Portland cement (PC) based concrete is the second most consumed commodity after water, and is the foundation of modern construction industry. The present status of the cement and concrete industry is unsustainable: the production of Portland cement (PC) accounts for ~8% of global anthropogenic CO2 emissions and the deterioration of existing reinforced concrete infrastructure is a constant, environmentally and economically costly problem. Optimizing the existing technology requires a breakthrough in the understanding of the cement hydration mechanisms, and in the structure-property correlation of the hydration products, especially at nano and atomistic scale. However despite decades of study, mechanisms underlying the cement hydration and all-stage performance remain unclear to a large extent, mainly due to the lack of experimental data at the fundamental level.

The aim of this dissertation is to provide new evidence to clarify unsolved questions along the service life timeline of PC concrete, using novel experimental methods that are based on synchrotron radiation. Three specific topics at distinct stage of concrete service life are investigated: 1) at early age the hydration mechanism of tricalcium aluminate (C3A) in the presence of gypsum. C3A is the most reactive clinker phase of PC whose rapid hydration greatly controls the workability of fresh concrete mixes, yet more evidence is needed to understand how C3A hydration is retarded by gypsum; 2) at mature age, the composition-structure-property correlation of calcium (alumino)silicate hydrates (C-(A-)S-H). C-(A-)S-H is the dominant binding phase of PC concrete, and therefore determines the mechanical property and volumetric stability of modern concrete infrastructure. It is hierarchically porous down to single-nanometer, making it rather difficult to experimentally probe the intrinsic mechanical properties of pore-free C-(A-)S-H; 3) at late age, the microstructure and chemistry of hydrated tricalcium silicate (C3S). C3S is the major clinker phase in PC and is the major source for the creation of C-S-H. Its chemistry and structure have been extensively studied for the first few months of hydration, but the long-term (beyond 10 years) characterization is rarely done.

The research reported in the present dissertation uses synchrotron-radiation-based characterizing techniques to provide multi-scale evidence to answer the above mentioned questions. In-situ Wide and Small Angle X-ray Scattering, X-ray spectromicroscopy coupled with the single-nanometer resolved X-ray Ptychography imaging tool, and nano-scale 3D Tomography are used, to investigate the dissolution-precipitation front of partially hydrated C3A particles. The crystal-chemistry and 3D-morphology of the hydration products are quantified. Ettringite is found to be the only stable phase during the induction period. Regardless of the water to cement ratio, ettringite needles grow to a maximum length of 1-2 μm. Ettringite crystallization is not homogeneous on the C3A surface, and the crystallization rate depends on the amount of relative surface site. Surface of pores/cavities inside the anhydrous C3A particles contributes significantly to the hydration reaction, as soon as the dissolution connects them with the solution.

The knowledge of both the properties of the (pore-free) material at fundamental scale, and the multi-scale pore structure, are critical to the theoretical prediction of the macro-scale properties of concrete. Here, high pressure X-ray diffraction is utilized to study the anisotropic nanomechanical property of C-(A-)S-H, by tracking the response of the nanocrystal lattice to hydrostatic pressure up to ~10 GPa. For the first time, the nanomechanical properties of pore-free C-(A-)S-H are investigated as a function of chemistry and atomic configuration. The experimental results show that, contrary to the predictions of most molecular simulations, the stiffness of C-S-H increases along with its bulk Ca to Si molar ratio (Ca/Si). Under hydrostatic pressure, the c-axis (perpendicular to the layer structure) deforms much more readily, compared with the a- and b-axis (i.e. the in-plane directions of the layer structure). The densification of the interlayer spacing significantly decreases the incompressibility along the c-axis, and thus increases the overall bulk modulus. When Al is incorporated, the Al-induced crosslinking of adjacent dreierketten chains increases the stiffness of C-(A-)S-H. Molecular simulations in this dissertation unveil the atomistic mechanism for the C-S-H deformation, that the in-plane deformation of the silicate tetrahedra chain is through rigid rotation instead of the shortening of the stiff Si-O bond. Therefore neither the vacancies nor the Al incorporation in the silicate chain, alters the in-plane stiffness of C-(A-)S-H layer structure.

Lastly, Scanning Transmission X-ray Microscopy is integrated with conventional methods (lab XRD, SEM, TEM and NMR) to study the composition and structure of a late age (50 years) hydrated C3S paste. After 50 years of curing, the mean chain length (MCL) of the C-S-H is 4.18, and the XRD reveals no resolved diffraction peaks for C-S-H. This indicates that the crystallization of C-S-H under ambient condition is negligible during concrete service life. Although the average Ca/Si of the C-S-H is ~1.7, much higher than that of tobermorite (0.67-0.8) but closer to that of jennite (1.5), the coordination environment of Ca in C-S-H is similar to that of tobermorite rather than jennite.

The results in this dissertation enhance the understanding of cement chemistry, as well as provide direct validation to existing cement hydration modelling and molecular level C-(A-)S-H modelling. The work also develops protocols for systematically studying modern construction material using innovative synchrotron-radiation-based methods.

An average of 0.9 tonne of CO2 is released for each tonne of clinker produced worldwide. Around 0.5 tonne of CO2 originates from the decarbonation of limestone, and on aggregate are considered to be the largest non-combustion source of CO2 emissions from industrial production. The remaining CO2 releases are mainly due to the fuel combusted during manufacturing of clinker and in the corresponding supply chains. In light of the scale and urgency of climate change, development of environmentally less damaging cements as well as improvements in concrete production have been explored by many researchers. While upgrading current plants with new technologies is costly and has already been done in most countries, the use of supplementary cementitious materials (SCMs) has shown to be the most cost-effective approach to reducing emissions from cement manufacturing. However, slag and fly ash, the two most used SCMs of suitable quality for clinker replacement, are concentrated in regions where coal power plants and steel plants are located, and the transition to cleaner and more efficient technologies for energy generation and steel manufacturing are limiting their use as a long-term solution. For this reason, there is a vital need to find readily available alternative materials for widespread use as partial replacement of clinker.

In this dissertation, the use of metakaolin (calcined clay) coupled with limestone as SCMs for partial replacement of clinker in cement is explored. Both their mechanical performance in mortar and concrete mixtures and their effects in reducing cement’s and concrete’s environmental burden are the main foci of this thesis. The importance of calculating the environmental impact of concrete binder made of cement, limestone, and metakaolin would allow us to understand their performance in reference to conventional concrete made of portland cement and commercially available Pozzolan-Portland-Cement concretes. In order to evaluate their environmental performance, Life-Cycle Assessment (LCA) of cements containing metakaolin is developed, along with a Life-Cycle Inventory (LCI) of each of its components. To this extent, a detailed description of an Excel-based tool, the MK-LCA Tool, is presented as a method to evaluate the cradle-to-gate environmental impacts of metakaolin by unit of mass. The environmental evaluation accounted not only for the direct emissions obtained from the production process, but also for indirect, supply-chain impacts of electricity generation and fuel pre-combustion. With an accurate evaluation of the emissions originating from manufacturing, the application of these materials in clinker replacement can be evaluated as a method to reduce cement’s, and in turn, concrete’s environmental footprint. Applications of the tool with a case study located in California showed a strong correlation between the global warming potential (GWP) of metakaolin’s production and the fuel used for its calcination, with values ranging from 340 kg CO2-eq/tonne to 46 kg CO2-eq/tonne when petroleum coke and waste wood were used, respectively. When included as SCM in ternary cement blend comprising clinker and limestone with a total substitution of 50% (i.e., 35% metakaolin and 15% limestone), significant reductions of greenhouse gas (GHG) emissions and life-cycle energy demand were achieved compared to portland cement (36% lower emissions and 19% lower energy demand when metakaolin was calcined using a dry rotary kiln, and 39% lower GHG emissions and 24% lower energy demand when using a flash calciner). When compared to commercially available blended cements, the ternary blends with metakaolin and limestone showed the lowest GWP, and energy demand was surpassed only by slag cement since the latter comprises an average replacement ratio of 85% of clinker by slag. However, all of the blended cements containing fly ash showed higher GHG intensities compared to the ternary blend with metakaolin and limestone cement (i.e., 16-42% increased GHG emissions). Additionally, data on the effect of the metakaolin-to-limestone ratio of ternary mortar blends on the mechanical performance and resistance to chloride penetration are also presented. It was observed that compressive strength is improved with the increase of limestone even for lower grades of calcined clay (i.e., low calcined kaolinite content). Furthermore, the transport properties are much improved with respect to those from portland cement concretes, which translates to more durable cement binders, demonstrating the ample applicability of these ternary cement blends. The development of ternary blends studied in this dissertation made with portland cement, calcined clay, and limestone at industrial scale will have the potential to contribute to 30-50% global reduction of GHG emissions from the cement industries with a focus on emerging economies that would benefit from low-carbon cements without the technical and economic challenges that limit the implementation of other highly technical solutions such as carbon capture and storage.

The objective of this study was to perform a laboratory investigation of the corrosion performance of several types of steel dowels embedded in concrete beams. The concrete beams and dowels were subjected to exposure to concentrated chloride solutions intended to accelerate corrosion and simulate environmental conditions. The purpose of this investigation was to develop recommendations for use of different types of dowels for different environmental risk conditions. To provide an indication of the aggressiveness of the laboratory conditions relative to field conditions, a set of field slabs from Washington State was examined, and chloride content analyses were performed on the concrete beams used in the laboratory studies and on cores taken from nine locations in Washington State.