Does a dissolution-precipitation mechanism explain concrete creep in moist environments?

Long-term creep (i.e., deformation under sustained load) is a significant material response that needs to be accounted for in concrete structural design. However, the nature and origin of creep remains poorly understood, and controversial. Here, we propose that concrete creep at RH (relative humidity) > 50%, but fixed moisture-contents (i.e., basic creep), arises from a dissolution-precipitation mechanism, active at nanoscale grain contacts, as is often observed in a geological context, e.g., when rocks are exposed to sustained loads, in moist environments. Based on micro-indentation and vertical scanning interferometry experiments, and molecular dynamics simulations carried out on calcium-silicate-hydrates (C-S-H's), the major binding phase in concrete, of different compositions, we show that creep rates are well correlated to dissolution rates - an observation which supports the dissolution-precipitation mechanism as the origin of concrete creep. C-S-H compositions featuring high resistance to dissolution, and hence creep are identified - analysis of which, using topological constraint theory, indicates that these compositions present limited relaxation modes on account of their optimally connected (i.e., constrained) atomic networks

Nanostructure and engineering properties of basic and modified calcium-silicate-hydrate systems

The nanostructure, physical properties and mechanical performance of C-S-H, the principal component in cement-based materials, was studied. Synthetic C-S-H of variable composition was examined as a model system in comparison with that produced in the hydration of Portland cement. The current doctoral thesis is comprised of several research chapters designed to probe some of the ambiguous aspects of the C-S-H at the nano level. Several advanced analytical tools and novel approaches were utilized in order to elucidate various controversial issues in cement and concrete science. The studied topics include three areas of C-S-H investigation: nanostructural features, engineering properties and modified systems. Nanostructural Features -- The C-S-H (I) was categorized in two main classes separated at a C/S ratio of about 1.1. These exhibit distinct properties as determined by XRD, Helium Inflow, 29Si NMR and 43Ca NMR. New evidence was provided supporting the layered nature of the C-S-H. The role of interlayer water and calcium ions as well as silicate tetrahedra on physical properties of C-S-H analogs were evaluated. It was mainly demonstrated that the high C/S ratio C-S-H (I) can be considered as a viable model for the nanostructure of the C-S-H in hydrated cement paste. Engineering Properties - The relation between the chemistry and mechanical performance of the phase pure C-S-H systems was investigated. The dynamic mechanical response and stress relaxation of the synthetic C-S-H as well as C-S-H in hydrated Portland cement were examined at various moisture contents. A unique oscillatory response in the storage modulus and internal friction of the C-S-H materials was identified. Viscoelastic behavior of C-S-H was associated with the sliding of the C-S-H sheets. A mechanistic model as proposed to explain the observed changes in the mechanical properties of layered C-S-H materials. Modified Systems - Two approaches were evaluated in order prepare and characterize nanohybrid C-S-H phases: organic modification of C-S-H, and metamorphosis of C-S-H in cement paste. In situ polymerized C-S-H/polyaniline nanostructures demonstrated enhanced physical and mechanical properties attributable to the interaction of polymer molecules with the silicate structure of the inorganic host. C-S-H seeding was also employed in order to tailor the nature of the C-S-H product. It was shown that the chemical properties of C-S-H can be readily controlled depending on the stoichiometry of the seed. This offers a unique method in order to engineer the hydrated cement and concrete materials for improved sustainability

Natural abundance ultrahigh-field 43Ca solid-state NMR in cement-based materials

Portland cement is a primary construction material with an annual production and consumption in billions of tons. Considering the great demand for this material, it is necessary to have a deep understanding of its nanostructure and detailed knowledge of the chemical transformations that occur during hydrolysis. Solid State (SS) NMR has had a significant role in studies of cement-based materials and processes. Until now, most of the SS NMR studies concentrated on such nuclei as 29Si, 27Al, 17O and 1H. All these nuclei are important part of the hydrated cement framework and the related studies have provided a wealth of information regarding the nanostructure and hydrolysis in cement systems.1,2 The properties of cement systems, however, depend greatly on the coordination and local environment of calcium. The 43Ca NMR could therefore play a significant role in structural and chemical studies of these materials. Recent 43Ca SS NMR studies have demonstrated impressively that the technique is suitable for solving complicated structural problems not accessible by other methods.3,4 Great difficulties in obtaining natural abundance 43Ca SS NMR, however, have limited its applications mostly to pure compounds.The current work presents results of a systematic 43Ca ultra high field solid state NMR study on a series of cement based materials, aimed at determining the possibilities and the limitations of the method in cement and concrete research. The low natural abundance (0.135%) and small gyromagnetic ratio of 43Ca present a serious challenge even at a magnetic field of 21.1T. In the initial phase of this research we examined the spectra of a number of anhydrous cement related compounds of known structure and composition. The spectra of several materials of significance in cement research, such as beta dicalcium (b-C2S) and tri-calcium (C3S) silicate, and tri-calcium aluminate (C3A), were obtained for the first time. The relation of spectroscopic and structural parameters was at the center of this study and the assignment of the signals was assisted by the first principles calculations. Further, the method was extended to the study of hydrated cement phases. The calcium environment in the synthetic calcium silicate hydrate (C-S-H) of variable composition was probed in comparison to the C-S-H formed in the hydration of C3S. The 43Ca NMR spectra of hydrated C3S suggested similar nanostructural features to those of more-ordered analogs including the synthetic C-S-H samples and 11 \uc5 tobermorite mineral. This observation supports the validity of using layered crystalline C-S-H systems as structural models for the nearly amorphous C-S-H that forms in the hydration of Portland cement. This study demonstrated that in-spite of the great complexity of the calcium silicate chemistry, 43Ca SS NMR is a useful and informative tool that provides valuable information in cement research.Le ciment Portland est un mat\ue9riau de construction primaire dont la production et la consommation annuelles totales s?\ue9l\ue8vent \ue0 des milliards de tonnes. \uc9tant donn\ue9 la forte demande pour ce mat\ue9riau, il devient n\ue9cessaire de comprendre \ue0 fond sa nanostructure et de conna\ueetre en d\ue9tail les transformations chimiques qui se produisent au cours de l?hydrolyse. La r\ue9sonance magn\ue9tique nucl\ue9aire (RMN) en phase solide a jou\ue9 un r\uf4le significatif dans les \ue9tudes des mat\ue9riaux et des proc\ue9d\ue9s \ue0 base de ciment. Jusqu?\ue0 maintenant, la plupart des \ue9tudes articul\ue9es sur la RMN en phase solide se sont int\ue9ress\ue9es \ue0 des nucl\ue9i tels que 29Si, 27Al, 17O et 1H. Tous ces nucl\ue9i constituent une partie importante de la structure du ciment hydrat\ue9, et les \ue9tudes connexes ont g\ue9n\ue9r\ue9 toute une masse de renseignements concernant la nanostructure et l?hydrolyse dans les formules de ciment. Les propri\ue9t\ue9s des formules, cependant, d\ue9pendent dans une large mesure de la coordination et de l?environnement calcique local. La RMN du 43Ca pourrait donc jouer un r\uf4le important dans les \ue9tudes structurales et chimiques de ces mat\ue9riaux. De r\ue9centes \ue9tudes de RMN du 43Ca en phase solide ont d\ue9montr\ue9 de fa\ue7on \ue9clatante que la technique convient \ue0 la r\ue9solution de probl\ue8mes structuraux complexes, que n?autorisent pas les autres m\ue9thodes. Des difficult\ue9s majeures li\ue9es \ue0 l?obtention de la RMN d?une teneur isotopique naturelle de 43Ca en phase solide ont toutefois limit\ue9 ses applications essentiellement aux compos\ue9s purs.Les travaux actuels pr\ue9sentent les r\ue9sultats d?une \ue9tude syst\ue9matique de RMN \ue0 champ ultra-\ue9lev\ue9 du 43Ca en phase solide, pour une s\ue9rie de mat\ue9riaux cimentaires, visant la d\ue9termination des possibilit\ue9s et des limitations de la m\ue9thode dans le cadre de la recherche sur les ciments et les b\ue9tons. La faible teneur isotopique naturelle (0,135 %) et le rapport gyromagn\ue9tique peu \ue9lev\ue9 du 43Ca pr\ue9sentent un d\ue9fi de taille, m\ueame \ue0 un champ magn\ue9tique de 21,1 T. Dans la phase initiale de cette recherche, nous avons examin\ue9 les spectres d?un certain nombre de substances voisines du ciment anhydre, d?une structure et d?une composition connues. Les spectres de plusieurs mat\ue9riaux importants dans la recherche sur le ciment, comme le silicate de b\ueata-dicalcium ( f-C2S), le silicate de tri-calcium (C3S) et l?aluminate de tri-calcium (C3A), ont \ue9t\ue9 obtenus pour la premi\ue8re fois. La relation entre les param\ue8tres spectroscopiques et structuraux \ue9tait au centre de notre \ue9tude, et l?assignation des signaux a \ue9t\ue9 r\ue9alis\ue9e \ue0 l?aide des calculs des premiers principes. En outre, on a \ue9largi cette m\ue9thode \ue0 l?\ue9tude des phases du ciment hydrat\ue9. L?environnement calcique dans l?hydrate de silicate de calcium (C-S-H) synth\ue9tique de diverses compositions a \ue9t\ue9 sond\ue9 en comparaison avec le C-S-H form\ue9 lors de l?hydratation du C3S. Les spectres de RMN du 43Ca du C3S hydrat\ue9 ont sembl\ue9 indiquer des caract\ue9ristiques nanostructurales semblables \ue0 celles d?analogues plus ordonn\ue9s, y compris les \ue9chantillons de C-S-H synth\ue9tiques et le min\ue9ral tobermorite \ue0 11 \uc5. Cette observation admet la validit\ue9 de l?utilisation de syst\ue8mes de C-S-H cristallins en couches comme mod\ue8les structuraux pour le C-S-H quasi-amorphe qui se forme au cours de l?hydratation du ciment Portland. Cette \ue9tude a d\ue9montr\ue9 qu?en d\ue9pit de la grande complexit\ue9 de la chimie des silicates de calcium, la RMN du 43Ca en phase solide demeure un outil utile et profitable qui g\ue9n\ue8re une information fort pr\ue9cieuse au domaine de la recherche sur les ciments.A version of this document is published in: 2008-2009 Annual Report, National Ultrahigh-Field NMR Facility for Solids, pp. 36-37Une version de ce document se trouve dans: 2008-2009 Annual Report, National Ultrahigh-Field NMR Facility for Solids, pp. 36-37Peer reviewed: NoNRC publication: Ye

Volume stability of calcium sulfoaluminate phases

The volume stability of calcium sulfoaluminate phases exposed to water, lime, and gypsum environments was investigated. The length changes of compacted specimens of synthetic monosulfate and ettringite were monitored in distilled water, lime-saturated water, gypsum-saturated water, and saturated water vapor. The X-ray diffraction analysis was also performed on the samples to assess the changes in the crystalline structure of each phase. Evidence was provided in support of the significant role of dissolution of monosulfate and ettringite, and the leaching of their constituent ions, on the expansion of these phases. \ua9 2012 The American Ceramic Society.Peer reviewed: YesNRC publication: Ye

Volume stability of calcium-silicate-hydrate/polyaniline nanocomposites in aqueous salt solutions

The volume stability of phase pure calcium-silicate-hydrates (C-S-H) and C-S-H/polyaniline nanocomposites prepared with two CaO-SiO2 molar ratio (C/S) variations (0.8 and 1.2) was assessed in MgSO4, MgCl2, LiCl, and NaCl aqueous solutions. The change in the crystalline structure of the samples with the time of immersion was also explored using X-ray diffraction, scanning electron microscopy, and thermal gravimetric analysis techniques. It was observed that the modification of the C-S-H samples with polyaniline significantly enhanced their volume stability and durability in all the salt solutions. The beneficial effect of the polyaniline modification was more pronounced in the C-S-H host with higher C/S (C/S = 1.2). The longitudinal expansion of the C-S-H/polyaniline nanocomposites with C/S = 1.2 in the salt solutions was about 30% of that of the phase pure C-S-H with a similar C/S ratio. In addition, the polyaniline modification of C-S-H samples reduced the rate of formation of gypsum, brucite, and other reaction products in the samples.Peer reviewed: YesNRC publication: Ye

Cement and Concrete Nanoscience and Nanotechnology

material

C-S-H\u2013Polyaniline nanocomposites prepared by in situ polymerization

Synthesis and characterization of a new cementbased polymer nanocomposite is reported. Calcium silicate hydrate (C\u2013S\u2013H) was prepared in the presence of aniline monomer followed by in situ polymerization to increase the degree of interaction between inorganic and organic phases. Two stoichiometrically different C\u2013S\u2013H systems were used. The properties of the C\u2013S\u2013H/polyaniline materials were studied using several analytical techniques including SEM, XRD, TGA, 29Si MAS NMR and FTIR. It is suggested that the in situ polymerization method can effectively be employed for producing a C\u2013S\u2013H/Polymer nanocomposite. The extent of molecular interaction with the polymer depends on the chemical composition of the C\u2013S\u2013H. Production of a new range of polymer-modified cement-based systems having improved environmental stability and mechanical performance is promising.Peer reviewed: YesNRC publication: Ye

Interaction of 2-, 3- and 4-nitrobenzoic acid with the structure of calcium\u2013silicate\u2013hydrate

Interaction of nitrobenzoic acid (NBA) with the structure of calcium\u2013silicate\u2013hydrate (C\u2013S\u2013H) was investigated. Phase pure C\u2013S\u2013H and C\u2013S\u2013H with 2-, 3- or 4NBA (0.01 and 0.02 mol/mol of Ca) were synthesized, and characterized by the X-ray diffraction, thermal analysis, Fourier transform infrared spectroscopy and scanning electron microscope techniques. Nitrogen adsorption measurements were also performed to estimate the surface area of the samples. It is suggested that all NBA isomers with the concentration of 0.01 mol/mol of Ca were able to fill the defects on the surface of the C\u2013S\u2013H layers, block access to the space between the stacked C\u2013S\u2013H layers, and possibly partially intercalate the layered structure. The C\u2013S\u2013H-based samples prepared with different types of NBA compounds, however, had different characteristics. The interaction of organic and inorganic phases was limited in the samples incorporating higher concentration of NBA. Traces of unreacted NBA were detected in a few samples with 0.02 mol of NBA/mol of Ca.Peer reviewed: YesNRC publication: Ye

Durability and mechanical properties of C\u2013S\u2013H/nitrobenzoic acid composite systems

The influence of nitrobenzoic acid on the nanostructure of calcium-silicate-hydrate (C\u2013S\u2013H) systems has been recently investigated by the authors. This study focuses on the assessment of durability and mechanical performance of the C\u2013S\u2013H/nitrobenzoic acid composite systems. In this context different nitrobenzoic acid isomers in various concentrations were studied. The C\u2013S\u2013H-based preparations were compacted into porous bodies. Their dimensional stability and the leaching of calcium ions in aqueous salt solutions containing Mg\ub2\u207a, Li\u207a, Cl\u207b or SO\u2084\ub2\u207b ions were evaluated. The resistance of the compacted samples to the diffusion of isopropyl alcohol was also obtained by the mass-change measurements. The microindentation technique was used to measure the creep modulus and the hardness of the samples. Evidence was obtained that nitrobenzoic acid has the potential to significantly improve the durability and mechanical properties of the C\u2013S\u2013H systems. This improvement, however, only occurred in the systems with the lower concentration of nitrobenzoic acid. The systems with higher concentration of nitrobenzoic acid had reduced durability and mechanical performance due to the limited interaction of the organic and inorganic phases in these systems.Peer reviewed: YesNRC publication: Ye

Volume stability of calcium-silicate-hydrate/polyaniline nanocomposites in aqueous salt solutions

The volume stability of phase pure calcium-silicate-hydrates (C-S-H) and C-S-H/polyaniline nanocomposites prepared with two CaO-SiO2 molar ratio (C/S) variations (0.8 and 1.2) was assessed in MgSO4, MgCl2, LiCl, and NaCl aqueous solutions. The change in the crystalline structure of the samples with the time of immersion was also explored using X-ray diffraction, scanning electron microscopy, and thermal gravimetric analysis techniques. It was observed that the modification of the C-S-H samples with polyaniline significantly enhanced their volume stability and durability in all the salt solutions. The beneficial effect of the polyaniline modification was more pronounced in the C-S-H host with higher C/S (C/S = 1.2). The longitudinal expansion of the C-S-H/polyaniline nanocomposites with C/S = 1.2 in the salt solutions was about 30% of that of the phase pure C-S-H with a similar C/S ratio. In addition, the polyaniline modification of C-S-H samples reduced the rate of formation of gypsum, brucite, and other reaction products in the samples.Peer reviewed: YesNRC publication: Ye