Nanotechnology in concrete – A review

Geopolymer concrete: A review of some recent developments

The microstructural evolution of alkali-activated binders based on blast furnace slag, fly ash and their blends during the first six months of sealed curing is assessed. The nature of the main binding gels in these blends shows distinct characteristics with respect to binder composition. It is evident that the incorporation of fly ash as an additional source of alumina and silica, but not calcium, in activated slag binders affects the mechanism and rate of formation of the main binding gels. The rate of formation of the main binding gel phases depends strongly on fly ash content. Pastes based solely on silicate-activated slag show a structure dominated by a C–A–S–H type gel, while silicate-activated fly ash are dominated by N–A–S–H ‘geopolymer’ gel. Blended slag-fly ash binders can demonstrate the formation of co-existing C–A–S–H and geopolymer gels, which are clearly distinguishable at earlier age when the binder contains no more than 75 wt.% fly ash. The separation in chemistry between different regions of the gel becomes less distinct at longer age. With a slower overall reaction rate, a 1:1 slag:fly ash system shares more microstructural features with a slag-based binder than a fly ash-based binder, indicating the strong influence of calcium on the gel chemistry, particularly with regard to the bound water environments within the gel. However, in systems with similar or lower slag content, a hybrid type gel described as N–(C)–A–S–H is also identified, as part of the Ca released by slag dissolution is incorporated into the N–A–S–H type gel resulting from fly ash activation. Fly ash-based binders exhibit a slower reaction compared to activated-slag pastes, but extended times of curing promote the formation of more cross-linked binding products and a denser microstructure. This mechanism is slower for samples with lower slag content, emphasizing the correct selection of binder proportions in promoting a well-densified, durable solid microstructure.

Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash

Highly dispersed carbon nanotube-reinforced cement-based materials

This paper presents a review of alkali-activation technology, moving from the atomic scale and chemical reaction path modelling, towards macroscopic observables such as strength and durability of alkali-activated concretes. These properties and length scales are intrinsically interlinked, and so the chemistry of both low-calcium (‘geopolymer’) and high-calcium (blast furnace slag-derived) alkali-activated binders can be used as a starting point from which certain engineering properties may be discussed and explained. These types of materials differ in chemistry, binder properties, chemical structure and microstructure, and this leads to the specific material properties of each type of binder. The secondary binder products formed during alkali-activation (zeolites in low-Ca systems, mostly layered double hydroxides in alkali-activated slags) are of significant importance in determining the final properties of the materials, particularly in the context of durability. The production of highly durable concretes must remain the fundamental aim of research and development in the area of alkali-activation. However, to enable the term ‘highly durable’ to be defined in a satisfactory way, the underlying mechanisms of degradation—which are not always the same for alkali-activated binders as for Portland cement-based binders, and cannot always be tested in precisely the same ways—need to be further analysed and understood. The process of reviewing a topic such as this will inevitably raise just as many questions as answers, and it is the intention of this paper to present both, in appropriate context.

/pdf/geopolymers-and-other-alkali-activated-materials-why-how-and-32n3zxweey.pdf

Geopolymers and other alkali activated materials: why, how, and what?

Role of slag in microstructural development and hardening of fly ash-slag geopolymer

It is well recognized that the use of mineral admixtures such as silica fume enhances the strength and durability of concrete. This research compares the effects of adding silica fume and nanosilica to concrete and provides a better understanding of the changes in the concrete nanostructure. Nanoindentation with scanning probe microscopy imaging was used to measure the local mechanical properties of cement pastes with 0% and 15% replacement of cement with silica fume. A reduction in the volume fraction of calcium hydroxide in a sample with silica fume provides evidence of pozzolanic reaction. Furthermore, replacing 15% cement by silica fume increased the volume fraction of the high-stiffness calcium silicate hydrate (C-S-H) by a small percentage that was comparable with the decrease in the volume fraction of calcium hydroxide. A parallel study of cement pastes with nanosilica showed that nanosilica significantly improves durability of concrete. This research provides insight into the effects of nanosilica on cement paste nanostructure and explains its effect on durability of concrete. The nanoindentation study showed that the volume fraction of the high-stiffness C-S-H gel increased significantly with addition of nanosilica. Nanoindentation results of cement paste samples with similar percentages of silica fume and nanosilica were compared. Samples with nanosilica had almost twice the amount of high-stiffness C-S-H as the samples with silica fume.

Comparative Study of the Effects of Microsilica and Nanosilica in Concrete

It is widely believed that the fundamental properties of concrete are affected by the material properties at the nanoscale. Hence, to improve cement and concrete properties, it is necessary to first understand the nanoscale properties. In this research, sample preparation techniques were developed to image the nano- and microstructure of hardened cement paste using atomic force microscopy (AFM). A special type of nanoindenter along with in-place scanning probe microscopy imaging has been used to determine the nanoscale local mechanical properties, including the Young's modulus of the interfacial transition zone.

Nanoscale Characterization of Cementitious Materials

Fiber reinforcements provide many benefits to cementitious composites, including reduction of crack widths and increases in ductility. However, the interfacial transition zone between fibers and hydrated cement can contain a high proportion of calcium hydroxide and porosity. With their high moduli of elasticity, carbon nanotubes and carbon fibers could provide substantial mechanical reinforcement at multiple length scales, but only if their bond to the matrix can be controlled. Surface treatments of fibers and addition of supplemental materials in the matrix can influence both the mechanical interaction at the interface and the dispersion of these relatively small reinforcements. We performed a broad study of Portland cement mortar mixtures containing silica fume, plain or silica-functionalized carbon nanotubes, and carbon fibers to characterize changes in fracture properties. The early age hydration kinetics of cement pastes containing carbon nanotubes were compared using isothermal calorimetry. Early age fracture surfaces of cement pastes containing carbon fibers were observed using a scanning electron microscope. The notched beam test method of the Two Parameter Fracture Model was used to determine the fracture properties of each mix. We observed that silica fume and silica functional groups improved the fracture performance of mixtures containing carbon nanotubes and carbon fibers. Further optimization of dosage, size, and interface strength is required to fully utilize carbon nanotubes in cementitious composites.

Effects of silica additives on fracture properties of carbon nanotube and carbon fiber reinforced Portland cement mortar

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.

Paramita Mondal

Papers

Role of slag in microstructural development and hardening of fly ash-slag geopolymer

Comparative Study of the Effects of Microsilica and Nanosilica in Concrete

Nanoscale Characterization of Cementitious Materials

Effects of silica additives on fracture properties of carbon nanotube and carbon fiber reinforced Portland cement mortar

Nanoscale Modification of Cementitious Materials