In-situ X-ray tomographic imaging of microstructure evolution of fly ash and slag particles in alkali-activated fly ash-slag paste

This paper presents an in-situ investigation of the microstructure evolution of fly ash and slag particles in alkali-activated fly ash-slag paste using X-ray microcomputed tomography. Results indicate that the dissolution of fly ash and slag particles is not uniform especially for the particles with large size due to the heterogeneous distribution of chemical composition and initial defects. The dissolution of the particles with small size is faster than that of large particles owing to the relatively high specific area. The formation of reaction products (i.e., inner products) is mostly accumulated within the boundary of original particles, which become a barrier to prevent the further dissolution of unreacted particles. The fly ash-slag interaction in terms of microstructure evolution is not obvious at early ages (1–3 d) but becomes apparent at later ages (7–28 d), which can be attributed to the continuous transport of dissolved ions between fly ash and slag

Three-dimensional in situ XCT characterisation and FE modelling of cracking in concrete

Three-dimensional (3D) characterisation and modelling of cracking in concrete have been always of great importance and interest in civil engineering. In this study, an in situ microscale X-ray computed tomography (XCT) test was carried out to characterise the 3D microscale structure and cracking behaviour under progressive uniaxial compressive loading. The 3D cracking and fracture behaviour including internal crack opening, closing, and bridging were observed through both 2D tomography slices and 3D CT images. Spatial distributions of voids and cracks were obtained to understand the overall cracking process within the specimen. Furthermore, the XCT images of the original configuration of the specimen were processed and used to build microscale realistic 3D finite element (FE) models. Cohesive interface elements were inserted into the FE mesh to capture complicated discrete crack initiation and propagation. An FE simulation of uniaxial compression was conducted and validated by the in situ XCT compression test results, followed by a tension simulation using the same image-based model to investigate the cracking behaviour. The quantitative agreement between the FE simulation and experiment demonstrates that it is a very promising and effective technique to investigate the internal damage and fracture behaviour in multiphasic composites by combining the in situ micro XCT experiment and image-based FE modelling

Direct Three Dimensional Observation of the Microstructure and Chemistry of C3s Hydration

Although portland cement has been used for over a hundred years as the binder in concrete, the basic mechanism of hydration is still not well understood. Progress has been halted for the fact that it is challenging for most current experimental techniques to give direct observation of the hydration process in-situ and provide quantitative measurement on the microstructure and chemistry at the nano-length scale. Recent advances of nano scale X-ray imaging make nano-tomography and nano-X-ray fluorescence reality. The nano-scale X-ray beams in these techniques allow the sample to be imaged nondestructively and provide a high transmission of signal that penetrate through both sample materials and a possible solution environment, which could make themselves in-situ techniques. Moreover, these techniques can be combined to enrich both datasets to become a more powerful technique. In this dissertation, the applications of both techniques have been established from micron lab scale experiment to nano-synchrotron investigation for studying cementitious materials. The progresses have been shown from first application on 3D chemical characterization of fly ash particles at the nanoscale to later updated versions of in-situ experiments for studying cement hydration, which allow quantitative measurements on 3D structure, chemistry and mass density of hydration products at different hydration periods. These unprecedented discoveries could lead to a breakthrough for both nanoscale analysis of any material and cement hydration research.Civil Engineerin

Multiscale Characterisation of Microstructure and Mechanical Properties of Alkali-Activated Fly Ash-Slag Concrete

Alkali-activated fly ash-slag (AAFS) concrete manufactured through the reaction of alkaline activator with industrial aluminosilicate by-products (fly ash and slag) is considered as a promising alternative to Portland cement (PC) concrete because of its environmental benefits (e.g. low CO2 emission and low consumption of natural resources) and superior engineering properties under ambient curing condition. There is about 55% less CO2 emissions in the production of AAFS comparing with the production of PC concrete. In addition, AAFS concrete can achieve a good synergy between fresh properties, mechanical properties and durability under ambient curing which cannot be achieved by the sole alkali-activated concrete, e.g., alkali-activated fly ash (AAF) and alkali-activated slag (AAS). AAF needs to be cured under an elevated temperature (60 ~ 85 °C) to gain early-age strength, whereas AAS concrete has some drawbacks including poor workability and quick setting. The mechanical properties of AAFS concrete are highly dependent on its heterogeneous microstructure with multiscale (nano- to macro-scale) and multiphase (pore, reaction products, unreacted fly ash and slag particles, and aggregate). Although the microstructure and mechanical properties of AAFS concrete have been studied for decades, a systematic understanding of the microstructure and micromechanical properties of individual phases within AAFS concrete and their corresponding relationships with the macroscopic mechanical properties is still lacking to date. More specifically, the following aspects for AAFS concrete have not been fully understood: (i) reaction mechanism of fly ash and slag particles in AAFS system; (ii) microstructure evolution of interfacial transition zone (ITZ) in AAFS concrete; (iii) multiscale micromechanical properties of AAFS concrete; (iv) multiscale microstructure-mechanical properties relationship in AAFS concrete. To fill these research gaps, this thesis aims to systematically characterise the microstructure and mechanical properties of AAFS concrete cured at ambient temperature at multiscale from nano- to macro-scale and to investigate the microstructure-mechanical properties relationship in AAFS concrete in depth. The multiscale features of AAFS concrete are identified based on four length levels: Level 0 (solid gel particle: 1 nm ~ 10 nm), Level I (gel matrix: 10 nm ~ 1 µm), Level II (paste: 1 µm ~ 100 µm), and Level III (concrete: 1 mm ~ 10 cm). Regarding the multiscale characterisation of microstructure, the nanostructure of solid gel particle at Level 0 is characterised using nuclear magnetic resonance (NMR), while the chemical composition of gel matrix at Level I is evaluated by means of X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR). The in-situ monitoring of microstructure evolution of fly ash and slag particles in AAFS paste at Level II is achieved using X-ray microcomputed tomography (XCT), providing new insights into their reaction mechanism. The microstructure evolution of ITZ in AAFS concrete at Level III is characterised using backscattered scanning electron microscopy (BSEM) and energy dispersive spectrometry (EDS), which delivers needed insight into the mechanism of ITZ evolution. The results of microstructural characterisation provide a systematic understanding of the microstructure of individual phases in AAFS concrete and their inherent relationships at different length scales. With respect to the multiscale characterisation of micromechanical properties, nanoindentation is used to evaluate the micromechanical properties (elastic modulus and hardness) of individual phases at Level I. It is the smallest material length scale that can be measured through experimental tests. The effective mechanical properties of AAFS paste at Level II are estimated using the self-consistent continuum micromechanics model by assuming that each nanoindentation test serves for a single phase in the material. Afterwards, the micromechanical properties of ITZ in AAFS concrete at Level III are evaluated through a series of statistical analysis. The multiscale micromechanical analysis offers the first-hand information of micromechanical properties of different phases in AAFS concrete and their contributions to the macroscopic mechanical properties of AAFS concrete. Lastly, the relationships between chemistry, microstructure, and mechanical properties of AAFS concrete from Level 0 to Level III are established based on the experimental results obtained above, which enable us to better understand the development of overall mechanical properties of this new type of concrete. The experimental and simulated results indicate that the dissolutions of fly ash and slag particles in AAFS system are not uniform due to their inherently heterogeneous characteristics, which would consequently lead to the formation of non-uniform reaction products, mostly accumulating within the boundary of the original particles. The polymerisation degree and cross-linking of reaction products are improved over curing age, potentially through the initial formation of C-A-S-H gels followed by the gradual development of N-A-S-H and N-C-A-S-H gels with a higher cross-linking degree. Within these three types of reaction products, the N-A-S-H gels have a relatively low elastic modulus due to their high level of structural disorder and gel porosity. In addition, it is found that the elasticity of reaction products and their relative volumetric proportions mainly determine the macroscopic elasticity of AAFS paste, while the porosity and pore size distribution primarily condition its macroscopic strength. Furthermore, it is also observed that ITZ formed in AAFS concrete has a comparable microstructure and micromechanical properties to the paste matrix, which indicates that ITZ might be not the weakest region within this new type of concrete. The ITZ with compact microstructure and high micromechanical properties would help to improve its macroscopic mechanical strength, especially for the fracture properties

Characterising the microstructure of cement-based materials using laser scanning confocal microscopy

Three-dimensional (3D) pore characterisation of cement-based materials is essential for understanding the influence of topological pore parameters such as connectivity and tortuosity on transport processes. The main objective of this thesis was to develop laser scanning confocal microscopy (LSCM) for 3D imaging and quantification of pore structure of cement-based materials at submicron resolution. To enable this, a novel approach to reconstruct large volumes of cement-based materials at submicron resolution was developed by combining serial sectioning with LSCM. The method uses a series of Z-stacks with overlapping regions for stitching based on phase correlation. With this method, no information is lost and the spatial resolution is maintained with increase in image size. The effects of axial distortion in LSCM images caused by mismatch of refractive indices between immersion medium and different phases within cement-based materials on various pore attributes were examined. Results indicated that parameters including porosity, specific surface area, percolation connectivity, scalar diffusion tortuosity and formation factor are not significantly affected by axial distortion. A generic correction method was proposed based on measuring the aspect ratio of pulverised fuel ash (PFA) particles in hardened blended pastes. The representative elementary volume for 3D pore characterisation of different cementitious systems was also investigated using a statistical approach. For a given number of realisations, an image volume of 1003 μm3 was found to give comparable porosity to that measured by 2D backscattered electron (BSE) microscopy. BSE signal variation across pore-solid boundaries was simulated using a 3D Monte Carlo technique to enhance image analysis of the pore structure. It was found that a single pore of down to 1 nm can be resolved with field emitters under ideal imaging conditions. The Overflow method was also found to be able to accurately segment pores larger than 1 μm with errors of ~1% and randomly inclined pores with an average error of ~5.2%. Effects of supplementary cementitious materials including silica fume (SF), pulverised fly ash (PFA) and ground granulated blastfurnace slag (GGBS) on the 3D pore structure of cement pastes were investigated using LSCM in conjunction with BSE microscopy. Generally, results from both techniques showed that SF enhances the pore structure (i.e. decreased porosity and percolation connectivity, and increased diffusion tortuosity) from early ages whereas PFA and GGBS show improvements at later ages. The percolation connectivity decreases while diffusion tortuosity increases drastically, as porosity reduces to ~15%. Measured 3D pore characteristics were used as inputs to simple analytical equations for predicting transport properties. Predicted results agreed reasonably well with measured values, mostly within a factor of five. An exploratory study into the application of fluorescence LSCM for real-time imaging of early cement hydration is also presented. Qualitative and quantitative analyses of microstructural developments in different hydrating cementitious systems were made. The advantages and limitations of LSCM for such application are also discussed.Open Acces

Feature investigation using micro computed tomography within materials

This work uses micro computed tomography, as a direct non-destructive tool, to investigate the internal 3D microstructure of different materials at multiple length scales to provide insights that are of interest to the scientific community on a variety of different problems.In the first chapter, the 3D microstructure of three types of manufactured biomass microsphere particles was investigated before and after fast pyrolysis. The results show that the particle size, biomass components, and volume of air decrease during fast pyrolysis.Next, concrete mixtures with different aggregate gradations and workability were prepared to study the aggregate packing in hardened concrete. The results show that low distance between aggregates and areas where no aggregates are observed correspond with mixtures of poor workability in the fresh concrete. These findings suggest that segregation of the coarse aggregate plays an important role in the workability of fresh concrete.In addition, the role of critical degree of saturation and air void system on the crack propagation of cement mortar subjected to freeze-thaw cycles was investigated. The results show that cracking occurred in non-air entrained mortar subjected to a single freeze-thaw cycle when the critical degree of saturation was near 100%. These microcracks mostly initiate and propagate from the paste-aggregate interface or from within aggregate. In addition, materials were observed to form within the pores after freezing.Finally, the role of air void system on the freeze-thaw damage of the cement paste was investigated. The results show that severe frost damage occurred in the surface of the non-air entrained cement paste ponded with KI solution after 63 freeze-thaw cycles. It was also observed that the average distance between air voids in the non-air-entrained was ~ 1.8x higher than the average distance between air voids in the air-entrained samples. In addition, most of the air voids (~75%) in both non-air-entrained and air-entrained samples are distributed in size ranges between 15 to 60 um. These observations show that X-ray imaging is a powerful method that provides new insights into physical properties and morphology of biomass particles, workability of flowable concrete, and freeze-thaw performance of concrete

Direct observations on microstructure evolution of cement systems at early ages

There is limited understanding of the mechanisms or direct measurements of cement paste hydration, as the main component that determines mechanical, durability, and rheology properties of concrete. This dissertation uses non-destructive in-situ X-ray imaging at multiple length scales (from 15.6 nm/pixel to 1.45 microm/pixel) to follow the three dimensional microstructural evolution and chemical composition change of ordinary portland cement (OPC) and monoclinic tricalcium silicate (mC3S) paste at early ages.Microscale resolution observations are made on paste at industrially relevant water-to-solids ratios between 0.40 and 0.70 to investigate the solidification of cement paste, evolution of air-filled void system, and stress induced change in kinetics of hydration. Complementary nanoscale resolution measurements were also made for a collection of OPC and mC3S particles at higher w/s to obtain microstructural and chemical information during hydration.The results from multiple techniques and different solution environments show that during the first hours of the reaction, hydration products with Ca/Si>3 form on and near the surface of the hydrating particles. These hydration products seem to change in chemistry and density over time. This process seems to be important to the induction and acceleration period of cement hydration.On the other hand, at the end of the induction period, the volume of air-filled voids reaches a maximum value and then decreases during the acceleration period and stays constant. The void distribution changes from a few coarse voids to a large number of smaller and more uniformly distributed voids after 10 h. This behavior is suggested to be controlled by changes in the ionic strength that cause exsolution of dissolved air from the pore solution.The results obtained from the loading experiments show that stress applied between 24 h and 60 h alters hydration kinetics of portland cement. The application of load caused an increase in stiffness, early age creep, and dissolution of individual cement particles in the highly loaded samples. These measurements provide insights into the microstructural changes that occur due to early age stress applications during the hydration of portland cement. Mechanisms are presented that discuss the observed behaviors

Synchrotron Microtomography and Neutron Radiography Characterization of the Microstruture and Water Absorption of Concrete from Pompeii

There is renewed interest in using advanced techniques to characterize ancient Roman concrete. In the present work, samples were drilled from the "Hospitium" in Pompeii and were analyzed by synchrotron microtomography (uCT) and neutron radiography to study how the microstructure, including the presence of induced cracks, affects their water adsorption. The water distribution and absorptivity were quantified by neutron radiography. The 3D crack propagation, pore size distribution and orientation, tortuosity, and connectivity were analyzed from uCT results using advanced imaging methods. The concrete characterization also included classical methods (e.g., differential thermal-thermogravimetric, X-ray diffractometry, and scanning electron microscopy). Ductile fracture patterns were observed once cracks were introduced. When compared to Portland cement mortar/concrete, Pompeii samples had relatively high porosity, low connectivity, and similar coefficient of capillary penetration. In addition, the permeability was predicted from models based on percolation theory and the pore structure data to evaluate the fluid transport properties

Investigation of Delayed Ettringite Formation Damage Process Using Simultaneous Neutron and X-ray Tomography

Delayed ettringite formation (DEF) is a significant deterioration process in concrete which involves the growth of ettringite [Ca6Al2(SO4)3(OH)12 ·26H2O] crystals leading to cracking and reduction of compressive strength. Conditions leading to DEF are well known and include among others cement chemistry, presence of humidity, heat curing of concrete structures, and the presence of cracks. The mechanisms and kinetics by which deterioration occur is still not well understood despite numerous investigations. Understanding the mechanism and kinetics of concrete deterioration due to DEF is important in order to prevent such costly deterioration and to improve concrete durability. In this research, concrete specimens were prepared with type III Portland cement and under different conditions that were designed to either promote or inhibit DEF. These consisted of a control set, a set subjected to a heat cycle and a third set made with elevated potassium content of 1.72% and also thermally cycled. They were tested periodically up to 380 days by conventional methods such as expansion and weight change measurements and compressive strength testing. Scanning electron microscopy with energy dispersive X-ray analysis (SEM-EDAX) confirmed the presence and the morphology of ettringite in voids at different ages. Simultaneous neutron and X-ray tomography, a new nondestructive microscopic method was used to scan the specimens at regular intervals in order to assess the feasibility of the method in monitoring the progress and characterizing DEF induced damages. The linear regression analysis of the correlation of expansion with weight change data revealed that expansion and deterioration process occurred in three distinct successive stages. In the first stage, the ettringite fills the pores with little or no expansion; in the second, the expansion appears to be creep due to expansive stresses in the filled pores and in the third stage, crack propagation leads to significant expansion and loss of compressive strength. The results of the linear regression also revealed that the mechanism of DEF is the replacement of pre-existing calcium hydroxide crystals. Through non-linear curve fitting, the kinetic of deterioration was modeled using the Kolmogorov-Avrami-Johnson-Miehl model. The simultaneous neutron and X-ray tomography allowed visualization of the interior of the specimen due to enhance phase segmentation. MATLAB routines were developed to allow for correction for beam hardening and to enhance phase segmentation. The study showed that with improved resolution, proper sample sizing, the method can be effectively used to characterize concrete damage due to expansive phases