摘要：

The tensile strength of cement paste is one of the most important mechanical properties that influence shrinkage cracks in cementitious materials. Cement pastes that exhibit low tensile strength tend to exhibit greater shrinkage crack potential and reduced durability. Increasing the tensile strength in cement paste can minimize the shrinkage cracking potential. It is believed that the strength and cohesion of cement paste are controlled by the formation of calcium silicate hydrate (C-S-H) gel. To enhance macroscopic mechanical properties (tensile strength), it is necessary to understand the structure and behavior of C-S-H gel at the atomic level. Previously, molecular statics was used to determine minimal potential energy and the mechanical properties of crystalline C-S-H structures. From this study, a plausible atomic structure of C-S-H gel is proposed. This research effort builds on the aforementioned work by using molecular dynamics to derive tensile and compressive strengths of C-S-H structures from uniaxial stress–strain data. The results from the molecular dynamics simulations showed that the maximum strengths (i.e., compressive and tensile) for the proposed C-S-H structures are three orders of magnitude higher than the strength at the macrolevel. However, the tensile strength of the proposed C-S-H gel is 23% of the compressive strength. This research also concludes that electrostatic forces and bond forces in the silicate chains are the main contributors to cement strength at the atomic level and that breakage in silicate chains leads to low tensile strength in C-S-H gel.ReferencesSection:1. Popovics, S. Strength and Related Properties of Concrete: A Quantitative Approach. John Wiley and Sons Inc., New York, 1998. Google Scholar2. Taylor, H. F. W. Cement Chemistry, 2nd ed. Thomas Telford Ltd., London, 1997. Google Scholar, Crossref3. Pellenq, R. J.-M., Lequeux, N., and Van Damme, H. Engineering the Bonding Scheme in C-S-H: The Iono-Covalent Framework. Cement and Concrete Research, Vol. 38, No. 2008, pp. 159–174. Google Scholar, Crossref4. Pellenq, R. J.-M., and Van Damme, H. Why Does Concrete Set? The Nature of Cohesion Forces in Hardened Cement Based Materials. MRS Bulletin, Vol. 29, No. 5, 2004, pp. 319–323. Google Scholar, Crossref5. Richardson, I. G. The Nature of C-S-H in Hardened Cements. Cement and Concrete Research, Vol. 29, 1999, pp. 1131–1147. Google Scholar, Crossref6. Hubbard, A. T. Encyclopedia of Surface and Colloid Science, 1st ed. CRC Press, Boca Raton, Fla., 2002. Google Scholar7. Plassard, C., Lesniewska, E., Pochard, I., and Nonat, A. Investigation of the Surface Structure and Elastic Properties of Calcium Silicate Hydrates at the Nanoscale. Ultramicroscopy, Vol. 100, 2004, pp. 331–338. Google Scholar, Crossref8. Aligizaki, K. K. Pore Structure of Cement-Based Materials: Testing, Interpretation and Requirements. Taylor and Francis Group, Abingdon, United Kingdom, 2006. Google Scholar9. Hamid, S. A. The Crystal Structure of the 11 ? Natural Tobermorite Ca2.25[Si3O75 (OH)15]·H2O. Zeitschrift fur Kristallographie, Vol. 154, 1981, pp. 189–198. Google Scholar10. Merlino, S., Bonaccorsi, E., and Armbruster, T. Tobermorites: Their Real Structure and Order-Disorder (OD) Character. American Mineralogist, Vol. 84, 1999, pp. 1613–1621. Google Scholar, Crossref11. Merlino, S., Bonaccorsi, E., and Armbruster, T. The Real Structures of Clintobermorite and Tobermorite 9 ?: OD Character, Polytypes, and Structural Relationships. European Journal of Mineralogy, Vol. 12, 2000, pp. 411–429. Google Scholar, Crossref12. Jennings, H. M. A Model for the Microstructure of Calcium Silicate Hydrate in Cement Paste. Cement and Concrete Research, Vol. 30, 2000, pp. 101–116. Google Scholar, Crossref13. Donev, A., Cisse, I., Sachs, D., Variano, E. A., Stillinger, F. H., Connely, R., Torquato, S., and Chaikin, P. M. Improving the Density of Jammed Disordered Packings Using Ellipsoids. Science, Vol. 303, 2004, pp. 990–993. Google Scholar, Crossref14. Thomas, J. A. Colloidal Interpretation of Chemical Aging of the C-S-H Gel and

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