Accelrys Material Studio 5.5. VAMP module: Simulation of reactions and properties of molecule in gas phase or solvent phase. Run AM1, PM3 and PM6 ...
Semi-empirical AM1, PM3 and PM6 Calculations on the Metakaolin Molecular Structure: Practical Application in Geopolymer Cement Production Ojas A. Chaudhari1, Joseph J. Biernacki1 and Scott Northrup2 (1) Department of Chemical Engineering (2) Department of Chemistry Tennessee Technological University Cookeville, Tennessee, USA
AIChE Annual Meeting- October 2012
Need of geopolymer cement concrete • 80% less CO2 emission • Uses waste materials • Reduces pollution • High strength and durability • fire Resistant • Acid, base & salt resistant
Portland Cement Concrete Plant
World’s 3rd Largest CO2 Production
Source: http://media.treehugger.com/assets/images/2012/01/cement_plant.png.492x0_q85_cropsmart.jpg
Total CO2 production from OPC plant: 825kg CO2/ton PC In 2011, United States produced 68400 TMT and China 200,000TMT.
http://minerals.usgs.gov/minerals/pubs/commodity/cement/cemenmcs06.pdf
Geopolymer Cement Joseph Davidovits (1978) “geopolymeric cements” are inorganic polymers with a
3D framework structure having good mechanical and physical properties.
•
Metakaolin: • dehydroxylated form of clay mineral kaolinite • Highly reactive pozzolan and high rate of dissolution in
the alkali solution •
Higher % of alumino silicate material compare to fly ash
Caijun Shi, T., Jiménez A and Angel Palomo, "New cements for the 21st century: The pursuit of an alternative to Portland cement," Cement and Concrete Research, 41, 2011, pp 750-763.
Geopolymerization reaction Dissolution of
metakaolin = type and amount of geopolymer product It is difficult to get experimental data for each step individually
Semi empirical calculations of dissolution process
Descriptive model of geopolymerization reaction
Pozzolanic reaction under alkaline condition results in 3-D polymeric chain and ring structure with Si-O-Al-O bonds. Mn [-(SiO2) z-AlO2] n .wH2O Where: M= the alkaline element –sodium, potassium or calcium, N= degree of polymerization •
Caijun Shi, T., Jiménez A and Angel Palomo, "New cements for the 21st century: The pursuit of an alternative to Portland cement," Cement and Concrete Research, 41, 2011, pp 750-763.
Metakaolin (MK) Structure
Dehydroxylation of Kaolinite clay
X-ray amorphous structure Sheet molecular structure alumino-silicate material Various assumptions for small molecular units
Two sheet structure: SiO4 tetrahedral layer and distorted AlO4 layer Five-membered alumino-silicate single ring cluster (Xu and Devanter, 2000) Six membered alumino-silicate single ring (Yunsheng and Wei, 2007)
To built a structure representing metakaolin
Size of molecular cluster
Single 6-membered ring SiO4 and AlO4 tetrahedron structure terminated OH bonds. 6-membered SiO4 and AlO4tetrahedron ring cluster
•Xu, H. and J. S. J. Deventer, "Ab initio calculations on five- membered alumino-silicate frameworks rings model: implications for dissolution in alkaline solutions," Computers and Chemistry, 24, 2000, pp 391-404. •Yunsheng, Z. and S. Wei, "Semi-empirical AM1 calculations on 6- membered alumino-silicate rings model: implications for dissolution process of metakaolin in alkaline solutions " Journal Of Material Science, 42, 2007, pp 3015-3023.
Quantity
PM6
ΔHf
Simulation Outline
Bond Lengths Angles
PM3
AM1
Units
8.01
18.2
22.86 Kcal/mol
0.091
0.104
0.13 Angstroms
7.86
8.5
8.77 Degrees
Average Unsigned Errors in PM6 Predictions
Accelrys Material Studio 5.5 VAMP module: Simulation of reactions and
properties of molecule in gas phase or solvent phase
AM1
Run AM1, PM3 and PM6 methods Dissolution in water
NaOH and KOH alkaline environment
dissolution
PM3
Si(OH)4 and NaOH geometry optimized structure
AM1 104°
PM3 107°
http://cccbdb.nist.gov/exp2.asp?casno=1310732 http://openmopac.net/manual/index_accuracy.html
PM6 109°
PM6
Si(OH)4 and NaOH geometry optimized structures
Geometry optimization of SiO4 and AlO4 Software and Method
Bond Length (nm)
SiO4
Bond Angle
Si-Onbr
Si-Obr
Obr-Si-Obr
Obr-Si-Onbr
Onbr-Si-Onbr
Si-Obr-Si
DMOL3
0.167
0.164
107.470
110.100
109.141
139.713
MS AM1
0.156
0.161
110.223
109.295
109.59
171.441
MS PM3
0.169
0.165
113.255
108.241
107.16
137.553
MS PM6
0.156
0.162
109.960
109.332
109.689
175.372
Software and Method
Bond Length(nm)
Bond Angle
AlO4
Al-Onbr
Al-Obr
Obr-Al-Obr
Obr-Al-Onbr
Onbr-Al-Onbr
Al-Obr-Al
DMOl3
0.189
0.176
124.098
102.762
105.016
166.508
MS AM1
0.1802
0.1735
131.955
94.36
102.71
151.902
MS PM3
0.1858
0.1787
125.257
104.365
98.825
174.823
MS PM6
0.1851
0.1761
122.075
91.706
102.866
132.643
AM1
PM3
PM6
AM1
PM3
PM6
Dissolution in water (Si(OH2)O)6 + 2H2O → (OH)3Si-(Si(OH)2O)3-Si(OH)3
+ HO-Si-(OH)3 E1 (1) (Al(OH2)O)6 + 2H2O → (OH)3Al-(Al(OH)2O)3-Al(OH)3 + HO-Al-(OH)3 E2 (2) The reaction energy of AlO4 broken ring is -1477 KJ/mol (PM6) which is highly exothermic as compare to SiO4 broken ring (-19 KJ/mol, PM6).
Dissolution in alkaline solution Ion pairing reaction:
(Si(OH2)O)6 + 3NaOH → (OH)3Si-(Si(OH)2O)3-Si(OH)3 + HO-Si-(ONa)3 + H2O E3 Ion pairing and interaction with broken cluster:
(Si(OH2)O)6 + 4NaOH → (OH)3Si-(Si(OH)2O)3-Si(OH)2-ONa + HO-Si-(ONa)3 + 2H2O E5
Heat of reaction comparison Reaction in alkaline medium is highly exothermic [E1(-19
KJ/mol) > E3 (-230.44 KJ/mol)] The ion pairing reaction between HO-Si-O3- and Na+ gives energy |E3| (230.44 KJ/mol). Similarly |E4| (215 KJ/mol) is obtained with K+ ion pairing. More heat of reaction generated when silicate species reacted with smaller size alkali cation in ion paring reaction (Sawddle et al., 1994) Reaction of dissolution of MK in the NaOH is more favorable than reaction of MK in the KOH, which is consistent with published literature. PM6 Reaction heat (KJ/mol) E3 (Si1-Na)
-230.44
E4 (Si1-K) E5 (Si2-Na) E6 (Si2-K) E7 (Al1-Na) E8 (Al1-K) E9 (Al2-Na) E10 (Al2-K)
-215.99
-522.18
-442.25
-1388.50
-1380.84
-1453.59
-1477.92
•Sawddle, T., Salereno J and P. Tregloan, "Aqueous aluminates, silicates, and aluminosilicates," Chemical society review, 23, 1994, pp 319.
Bond lengths and angles after dissolution 200 180
Bond Angle (deg)
120
NaOH dissolution
100 80 60 40 20
0.19
Al-Obr-Al
Onbr-Al-Onbr
Obr-Al-Onbr
Obr-Al-Obr
Si-Obr-Si
Onbr-Si-Onbr
Obr-Si-Onbr
0
Obr-Si-Obr
bonds on the ring Si/Al-Obr are both stretched after dissolution in the highly alkaline solution. The angles on ring (Obr-Si-Obr, Obr-Al-Obr) and angles on bridge (Si-Obr-Si, Al-Obr-Al) are reduced after dissolution. Yunsheng and Wei (2007) reported that single 6-membered AlO4 tetrahedron ring model shows contraction after dissolution in the highly alkaline solution.
KOH dissolution
140
Local Dissolution KOH dissolution NaOH Dissolution
0.185 0.18
Bond Length (A⁰)
Bonds on corner Si/Al-Onbr and
160
Local Dissolution
0.175 0.17 0.165 0.16 0.155
0.15 0.145 0.14 Si-Onbr
Si-Obr
Al-Onbr
Al-Obr
Conclusion In order to know dissolution of metakaolin in alkaline solution process, semi-
empirical PM6, PM3 and AM1 calculations have been conducted on optimized geometry of 6- membered SiO4 and AlO4 single ring clusters. The geometry optimization shows that PM6 semi-empirical method gives better idea about SiO4 and AlO4 ring clusters in the metakaolin. PM6 method achieved much more realistic geometry optimization for NaOH and Si(OH)4 molecule. SiO4 and AlO4 ring clusters demonstrates contraction after dissolution in the highly alkaline solution, which indicates that structure of metakaolin will distort after dissolution. In the highly alkaline environment, AlO4 ring cluster is more reactive and easily dissoluble as compared to SiO4 ring cluster. Also small size Na+ cation shows easy ring breakage and ion pairing interaction than K+ cation. Therefore metakaolin dissolution in the NaOH is more favorable than KOH solution
Acknowledgement Dr. Biernacki1 and Dr. Northrup2, (1) Department of
Chemical Engineering (2) Department of Chemistry Tennessee Technological University, Tennessee Center of Energy Systems Research (CESR) for their financial assistance. Joel Seber and Michael Renfro All my friends and work colleagues.
Thank you.
