International Concrete Abstracts Portal

In this study, the development of cementitious nanocomposites reinforced with multiwall carbon nanotubes (MWCNTs) at water-cement ratios of 0.3 and 0.5 was investigated. The effect of carbon nanotubes at low concentrations on the fracture properties, nanoscale properties and microstructure of the nanocomposite materials was studied. The morphology and the microstructure of nanocomposite samples were investigated using an ultra high resolution field emission scanning electron microscope. A special type of nanoindenter, along with in-place scanning probe microscopy imaging, was used to determine the local nanoscale mechanical properties. The results show that the mechanical properties of cementitious matrices can be increased by the incorporation of very low amounts of CNTs. Nanoimaging of the fracture surfaces of cement nanocomposites have shown that CNTs reinforce cement paste by bridging the nanocracks and pores. Additionally, nanoindentation results suggest that CNTs modify and reinforce the nanostructure of cement paste by increasing the amount of high stiffness C-S-H and reducing the porosity.

Recent developments in nanotechnology paved the way for deepening the modeling level of science-based kernels and enabling new opportunities in terms of understanding the formation of hydration products and their contribution to the microstructure. Based on these developments next generation models are considered to have the potential to design new advanced materials that contribute to sustainable construction. One of the activities currently running in this field is the Codice (Computationally Driven design of Innovative Cement-based materials) project which is a multi-scale modeling research project established within the European research arena FP7. The project has the ambition to bridge the nanoscale to the microscale by applying advanced computational simulation modeling techniques for cementitious materials. Nano-based models will be connected to the microscale level within the framework of the Hymostruc model. It is the objective of the CODICE project to provide more insight into the role of the fundamental building blocks of CSH gel (basically 5nm sized nanoparticles) and the mechanisms that govern their aggregation into high-density (HD) and low-density (LD) C-S-H varieties. Thus, the project aims to refine the microstructure of the Hymostruc model (Breugel 1991) so as to recognize the two types of C-S-H aggregates. The new computational scheme is expected to be a perfect tool to design new cementitious materials with improved mechanical properties.

The effect of nano-anatase titanium dioxide (TiO2) powder on early age hydration kinetics of tricalcium silicate (C3S) was investigated. Isothermal calorimetry was performed on C3S pastes with 0, 10, and 15% of TiO2 addition by weight, and two mathematical models-the Avrami model and the boundary nucleation model (BN model)-were fitted to the data. The addition of TiO2 accelerated the rate of hydration, increased the peak reaction rate, and increased the degree of hydration at 12 and 24 hours. The model fits demonstrate that the BN model better captures the kinetics of the reaction, particularly in the deceleration period, than the Avrami model. The increase in the ratio of rate parameters (kB/kG) of the BN model with TiO2 addition suggests that hydration product is formed on or near the surfaces of TiO2 particles, as well as on the C3S surface. These results demonstrate that the addition of TiO2 nanoparticles accelerates the early hydration by providing additional nucleation sites, forming the foundation for future optimization of photocatalytic and other nanoparticle-containing cements.

Due to their excellent mechanical characteristics, carbon nanofibers (CNFs) and nanotubes (CNTs) are expected to enhance properties such as strength, ductility, and toughness in cementitious composites. However, such enhancements cannot be achieved unless the fibers are uniformly distributed in the composite and properly bonded to the matrix. CNT/Fs tend to agglomerate due to their high level of van der Waals interactions, and typically form a weak bond with hardened cement paste matrix. This work first presents a summary of the efforts made in the past to overcome these two problems. Some typical methods of qualifying/quantifying the dispersion of CNT/Fs either in the hardened cement paste or the mix water are discussed. To demonstrate the challenges associated with CNFs and their dispersion and interfacial bond with cementitious matrices, some of the results from an ongoing experimental program are presented. The experiments investigate the effect of surfactants on dispersion and their benefits and shortcomings when cementitious composites are concerned. It was shown that mixing cement and a well dispersed water-surfactant-CNF solution may not result in a uniform distribution of CNFs in the paste or an optimal CNT-matrix interfacial bond. However, it was also found that the interfacial bond can reach to a level high enough to prevent fiber pullout.

The effects of small dosages of nano-silica as a partial cement replacement material on the Ca ion leaching resistance of cement pastes exposed to deionized water is reported in this paper. Plain and modified cement paste specimens (containing either 6% or 9% of silica fume, or 0.5% or 1.5% of nano-silica) are subjected to leaching in deionized water for different durations after 56 days of curing in saturated limewater. The mass loss, change in porosity, and the changes in calcium hydroxide (CH) and C-S-H contents from thermogravimetric analysis between the specimens cured under saturated limewater for the entire duration and the specimens leached for different times are used to bring out the beneficial effects of these cement replacement materials when pastes are exposed to a leaching medium. The nano-silica modified cement pastes are observed to demonstrate lower mass loss and a lower increase in porosity when subjected to leaching. Using the changes in CH and C-S-H contents between the saturated and the leached pastes, it is shown that leaching and continuing cement hydration and/or pozzolanic reaction are essentially coupled, especially for the modified pastes. The paste with higher nano-silica content is seen to demonstrate increased C-S-H contents when undergoing leaching. The net Ca ion loss from both CH and C-S-H phases are seen to be lower for the pastes incorporating nano-silica as compared to those containing silica fume. The plain paste is seen to suffer the highest amount of Ca ion loss. A simplified method of calculating the apparent depth of the CH dissolution front is also reported, which is seen to highlight the influence of nano-silica and silica fume in improving the leaching resistance of pastes.