Maria S. Konsta-Gdoutos

Hydration and properties of novel blended cements based on cement kiln dust and blast furnace slag

The aim of the present paper is to address the key technical issues pertaining to the utilization of cement kiln dust (CKD) as an activator for ground granulated blast furnace slag (GGBFS) to create nonconventional cementitious binders for concrete. The relatively high alkaline content of CKD is the predominant factor preventing its recycling in cement manufacture. However, it was observed that depending on the water-soluble alkali and sulfate compounds, CKD could provide the environment necessary to activate latent hydraulic materials such as GGBFS. Binary blends containing slag and CKDs from different sources were characterized and compared in terms of the rates of heat evolution and strength development, hydration products, and time of initial setting. A study of the effects of the influencing factors in terms of soluble alkali content, particle size, and free lime content was undertaken. The results confirm the dependence of the dissolution rate of slag on the alkalinity of the reacting system, and the importance of the optimum lime content on the rate of strength gain.

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.

Nano-modification of cementitious material: toward a stronger and durable concrete

Nanotechnology in construction and building materials has attracted great attention in recent years. Results demonstrated that nanomodification of cementitious materials can lead to significant improvement of the mechanical property, compactness, and durability. In this paper, based on the major characteristics of nanomaterials when used in cement-based materials, the recent progresses of nanomodification of cementitious materials with the mostly used nanomaterials such as nano-SiO2, nanoclay, nano-Al2O3,and carbon nano materials (carbon nanotubes and nanofibers) are reviewed. Modification effects and their influencing mechanisms of nanomaterials introduced by their specific features, such as the hydration seeding effect, the filling effect, the thixotropy-modifying effect, and the chemical reactivity feature on the properties of cementitious materials are reviewed. It is suggested that a stronger, greener, and more durable cementitious material can be obtained through the help of nanomodification.

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.

Nanomaterials in self-consolidating concrete: a state-of-the-art review

Nanomaterials, thanks to their exceptional properties and high surface-to-volume ratio, are known to improve the pore structure of concrete, accelerate the C–S–H gel formation, and enhance the concrete’s mechanical and durability properties. This work highlights the state of knowledge in producing self-compacting concrete (SCC) using nanomaterials such as nanosilica, nanoclay, TiO2, Fe2O3, CuO, ZnO2, Al2O3, and ZrO2 nanoparticles. Specifically, we focus on three areas in nano-modified SCC research: (i) the technical challenges associated with utilization of nanoparticles; (ii) the key research efforts in processing and manufacturing nano-engineered SCC; and (iii) the effect of nanomaterials on the fresh state and hardened properties of nano-engineered SCC.

Recycling of cement industry wastes by grinding process

Abstract This paper presents the results of an investigation into the use of cement kiln dust and fly ash as a new cementitious material. The activation process chosen, in order to facilitate and enhance hydration of the two materials, was mechanochemical activation. Activation is accomplished through the use of attrition mill. The two materials were combined and subjected to various grinding regimes to cultivate mechanochemical activation. Activation was determined through the use of particle size distribution and X-ray diffraction testing. Properties of the cementitious paste were determined by heat of hydration and compressive strength testing of cubes. The results indicate that separate grinding of the two materials is more effective at activating the materials and provide the best properties of the paste.

MWCNT and CNF Cementitious Nanocomposites for Enhanced Strength and Toughness

Cementitious nanocomposites reinforced with carbon fibers at the nanoscale were fabricated and tested, exhibiting remarkably improved mechanical and fracture properties. The cementitious nanocomposites were reinforced with well dispersed multiwall carbon nanotubes (MWCNTs) and carbon nanofibers (CNFs). A dispersion method involving the application of ultrasonic energy and the use of a superplasticizer was employed to prepare the nanoscale fiber suspensions. Flexural strength and Young’s modulus were experimentally investigated and compared with similarly processed reference cement based mixes without the nano-reinforcement. The nanocomposites’ fracture properties were also determined using the two parameter fracture model (TPFM). The excellent reinforcing capability of MWCNTs and CNFs is demonstrated by a significant improvement in flexural strength (87 % for MWCNTs and 106 % for CNFs reinforcement), Young’s modulus (100 %), and fracture toughness (86 % for MWCNTs and 119 % for CNFs reinforcement).

Measurement and Modeling of the Elastic Modulus of Advanced Cement Based Nanocomposites

Carbon nanotubes and carbon nanofibers exhibit several distinct advantages as a reinforcing material for cementitious composites, as compared to more traditional fibers. They exhibit significant greater strength and stiffness, which greatly improve the composites’ mechanical behavior. In this research, an experimental study of the mechanical characterization of cement based nanocomposite materials reinforced with carbon nanotubes is presented. The classic micromechanical approach for fiber reinforced composites was employed to develop predictive models for the modulus of elasticity of the nanocomposites. Results reveal a good agreement between the experimental and predicted values, when using the Benveniste model with disk like inclusions.