Paramita Mondal

Nanoscale Characterization of Cementitious Materials

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 Youngs modulus of the interfacial transition zone.

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.

Comparative Study of the Effects of Microsilica and Nanosilica in Concrete

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.

Small Changes Can Make a Great Difference

Four different types of commercially available silica nanoparticles were added to ordinary portland cement pastes to study their effects. The subsequent multiscale characterization of the material revealed that the addition of the nanoparticles induced a pozzolanic reaction that increased the amount of calcium silicate hydrate (C-S-H) gel in the paste to the detriment of portlandite. This had important implications for the hydration kinetics and the microstructure of the paste, including an increase in the initial hydration rate. A reduction of the overall porosity was also observed. The C-S-H gel of the pastes with nanosilica also showed some particular features, such as greater aluminum content and longer silicate chains. This was especially relevant because nanoindentation measurements and atomistic calculations showed that this was bound to an improvement in the mechanical properties of the C-S-H gel itself. Finally, the sum of all these factors resulted in pastes with 30% more compressive strength, which proved that, effectively, small changes can make a great difference.

Nanomechanical Properties of Interfacial Transition Zone in Concrete

This research provides better understanding of the nanostructure and the nanoscale local mechanical properties of the interfacial transition zone (ITZ) in concrete. Nanoindentation with in-situ scanning probe microscopy imaging was used to compare the properties of the bulk paste with the properties of the ITZ between paste and two different types of aggregates. ITZ was found to be extremely heterogeneous with some areas as strong as the bulk matrix. Higher concentration of large voids and cracks along the interface was observed due to poor bonding. Nanoindentation results on relatively intact areas of the interface disagreed with the notion of increasing elastic modulus with distance from the interface. Depending on the aggregate type, average modulus of the ITZ was 70% to 85% of the average modulus of the paste matrix. The main problem the ITZ poses on the overall mechanical properties of concrete was concluded to be due to extreme heterogeneity within the interface and poor bonding between aggregate and paste. It was noted that the connectivity of the weaker areas such as large voids and cracks along the interface governs failure.

Characterization of silica-functionalized carbon nanotubes dispersed in water

Carbon nanotubes (CNTs) have the potential to enhance the strength, toughness, and multifunctional ability of composite materials. However, suitable dispersion and interfacial bonding remain as key challenges. Composites that are formed by reactions with water, like Portland cement concrete and mortar, pose a special challenge for dispersing the inherently hydrophobic nanotubes. The hydration of Portland cement also offers a specific chemical framework for interfacial bonding. In this study, nanoscale silica functional groups are covalently bonded to CNTs to improve their dispersion in water while providing interfacial bond sites for the proposed matrix material. The bond signatures of treated nanotubes are characterized using Fourier transform infrared spectroscopy. In situ dispersion is characterized using cryogenic transmission electron microscopy and point of zero charge (PZC) measurements. At the nanoscale, interparticle spacing was greatly increased. A slight increase in the PZC after treatment indicates the importance of steric effects in the dispersion mechanism. Overall, results indicate successful functionalization and dramatically improved dispersion stability in water.

Monitoring Setting of Geopolymers

The setting behavior of geopolymer pastes as a function of time was studied using two methods: penetration resistance and ultrasonic shear wave reflection. Several starting materials were included—metakaolin, Class C and Class F fly ashes, and slag—and chemical parameters known to affect set were varied. The geopolymers showed a wide range of set times compared to a reference Portland cement paste: some were much more rapid, some were similar, and some were much slower. Although most geopolymers formed gels that were both solid and had measurable strength, some, initially, formed a soft gel that had no measurable strength. Therefore, to fully characterize setting behavior, it is necessary to use both types of tests. It was seen that setting behavior was sensitive to chemical parameters, with setting delayed somewhat with increasing silica/alumina and increasing water/alkali, and accelerated substantially with calcium hydroxide substitution.

Effects of nanosilica addition on increased thermal stability of cement-based composite

Recently, nanosilica has been widely suggested as an excellent supplementary cementitious material due to its superior reactivity in comparison to other types of siliceous materials. Nanosilica is reported to increase the residual compressive strength of mortar after exposure to high temperatures. The addition of nanosilica has also been reported to cause fundamental changes in hydration products, increasing the average chain length of calcium silicate hydrate (C-S-H) and volume fraction of high-density C-S-H in cement paste in addition to decreasing calcium hydroxide content. However, it is not well understood exactly how nanosilica addition increases thermal stability of cement-based composites. In this study, the effects of replacement of a small amount of cement with nanosilica on the degradation of cement paste exposed to various heating and cooling regimes were investigated. Following heat treatment of cement paste samples with and without nanosilica up to 500°C (935°F), two different cooling regimes (cooling down to room temperature of 23± 2°C [73.4 ± 3.6°F] and prolonged heat treatment at 50°C [122°F] for 3 days) were followed. The residual states of each sample were analyzed by compression test, X-ray diffraction, scanning electron microscopy, and thermogravimetric analysis. The experimental results show that replacing 5% cement by weight with nanosilica leads to a 7 to 20% higher residual compressive strength after exposure to elevated temperatures. Furthermore, maintained exposure at above ambient temperature for long periods of time after exposure to high temperatures caused severe damage only to paste samples that did not contain nanosilica. This damage was not seen in samples containing nanosilica. Reduction in calcium hydroxide content due to the pozzolanic activity of nanosilica seems to be the primary reason for the minimized level of damage by reducing the degree of carbonation of hydration products immediately following heat treatment.

Use of biomineralisation in developing smart concrete inspired by nature

Recently, interest has focused on leveraging the biological functions of microorganisms to develop smart cement-based materials. This paper provides an overview of the calcium carbonate biomineralisation process in nature and presents a review of the work conducted by various groups around the world on biogenic calcium carbonate formation as it relates to the hydration, microstructure, properties, and performance of cement-based materials. Promises and concerns of applying biomineralisation in cement-based materials are also discussed, and directions for future research are explored.

Early-age dynamic moduli of crumbed rubber concrete for compliant railway structures

Heavy freight or high speed rail operations cause dynamic aggressive environments for structural materials in a railway system, resulting in higher wear and degradation rate of traditional brittle cementitious materials. As such, the improvement in fundamental dynamic properties of the material is crucial. In this study, the focus is placed on the development of a concrete damping enhancement using crumb rubber, which has significant potential to mitigate pressing issues in modern railway industry such as railseat abrasion and impact damages in concrete sleepers, thermal expansion crack of track slabs, and so on. Also indirectly, the utilization of crumb rubber will enhance the recycling options of rubber tire wastes, which are not biodegradable. This paper will focus only on the early-age dynamic moduli of the engineered concrete using modal excitation technique. The comparative study to evaluate the influence of crumb rubber participles highlights vital changes in dynamic elasticity and damping coefficients. This insight will establish a novel material design for precast and prestressed members where initial bonding and elastic shortening is critical for serviceability criteria.