible behaviour

C-S-H: range of compositions

Layers of Ca-O Tetrahera of Si-O4 linked in chains

Molecules of water 14Å

1.4nm

C-S-H, analogy with natural mineral Tobermorite

Ca/Si=0.833

- the abscence of some bridging tetrahedra (Q1/Q2 increases)

H

Ca Ca Ca Ca Ca Ca Ca

H H H H H H

H Ca/Si = 0.72

Ca

Ca/Si = 0.8 H

H H H H

H H H

Ca Ca Ca Ca Ca Ca Ca Ca

H H H H H H

H H Ca/Si = 0.9



The increase of Ca/Si ratio is attributed to three mechanisms :

- the substitution of a part of the protons by calcium ions


Ca H H H H

H H Ca/Si = 1

Ca

H H Ca/Si = 1.22


Ca Ca Ca

Ca

H H H H H H

H H Ca/Si = 0.9





- the presence of Ca-OH regions

- the substitution of a part of the protons by calcium ions



In presence of portlandite (CH) – high Ca/Si ratio:

• Almost exclusively dimers

• Almost all terminating Ca, not H

CL = 5 : pentamer

CL = 8 : octamer

CL = 2 : dimer

Change occurs very slowly, years; faster at higher temperatures

Also limited by portlandite presence

Silicate polymerisation during hydration

H+

- -

-

- - - -

Al

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

0.1

0.2

0.3

0.4

0.5 Ettringite 28 days 1 year 3 years

Monosulfate

CHCSH

S/C

a

Al/Ca

OPC pastes: Al/Ca ~ 0.05 ; Al/Si ~ 0.1

Higher with Al rich SCMs

C-S-H: Si substitution by Al

C-S-H




Evidence

• No long range order

• “intrinsic” porosity of 26-28% (Powers)

– “gel porosity”

- from drying – therefore upper limit

• Scattering experiments (neutron, X-ray) and proton NMR

indicate “characteristic size” of about 4-5 nm

Jennings model

Nano crystalline?

~5nm

Meso structure

Two interpretations of nanocrystalline nature:

1. Granules – independent blobs

2. Sheets with disorganised structure

Open question

General agreement that C-S-H consists of nanocrystalline

regions:

The main open question is whether they are discrete or linked by

sheets

Important issues for water transport through C-S-H

C-S-H




3 hrs

10 hrs

HVEM, wet cell, C3S, 24hrs

SEM fracture surface

Two microstructurally

distinct forms:

“Outer” or “early”

“inner” or “late”

TEM from Richardson

C-S-H summary

Atomic level structure fairly well understood:

CaO sheets with chains (dimers) of SiO4 tetrahedra attached

Al substitutes for Si, in bridging sites

Meso level structure less clear

Nanocrystallites or nanocrystalline regions with characteristic scale of about 5nm

Microstructure

Outer, formed early through solution

Inner formed later

ettringite

AFt – aluminate ferrite tri

Aluminate hydrates

2 24 3SO CO

Possible exchange

Ca3Si(OH)6(CO3)(SO4)·12H2O

(thaumasite)

AFm aluminate ferrite mono

[Ca2Al(OH)6]+ layers

Many inter layers ions:

24SO

3 32C A.3C$.H

hydrogarnet

katoite

monsulfate

8AlSi(OH) stratlingite

3 6C AH

2H Si

Possible exchange

OH 4 13C AH

23CO

Cl

monocarbonate

Friedel’s salt

C3A

+

water

stiffening

flash set

Fast reaction Large plates of hydrates

Calciumaluminate hydrates : C2AH8 , C4AH13 : AFm phases

then C3AH6

Hydration of aluminate phases : C3A

C3A + CaSO4Hx

_ _

3 2 3 32C A 3CSH 26H C A.3CS.H (ettringite)

_2 2

4 4 3 326Ca 2Al(OH) 3SO 4OH 26H Ca A.3CS.H

_ _

3 3 32 3 122C A C A.3CS.H 4H 3C A.CS.H

Exhaustion of sulfate

Calcium aluminate monosulfate

AFm phase stablised by solid solution

It forms in the presence of anhydrous C3A

In presence of CaCO3 (fine limestone)

monocarbonate forms instead of monosulfate

Setting regulation by sulfates


C3A + water + calcium sulfate

Note

Reaction of C3A is not blocked by ettringite

Reaction controlled by absorption of sulfate ions at reacting sites

Cement paste after 2-7 days -

local formation of monosulfate inside layer of C-S-H where there is C3A available.

Ettringite remains on exterior of grain.


2. Hydration Kinetics

3. Microstructure formation

equilibrium curve of the cement phase

equilibrium curve of the hydrate

condition of equilibrium of the

solids (thermodynamics)

precipitation [nucleation and

growth] with dissolution Path of solution composition resulting

from the dissolution of solids

[conc X]

[conc Y]

Hydrates form “through solution” due to difference in solubility of anhydrous compounds and hydrates

0

1

2

3

4

0 3 6 9 12 15 18 21 24

He

at F

low

(m

W/g

)

Age of Specimen (Hours)

sta

ge

1

sta

ge

2

sta

ge

3

sta

ge

4

Alite reaction

0

1

2

3

4

5

0 10 20 30 40 50

Heat

Evo

luti

on

Rate

[m

W/g

]

Time [h]

Secondary

formation

of ettringite

Formation

of AFm phase

Alite reaction

Portland cement: silicate plus aluminate

1μm

~10 h ~3 h ~10 m ~24 h

Heat flow

time

1μm

Initial

dissolution

Induction

Nucleation and growth

~10 h ~3 h ~10 m ~24 h time

Heat flow

Initial

dissolution

Induction

Nucleation and growth

12hr cement paste: ion thinned section TEM

For some (as yet unknown reason) the alite dissolves beneath the outer C-S-H,

leaving a gap or low density area

This hydrates of seems to grow the outside this “shell”,

small hollow shells persist in mature concrete

The presence of aluminate seems to be important as these shells are not so

clearly separated in the case of C3S

Formation of Hadley grains (hollow shells)

28 days, SEM

SCRIVENER, 1984

Summary Microstructural Development

Hydration of different phases

Ph D Vanessa Kocaba (NANOCEM, CP4)

1 10 100 0

20

40

60

80

100

alit

e (

%w

t)

1 10 100

0

20

40

60

80

100

be

lite

(%

wt)

Time (d)

1 10 100 0

20

40

60

80

100

alu

min

ate

(%

wt)

1 10 100 0

20

40

60

80

100

ferr

ite

(%

wt)

A

B

C

D

Time (d)

Reaction of Belite

Old concrete, only belite and ferrite apparent

Reaction of ferrite phase

XRD shows reaction but in BSE appears unreacted

Alumina and calcium “leach” out leaving Fe “relic” as

hydrous Fe hydroxide of iron rich hydrogarnet

Aluminate phases in hardened cement paste

1 mm

Intermixing of ettringite and/or

Monosulfate at submicron scale

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

0.1

0.2

0.3

0.4

0.5 Ettringite 28 days

Monosulfate

CHCSH

S/C

a

Al/Ca

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

0.1

0.2

0.3

0.4

0.5

0.6

28 days

CH C1.7

SH

Ettringite

Monosulfo

Al/C

a

(Si+Al)/Ca

28 days. Formation of C-S-H, CH

ettringite and then monosulfate

3 3 32 3 12. . . .2C A C A 3CS H 4H 3C ACS H

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

0.1

0.2

0.3

0.4

0.5 Ettringite 28 days 1 year 3 years

Monosulfate

CHCSH

S/C

a

Al/Ca

0.0 0.1 0.2 0.3 0.4 0.5 0.60.0

0.1

0.2

0.3

0.4

0.5

0.6

28 days 1 year 3 years

CH C1.7

SH

Ettringite

Monosulfo

Al/C

a

(Si+Al)/Ca

1, 3 years. Further formation of C-S-H, CH

Reaction of ettringite into monosulfate

Source thesis Severine Lamberet

Microprobe analyses

Ettringite is commonly found in old concrete through recystallisation Quantity may increase due to carbonation – sulfate released as monosulfate converts to moncarbonate

This

reaction is

not

damaging

Impact of temperature on microstructural development

PhD Xinyu Zhang http://library.epfl.ch/theses/?nr=3725

http://library.epfl.ch/theses/?nr=3725

0 5 10 15 20 25 30

-5

0

5

10

15

20

25

30

He

at e

vo

lutio

n r

ate

(m

W/g

)

Time (hours)

20°C

40°C

55°C

55°C

40°C

20°C

Effect of temperature

1 2 3 4 5 6 7 8 910 20 30

0

50

100

150

200

250

300

55°C

20°C

Cu

mu

lative

he

at (J

/g)

Time (hours)

20°C

40°C

55°C

40°C

Arrhenius’ equation

k = A exp(-Ea/(RT))

ln(k) = ln(A) - Ea/(RT)

k rate of reaction

A constant

Ea apparent activation energy

R gas constant,

8.31 J/K mole

T temperature in Kelvin

Temperatures does not affect mechanical properties and “durability” in the same way

0 2 4 6 8 10 12 14 16 18 20

0

1

2

3

4

5

6

7

8

Wa

ter

so

rptivity (

ms

ec

-1/21

0-6)

Time (d1/2

)

5°C

20°C

40°C

60°C

0 2 4 6 8 10 12 14 16 18 20

0

5

10

15

20

25

30

35

40

45

50

55

Co

mp

ressiv

e s

tre

ng

th (

MP

a)

Time (d1/2

)

5°C

20°C

40°C

60°C

strength Water absorption

Why?

Lower final degree of hydration at higher temperatures?

Different hydration products?

Need strength as a function of hydration degree not time

0

10

20

30

40

50

60

20 40 60 80 100

hydration degree (%)

co

mp

res

siv

e s

tre

ng

th (

MP

a)

20°C

40°C

60°C

5°C

(w/c=0.5)

90d

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

mix1 mix5 mix4 mix2 mix3

20°C

40°C

60°C

inner CSH is very

thin

w/c=0.6

CEM42.

5

w/c=0.35

CEM52.5

R

20°C

60°C

CSH relative density

Due to lower microporosity in C-S-H

– more capillary porosity at higher

temperatures.

Building Materials Analysis 2009

C-S-H formed at 90°C

Brighter = more dense;

less microporosity

C-S-H formed at 20°C

Darker = less dense;

more microporosity

58

Cement Chemistry for Engineers, Cape Town 31st January 2013

End Lecture 2

Lecture 2: Cement Hydration - CoMSIRU

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