ab-initio modelling as a tool for understanding c-s-h...

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CENTRO DE APLICACIONESDE NANOMATERIALESEN CONSTRUCCIONUnidad

Asociada

Ab-initio modelling as a tool for Understanding C-S-H gel

J.S. [email protected]

Unidad AsociadaNANOC®

CENTRO DE APLICACIONESDE NANOMATERIALESEN CONSTRUCCION

Contents

IntroductionAb-initio calculations

Ab-initio calculations for the silicate chains that make up the C-S-H gel.

StabilityFormation pathwaysGuest ions (Al ions)

Conclusions



Ab-initio Modelling

• Very Accurate :Schrodinger equations•Suited to nano-scale phenomena. • The basic pieces of C-S-H gel are nano-scale hydration products of Cement

•Example: Ab-Initio Study of Silicate ChainFormation in C-S-H gel

nm



Silicate chains of C-S-H gel

ImportanceThese chains can bee seen as the ultimate components of the cementitiousmatrix and many of its ulterior properties depend on them.

To learn about the way in which these chains polymerise is an unavoidable technological need!

Aims•To determine which species are stable• Different pathways of formation • To determine the influence of Al on the growth

MethodologyGAMESS code and SIESTA Code a) Hartree-Fock Molecular Orbital Theoryb) Density Functional Theory calculations



STABILITY




Experimental evidences

•Nuclear Magnetic Resonance (NMR) evidence: Presence of silicate chains of different lengthsExperimentally derived equation: m = 3n - 1(m is number of Si atoms in chain)

• Dimer is formed in the initial stage of hydration

• Dimeric units link together by insertion of a monomer to give a pentamer




But where do these silicate chains come from? SiO2+2H2O Si(OH)4 = (1)

Condensation reactions

Si(OH)4 + Si(OH)4 <=> (OH)3SiOSi(OH)3 + H2O(1) + (1) <=> (2) + H20

(2) + (1) <=> (3) + H20

(3) + (1) <=> (4) + H20

(m) + (1) <=> (m+1) + H20 (m-1) (m) (m+1)

Similar to a Sol-Gel gel process!!!




Sol-Gel gel parallelism…

1) Highly basic mediaCharged states

(m)(0) , (m)(-), …






2) calcium presence…

Geometrical Constrain =DREIERKETTEN arrangement(m)(0) , (m)(-), …






2) The calcium presence…

Geometrical Constrain =Dreierketten arrangement(m)(0) , (m)(-), …

Si(OH)4 Si(OH)3O Si(OH)5(OH)3SiOSi(OH)3 (OH)3SiOSi(OH)2O

Si5O4(OH)12 Si5O4(OH)11O

We have calculated - The energies ( E(m)(a) ) of the chains- The Optimised geometries



Stability{ E(m)(a) } Stability (m)?



Stability{ E(m) } Stability?

STABILITY INDEXGrowthchannel (m-1) (m) (m+1)

∆2E(m) = E(m+1)+E(m-1) - 2 E(m)

Local maxima of ∆2E give the stable chains at thermal equilibrium



Stability

(m-1)0 (m)0(1)0+ + H2O

(m)0 (m+1)0(1)0+ + H2O

(m-1)0 (m)¯(1)¯+ + H2O(m-1)¯ (m)¯(1)0+ + H2O

(m)0 (m+1)¯(1)¯+ + H2O

(m)¯ (m+1)¯(1)0+ + H2O

(m-1) (m) (m+1)

(m-1)0 (m)0 (m+1)0

(m-1)- (m)- (m+1)-(1)-

(1)0(1)0

(1)0 (1)0

(1)-

∆2E(m) = Σchannel [∆2E(m)channel]E((m-1)¯) + 2 E((m-1)0) + 2 E((m+1)¯) + E((m+1)0) - 3 E((m-1)¯) -3 E((m-1)¯)



Stability

2

Results:m = 2, 5 and 8are stable species

in agreement withm = 3n-1

L1 3 4 5 6 7 8 9

-3

-2

-1

0

1

2

3

4

5

1 2 3 4 5 6 7 8 9

m (number of silicon atoms in chain)

Stab

ility

inde

x(e

V)

2

5

8



-3

-2

-1

0

1

2

3

4

1 2 3 4 5 6 7 8 9

ChargedNeutral-ChargedNeutralTotal

Stab

ility

Inde

x (e

V)

m (number of silicon atoms in chain)

The charged channel representsthe most important contribution

The charged chains are predominant over the neutral species

(m-1)0 (m)0 (m+1)0

(m-1)- (m)- (m+1)-(1)-

(1)0(1)0

(1)0 (1)0

(1)-

Stability

SILICATE CHAIN FORMATION IN THE NANOSTRUCTURE OF CEMENT-BASED MATERIALS. Submitted to Nature MaterialsA. Ayuela, J. S. Dolado, I. Campillo, Y. R. de Miguel, E. Erkizia, D. Sánchez-Portal, A. Rubio, A. Porro and P. M. Echenique



FORMATIONPATHWAYS



Pathways of Chain Formation

• The initial formation of dimers has to come from monomers, which come in turn from the hydration of the cement components (C3S, C2S)

Growth: m =1 m = 2

• However, another path needs to be taken into account for the formation of pentamers



Pathways of Chain Formation

Pathway = Merging

+ +

a

b

c

-

a

b

c

Silicio

Oxígeno

Hidrógeno

∆E

- 0.12 eV

- 0.37 eV

- 0.57 eV

c

b

a

Charged monomerspromote the larger energy gain



GUEST IONS(Al ions)



Guest ions

Experimental evidences

* NMR and TMS-GCP have shown that in systems where there is significant substitution of Al ions for Si, the m=3n-1 sequence is maintained.

* The Al atoms seem to substitute for Si atoms in the bridging tetrahedra.

(*) P. Faucon, A. Delegrave, J. C. Petit, C. Richet, J. M. Marchand and H. Zanni. J. Phys. Chem. B 1999, 103, 7796-7802(**) I. G. Richardson. Cement abd Concrete Research 34 (2004) 1733-1777



Guest ions

(m) (m+1)(m-1)

(m-1)* (m)* (m+1)*

(1) (1)

(1)* (1)* = [Al(OH)4] -(1)* =

There are new growth channels

∆2E(m) = ∆2E(m)[Si_Channel] + ∆2E(m)[Al_Channel]

∆2E(m)[Al_Channel] = 2E((m+1)*)+E((m-1)*)+ E((m-1))- E((m)) -3 E((m)*)



Guest ions

-3

-2

-1

0

1

2

3

4

5

1 2 3 4 5 6 7

Al_ChannelSi_ChannelTotal

Stab

ility

Inde

x (e

V)

m

o The m=3n-1 rule is maintained !!

o This channel enhances the m=5 stability,whereas its contribution to the stability of dimers is almost negligible.

This last fact suggests that Al(OH)4-

hardly contributes to the formation of dimers.



Guest ions

MERGING by means of [Al(OH)4] (-1)

These merging processes might explain the preference of Al for Bridging sites!!!

[Al(OH)4] (-1) favours the merging of two dimers

even more than the Si-monomers !!!!

D+D+ [Al(OH)4] (-1) P(-)(Al) + H2O

∆E ~ -0.8 eV



Conclusions

The empirical equation m=3n-1 can be predicted from the ab-initio calculated Stability Index

The pathways for the formation of the silicate chains: Dimers growth Pentamers merging.

Charged monomers promote the larger energy gain

Our calculation are consistent with the preference of Al ions for bridging sites.

Ab-initio modelling is a powerful tool in the study of the complex structure of C-S-H gel



Acknowledgements* NANOC Team

Dr. Antonio Porro (head)Dr. Igor CampilloDr. Yolanda de MiguelDr. Edurne ErkiziaDr. José Antonio IbáñezDr. María MoraguesY. Saez de IbarraJ. J GaiteroI. BerraH. ManzanoA. MonteroR. VegaE. Velasco

* DIPC

Dr. Andrés AyuelaDr. Daniel Sánchez-Portal

FundingBasque Government under the ETORTEK and SAIOTEK Programs.

EU : NANOQUANTA Network of Excellence (NMP4-CT-2004-500198)



Thanks for your attention!

ab-initio modelling as a tool for understanding c-s-h...

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