Zoi S. Metaxa

Nanoscale Modification of Cementitious Materials

This research investigates changes in the nanostructure and the nanoscale local mechanical properties of cement paste with micro- and nano-modifiers. Silica fume and multiwall carbon nanotubes (MWCNTs) were used as micro- and nano-modifiers. An effective method of dispersing CNTs in cement matrix was developed. A detailed study on the effects of CNTs concentration and aspect ratio on the fracture properties, nanoscale properties and microstructure of nanocomposite materials, was conducted. Significant improvements on the macro and nano-mechanical properties of cement paste were observed with the incorporation of CNTs. Results suggest that CNTs can strongly modify and reinforce the cement paste matrix at the nanoscale.

Carbon Nanofiber–Reinforced Cement-Based Materials

Fibers are incorporated into cementitious matrices to help control cracking by bridging and providing load transfer across cracks and pores. Typical reinforcement of concrete is usually provided at the macro- and microscales by using macrofibers and microfibers, respectively. Although microfibers delay the propagation of microcracks, they do not stop their initiation. Theory suggests that nanofibers delay the formation of nanocracks, thus requiring higher loads to initiate cracking, which improves the weak tensile strength of the cementitious matrix. In this paper, a detailed investigation of the effects of carbon nanofibers (CNFs) on the flexural strength and nanostructure of the cement matrix was conducted. An ultra-high-resolution field emission scanning electron microscope was used to investigate the morphology of the nanocomposites. Nanoimaging of the fracture surfaces of cementitious nanocomposites reinforced with CNFs at different concentrations has shown that CNFs are able to bridge nanocracks and pores and achieve good bonding with the cement hydration products. As a result, the incorporation of CNFs was shown to improve significantly the flexural strength of the cementitious matrix.

Mechanical Properties and Nanostructure of Cement-Based Materials Reinforced with Carbon Nanofibers and Polyvinyl Alcohol (PVA) Microfibers

Synopsis: There have been numerous studies that have aimed at improving the low tensile strength, stiffness, and toughness of cementitious materials. This study aims to show that all of these characteristics can be greatly improved by the addition of ladder scale reinforcement at the nano and micro scale. Carbon nanofibers (CNFs) as well as polyvinyl alcohol (PVA) microfibers were used as reinforcement. The mechanical properties of the nanocomposites were investigated by fracture mechanics three-point bending test. The microstructure and the morphology of nanocomposite samples were studied using an ultra high resolution scanning electron microscope (SEM). The results clearly illustrate that the incorporation of nanofibers and microfibers greatly improves the flexural strength, Young’s modulus, and toughness of the cement matrix.

Advanced Cement Based Nanocomposites

Considerable research and development efforts have been directed towards high strength/high performance concrete with engineered properties, using three main concepts: a low water to binder ratio (w/b), and the partial replacement of cement by fine supplementary cementitious or pozzolanic materials and/or fibers. To better understand how material composition and microstructural modifications determine the concrete structural performance, and to develop new materials with specific properties, researchers at ACBM have taken a materials science approach with an application to nanotechnology to optimize the processing and micro/nanoscale structure of cement based materials. In particular, due to their exceptional mechanical properties, the reinforcing effect of highly dispersed multiwall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs) in cement paste matrix was investigated. The major challenge however, associated with the incorporation of MWCNTs and CNFs in cement based materials is poor dispersion. In this study, effective dispersion of different length MWCNTs in water was achieved by applying ultrasonic energy and with the use of a surfactant. The excellent reinforcing capabilities of the MWCNTs are demonstrated by the enhanced fracture resistance properties of the cementitious matrix. Additionally, nanoindentation results suggest that the use of MWCNTs can increase the amount of high stiffness C-S-H and decrease the porosity. Besides the benefits of the reinforcing effect, autogenous shrinkage test results indicate that MWCNTs can also have a beneficial effect on the early strain capacity of the cementitious matrix, improving this way the early age and long term durability of the cementitious nanocomposites.