Numatsmail
Numatsmail
Autor(a/res): Mostafa Aboelkheir, Celeste Y. S. Siqueira, Fernando G. Souza Jr., Romildo D. Toledo Filho
Resumo: Flexible cement sheath can eliminate the formation of cracks caused by the thermal recovery in the EOR methods. SBR-modified slurries were fabricated by adding Styrene-Butadiene Rubber (SBR) in latex to the cement matrix. In this article, the influence of this latex on the hydration kinetics of the cement slurry was investigated. SBR (≈83% of Styrene) was added by 6%, 10%, and 13% (V/V) to prepare the slurries and the kinetics in both fresh and hardened states were accompanied by UCA and isothermal calorimetry. The results indicated that in the very early ages of hydration, Ca(OH)2 was consumed by SBR (demulsification phenomenon). A retarding effect was observed and the percolation threshold values were increased by 18.05%, 19.44%, and 27.77% for the SBR-modified slurries, whereas an overall higher accumulated heat could be observed in the isothermal calorimetry curves when compared to the reference slurry.
Acesse AQUI e confira o artigo.
Autor(a/res): Saulo Rocha Ferreira , Marco Pepe, Enzo Martinelli, Flávio de Andrade Silva, Romildo Dias Toledo Filho b
Resumo: The use of Natural Fibers in cementitious composites is an innovative technical solution but, they are characterized by a complex microstructure and significant heterogeneity, which influence their interaction with cementitious matrices, whose identification requires further advances in the current state of knowledge. The present study summarizes the results of a wide series of pull-out tests carried out on sisal, curaua and jute fibers. Then, the experimental results are employed in an inverse identification procedure aimed at unveiling the key features of the aforementioned bond-slip laws. Morphological, chemical, physical and mechanical characterization of the natural fibers were correlated with the resulting bond properties within the embedding matrix. The obtained results in terms of relevant parameters, such as bond strength and fracture energy (under pull-out stresses) of the fiber-matrix interface, pave the way for future studies intended for a better understanding of the structural response of Natural Fiber Reinforced Cementitious Composites.
Acesse AQUI e confira o artigo.
Autor(a/res): Alex Neves Junior, Romildo Dias Toledo Filho, Jo Dweck, Frank K. Cartledged e Eduardo de Moraes Rego Fairbairn.
Resumo: The microstructure of high early strength and sulfate-resistant Portland cement carbonated pastes at two different durations of carbonation (1 h and 24 h) was analysed under the best conditions for carbon dioxide capture as indicated in previous studies by the authors. The aim was to determine, by means of X-ray diffraction analysis, nuclear magnetic resonance spectroscopy and scanning electron microscopy, how different carbonation times affect the microstructure of the pastes and to explain their respective related mechanical and porosity properties. The microstructure of the reference paste was homogeneous. The 1 h carbonated paste showed a more porous region near its external surface, surrounding a much higher inner non-porous homogeneous phase, while the 24 h carbonated specimen showed a single and more porous phase. The longer the carbonation time, the higher was the microstructural porosity of the carbonated regions and the higher the condensed silicate structure of the matrix, as characterised by morphology modifications. The resultant mechanical and physical properties of the pastes were affected by these chemical changes.
Acesse AQUI e confira o artigo.
Autor(a/res): Oscar A. Mendoza Reales, Yhan Paul Arias Jaramillo, Juan Carlos Ochoa Botero, Carmen Alicia Delgado, Jorge Hernán Quintero, Romildo Dias Toledo Filho
Resumo: This work studies the effect of MWCNT/surfactant aqueous dispersions on the rheology of cement paste. Three types of surfactants (sodium dodecyl sulfate, cetylpyridinium chloride and triton TX-100) were used to prepare cement pastes with and without MWCNT. Three rheological parameters were determined for each sample: static yield stress, yield stress, and viscosity. The first was measured directly, while the other two were obtained by fitting a Bingham model to the descending portion of a flow curve. Additionally, X-ray diffraction and isothermal calorimetry were used to follow the hydration reaction of cement during the first hour. It was found that the MWCNT/surfactant dispersions generate an overall shift to higher yield stress values while maintaining viscosity, suggesting a modification of the interparticle attraction. It was concluded that the triple interaction MWCNT-surfactant-cement governs the rheology of cement pastes.
Acesse AQUI e confira o artigo.
Autor(a/res): Oscar Aurelio Mendoza Reales, Pedro Duda e Romildo Dias Toledo Filho.
Resumo: Multiwalled carbon nanotubes (MWCNT) are expected to enhance the load transfer and increase the flexural strength of a MWCNT-cement composite by working as bridges for load across cracks. For this, an adequate dispersion degree of nanotubes throughout the matrix must be obtained, surfactants being one of the most used chemical-dispersing aids to attain this goal. A commercial MWCNT/surfactant aqueous dispersion was used to produce cement pastes with MWCNT additions up to 0.25% by mass of cement. Rheological behavior and mechanical performance were used to characterize the influence of nanotubes on the cement paste. It was found that the MWCNT/surfactant dispersion studied decreased the workability and mechanical properties of the cement paste, regardless of micrographs showing that MWCNT acted as bridges across submicrometric cracks. This was due to a delay of the hydration reaction of cement and an increase of the amount of entrapped air caused by the presence of surfactant. It was concluded that the reinforcing effect of nanotubes was masked by the negative effects of the surfactant on the mechanical performance of the matrix.
Acesse AQUI e confira o artigo.
