International Concrete Abstracts Portal

With the advent of nano technology, materials have been developed that can be applied to high performance concrete mix designs. Nano silica reacts with calcium hydroxide to develop more of the strength carrying structure of cement paste, calcium silica hydrate. In this paper, predictive relationships have been developed to distinguish the strength benefits when using different sizes of nano silica in cement paste. An extensive regime of experimental analysis was carried out to determine the effect of nano silica in the cement paste. Through these experiments the heat of hydration of multiple cement mix designs were measured. After that, the concentration of calcium hydroxide was recorded through X-ray diffraction. Then, the crystallographic structures were examined through scanning electron microscopy. Finally, the compressive strength was determined for each cement paste mixture. Through these experiments it was found that as the silica particles decreases in size and incorporate a wider gradation of sizes, the calcium silicate hydrates became more rigid; this increased the compressive strength.

This work presents results from an on-going investigation on the microstructural properties of cement-based materials in the presence of very low concentration of polymeric nano-aggregates. The hereby discussed properties are mainly porosity and permeability of mortar specimens, containing 0.5 g/L (0.03 lb/ft3 ) (mixing water) Poly(ethylene oxide)-block-Polystyrene (PEO113-b-PS218) stabilized micelles. The coefficient of water permeability, K, for the samples cast with nano-aggregates was found to be significantly lower, 3.65 e–14 m/s, compared to the control specimen, 3.7 e–8 m/s. Apparently, even a very low concentration of PEO113-b-PS218 micelles is able to significantly reduce water permeability at early ages. The phenomena are most likely denoted to re-distribution and transformation of hydration products in the matrix, the micelles contributing to restructuring of the pore space and altering the cement hydration mechanisms due to their specific amphiphilic properties.

The development of nanotechnology has led to the ability to produce silicon dioxide in nano-sized particles of predictable size ranges. In this study, concrete mixtures were developed using silicon dioxide of various sizes. Compressive strength testing showed significant increase in strength with decrease in particle size of the silicon dioxide down to 12 nm (4.7 × 10–7 in.). However, the mixture under 12 nm (4.7 × 10–7 in.) had a slightly lower increase in strength. High vacuum SEM analysis was performed on the samples. High-resolution images at magnifications of 5000× to 60,000× were achieved. The photographs suggest that only the surface of silicon dioxide particles is involved in chemical reactions. The particles then appear to become nucleation sites for the development of CSH crystals. Fine silicon dioxide particles provide numerous and small nucleation sites. Silicon dioxide particles smaller than 12 nm (4.7 × 10–7 in.) do not appear to generate additional nucleation sites for CSH. SEM photos of the 7 nm (2.8 x 10–7 in.) mixture reveal a structure similar to that of the 150 nm (59.1 × 10–7 in.) mixture.

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.

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.