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

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1 Ab-initio modelling as a tool for Understanding C-S-H gel J.S. Dolado jsanchez@labein.es Unidad Asociada

2 Contents Introduction Ab-initio calculations Ab-initio calculations for the silicate chains that make up the C-S-H gel. Stability Formation pathways Guest ions (Al ions) Conclusions

3 Very Accurate :Schrodinger equations Suited to nano-scale phenomena. Ab-initio Modelling The basic pieces of C-S-H gel are nano-scale hydration products of Cement Example: Ab-Initio Study of Silicate Chain Formation in C-S-H gel nm

4 Silicate chains of C-S-H gel Importance These chains can bee seen as the ultimate components of the cementitious matrix 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 Methodology GAMESS code and SIESTA Code a) Hartree-Fock Molecular Orbital Theory b) Density Functional Theory calculations

5 STABILITY

6 Experimental evidences Silicate chains of C-S-H gel Nuclear Magnetic Resonance (NMR) evidence: Presence of silicate chains of different lengths Experimentally 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

7 But where do these silicate chains come from? SiO 2 +2H 2 O Si(OH) 4 = (1) Silicate chains of C-S-H gel Condensation reactions (2) + (1) <=> (3) + H 2 0 (3) + (1) <=> (4) + H 2 0 Si(OH) 4 + Si(OH) 4 <=> (OH) 3 SiOSi(OH) 3 + H 2 O (1) + (1) <=> (2) + H 2 0 (m) + (1) <=> (m+1) + H 2 0 (m-1) (m) (m+1) Similar to a Sol-Gel gel process!!!

8 Sol-Gel gel parallelism Silicate chains of C-S-H gel 1) Highly basic media Charged states (m) (0), (m) (-),

9 Sol-Gel gel parallelism Silicate chains of C-S-H gel 1) Highly basic media Charged states (m) (0), (m) (-), 2) calcium presence Geometrical Constrain = DREIERKETTEN arrangement

10 Sol-Gel gel parallelism Silicate chains of C-S-H gel 1) Highly basic media Charged states (m) (0), (m) (-), 2) The calcium presence Geometrical Constrain = Dreierketten arrangement Si(OH) 4 Si(OH) 3 O Si(OH) 5 (OH) 3 SiOSi(OH) 3 (OH) 3 SiOSi(OH) 2 O Si 5 O 4 (OH) 12 Si 5 O 4 (OH) 11 O We have calculated - The energies ( E(m) (a) ) of the chains - The Optimised geometries

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

12 { E(m) } Stability? Stability STABILITY INDEX Growth channel (m-1) (m) (m+1) 2 E(m) = E(m+1)+E(m-1) - 2 E(m) Local maxima of 2 E give the stable chains at thermal equilibrium

13 Stability (m-1) (m) (m+1) (m-1) 0 (1) 0 (m) 0 (1) 0 (m+1) 0 (1) - (1) - (m-1) - (m) - (m+1) - (1) 0 (1) 0 (m-1) 0 + (1) 0 (m) 0 + H 2 O (m) 0 + (1) 0 (m+1) 0 + H 2 O (m) 0 + (1) (m+1) + H 2 O (m-1) 0 + (1) (m) + H 2 O (m-1) + (1) 0 (m) + H 2 O (m) + (1) 0 (m+1) + H 2 O 2 E(m) = Σ channel [ 2 E(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) )

14 Stability L 5 Results: m = 2, 5 and 8 are stable species in agreement with m = 3n-1 Stability index (ev) m (number of silicon atoms in chain)

15 Stability 4 Stability Index (ev) Charged Neutral-Charged Neutral Total (m-1) 0 (1) 0 (m) 0 (1) 0 (m+1) 0 (1) - (1) - (m-1) - (m) - (m+1) - (1) 0 (1) 0 The charged channel represents the most important contribution The charged chains are predominant over the neutral species m (number of silicon atoms in chain) SILICATE CHAIN FORMATION IN THE NANOSTRUCTURE OF CEMENT-BASED MATERIALS. Submitted to Nature Materials A. Ayuela, J. S. Dolado, I. Campillo, Y. R. de Miguel, E. Erkizia, D. Sánchez-Portal, A. Rubio, A. Porro and P. M. Echenique

16 FORMATION PATHWAYS

17 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 (C 3 S, C 2 S) Growth: m =1 m = 2 However, another path needs to be taken into account for the formation of pentamers

18 Pathways of Chain Formation a Pathway = Merging b + + c a E c a - c ev b ev ev b Silicio Oxígeno Hidrógeno Charged monomers promote the larger energy gain

19 GUEST IONS (Al ions)

20 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, (**) I. G. Richardson. Cement abd Concrete Research 34 (2004)

21 Guest ions (m-1) (m) (1)* (1)* (m+1) (m-1) * (m) * (m+1) * (1) (1) (1)* = = [Al(OH) 4 ] - There are new growth channels 2 E(m) = 2 E(m) [Si_Channel] + 2 E(m) [Al_Channel] 2 E(m) [Al_Channel] = 2E((m+1)*)+E((m-1)*)+ E((m-1))- E((m)) -3 E((m)*)

22 Guest ions Stability Index (ev) Al_Channel Si_Channel Total 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. m

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

23 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) + H 2 O E ~ -0.8 ev

24 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 * Team Dr. Antonio Porro (head) Dr. Igor Campillo Dr. Yolanda de Miguel Dr. Edurne Erkizia Dr. José Antonio Ibáñez Dr. María Moragues Y. Saez de Ibarra J. J Gaitero I. Berra H.

25 Acknowledgements * Team Dr. Antonio Porro (head) Dr. Igor Campillo Dr. Yolanda de Miguel Dr. Edurne Erkizia Dr. José Antonio Ibáñez Dr. María Moragues Y. Saez de Ibarra J. J Gaitero I. Berra H. Manzano A. Montero R. Vega E. Velasco * DIPC Dr. Andrés Ayuela Dr. Daniel Sánchez-Portal Funding Basque Government under the ETORTEK and SAIOTEK Programs. EU : NANOQUANTA Network of Excellence (NMP4-CT )

26 Thanks for your attention!

AB-INITIO MODELLING AS A TOOL FOR UNDERSTANDING C-S-H GEL

AB-INITIO MODELLING AS A TOOL FOR UNDERSTANDING C-S-H GEL J. S. Dolado a, I. Campillo a, Y. de Miguel a, E. Erkizia a, A. Porro a, A. Ayuela b, and D. Sánchez-Portal b a. Fundación Labein, C/Geldo, Edif.